### 2016

- A. F. Kemper, M. A. Sentef, B. Moritz, T. P. Devereaux, and J. K. Freericks, Review of the theoretical description of time-resolved angle-resolved photoemission spectroscopy in electron-phonon mediated superconductors, 2016.

[Bibtex]`@misc{escidoc:2344298, title = {{Review of the theoretical description of time-resolved angle-resolved photoemission spectroscopy in electron-phonon mediated superconductors}}, author = {Kemper, A. F. and Sentef, Michael A. and Moritz, B. and Devereaux, T. P. and Freericks, J. K.}, language = {eng}, eprint = {1609.00087}, year = {2016}, abstract = {{We review recent work on the theory for pump/probe photoemission spectroscopy of electron-phonon mediated superconductors in both the normal and the superconducting states. We describe the formal developments that allow one to solve the Migdal-Eliashberg theory in nonequilibrium for an ultrashort laser pumping field, and explore the solutions which illustrate the relaxation as energy is transferred from electrons to phonons. We focus on exact results emanating from sum rules and approximate numerical results which describe rules of thumb for relaxation processes. In addition, in the superconducting state, we describe how Higg\textquotesingles oscillations can be excited due to the nonlinear coupling with the electric field and how pumping the system can enhance superconductivity.}}, }`

- T. Dimitrov, H. Appel, J. I. Fuks, and A. Rubio, “Exact maps in density functional theory for lattice models,” New Journal of Physics, vol. 18, iss. 8, 2016.

[Bibtex]`@article{escidoc:2344038, title = {{Exact maps in density functional theory for lattice models}}, author = {Dimitrov, Tanja and Appel, Heiko and Fuks, Johanna I. and Rubio, Angel}, issn = {1367-2630}, doi = {10.1088/1367-2630/18/8/083004}, publisher = {IOP Publishing}, address = {Bristol}, year = {2016}, abstract = {{In the present work, we employ exact diagonalization for model systems on a real-space lattice to explicitly construct the exact density-to-potential and graphically illustrate the complete exact density-to-wavefunction map that underly the Hohenberg\textendashKohn theorem in density functional theory. Having the explicit wavefunction-to-density map at hand, we are able to construct arbitrary observables as functionals of the ground-state density. We analyze the density-to-potential map as the distance between the fragments of a system increases and the correlation in the system grows. We observe a feature that gradually develops in the density-to-potential map as well as in the density-to-wavefunction map. This feature is inherited by arbitrary expectation values as functional of the ground-state density. We explicitly show the excited-state energies, the excited-state densities, and the correlation entropy as functionals of the ground-state density. All of them show this exact feature that sharpens as the coupling of the fragments decreases and the correlation grows. We denominate this feature as intra-system steepening and discuss how it relates to the well-known inter-system derivative discontinuity. The inter-system derivative discontinuity is an exact concept for coupled subsystems with degenerate ground state. However, the coupling between subsystems as in charge transfer processes can lift the degeneracy. An important conclusion is that for such systems with a near-degenerate ground state, the corresponding cut along the particle number N of the exact density functionals is differentiable with a well-defined gradient near integer particle number.}}, journal = {{New Journal of Physics}}, volume = {18}, number = {8}, eid = {083004}, }`

- F. G. Eich and F. Agostini, “The adiabatic limit of the exact factorization of the electron-nuclear wave function,” The Journal of Chemical Physics, vol. 145, iss. 5, 2016.

[Bibtex]`@article{escidoc:2326779, title = {{The adiabatic limit of the exact factorization of the electron-nuclear wave function}}, author = {Eich, Florian G. and Agostini, Federica}, language = {eng}, issn = {0021-9606}, doi = {10.1063/1.4959962}, eprint = {1604.05098}, publisher = {American Institute of Physics}, address = {Woodbury, N.Y.}, year = {2016}, date = {2016-08-07}, abstract = {{We propose a procedure to analyze the relation between the exact factorization of the electron-nuclear wave function and the Born-Oppenheimer approximation. We define the adiabatic limit as the limit of infinite nuclear mass. To this end, we introduce a unit system that singles out the dependence on the electron-nuclear mass ratio of each term appearing in the equations of the exact factorization. We observe how non-adiabatic effects induced by the coupling to the nuclear motion affect electronic properties and we analyze the leading term, connecting it to the classical nuclear momentum. Its dependence on the mass ratio is tested numerically on a model of proton-coupled electron transfer in different non-adiabatic regimes.}}, journal = {{The Journal of Chemical Physics}}, volume = {145}, number = {5}, eid = {054110}, }`

- M. Lubasch, J. I. Fuks, H. Appel, A. Rubio, I. J. Cirac, and M. Bañuls, “Systematic construction of density functionals based on matrix product state computations,” New Journal of Physics, vol. 18, iss. 8, 2016.

[Bibtex]`@article{escidoc:2332181, title = {{Systematic construction of density functionals based on matrix product state computations}}, author = {Lubasch, Michael and Fuks, Johanna I. and Appel, Heiko and Rubio, Angel and Cirac, J. Ignacio and Ba\~nuls, Mari-Carmen}, language = {eng}, issn = {1367-2630}, doi = {10.1088/1367-2630/18/8/083039}, eprint = {1603.06565}, publisher = {IOP Publishing}, address = {Bristol}, year = {2016}, abstract = {{We propose a systematic procedure for the approximation of density functionals in density functional theory that consists of two parts. First, for the efficient approximation of a general density functional, we introduce an efficient ansatz whose non-locality can be increased systematically. Second, we present a fitting strategy that is based on systematically increasing a reasonably chosen set of training densities. We investigate our procedure in the context of strongly correlated fermions on a one-dimensional lattice in which we compute accurate training densities with the help of matrix product states. Focusing on the exchange-correlation energy, we demonstrate how an efficient approximation can be found that includes and systematically improves beyond the local density approximation. Importantly, this systematic improvement is shown for target densities that are quite different from the training densities.}}, journal = {{New Journal of Physics}}, volume = {18}, number = {8}, eid = {083039}, }`

- P. Wopperer, U. De Giovannini, and A. Rubio, Efficient and accurate modeling of electron photoemission in nanostructures with TDDFT, 2016.

[Bibtex]`@misc{escidoc:2328295, title = {{Efficient and accurate modeling of electron photoemission in nanostructures with TDDFT}}, author = {Wopperer, Philipp and De Giovannini, Umberto and Rubio, Angel}, language = {eng}, eprint = {1608.02818}, year = {2016}, abstract = {{We review different computational methods for the calculation of photoelectron spectra and angular distributions of atoms and molecules when excited by laser pulses using time-dependent density-functional theory (TDDFT) that are suitable for the description of electron emission in compact spatial regions. We derive and extend the time-dependent surface-flux method introduced in L. Tao and A. Scrinzi 2012 New J. Phys. 14 013021 within a TDDFT formalism and compare its performance to other existing methods. We illustrate the performance of the new method by simulating strong-field ionization of C\textlesssub\textgreater60\textless/sub\textgreater fullerene and discuss final state effects in the orbital reconstruction of planar organic molecules.}}, }`

- J. I. Fuks, S. E. B. Nielsen, M. Ruggenthaler, and N. T. Maitra, “Time-dependent density functional theory beyond Kohn\textendashSham Slater determinants,” Physical Chemistry Chemical Physics, vol. 18, iss. 31, pp. 20976-20985, 2016.

[Bibtex]`@article{escidoc:2328218, title = {{Time-dependent density functional theory beyond Kohn\textendashSham Slater determinants}}, author = {Fuks, Johanna I. and Nielsen, S\oren Ersbak Bang and Ruggenthaler, Michael and Maitra, Neepa T.}, language = {eng}, issn = {1463-9076}, doi = {10.1039/C6CP00722H}, eprint = {1603.01176}, publisher = {Royal Society of Chemistry}, address = {Cambridge, England}, year = {2016}, date = {2016-08-21}, abstract = {{When running time-dependent density functional theory (TDDFT) calculations for real-time simulations of non-equilibrium dynamics, the user has a choice of initial Kohn\textendashSham state, and typically a Slater determinant is used. We explore the impact of this choice on the exchange\textendashcorrelation potential when the physical system begins in a 50 : 50 superposition of the ground and first-excited state of the system. We investigate the possibility of judiciously choosing a Kohn\textendashSham initial state that minimizes errors when adiabatic functionals are used. We find that if the Kohn\textendashSham state is chosen to have a configuration matching the one that dominates the interacting state, this can be achieved for a finite time duration for some but not all such choices. When the Kohn\textendashSham system does not begin in a Slater determinant, we further argue that the conventional splitting of the exchange\textendashcorrelation potential into exchange and correlation parts has limited value, and instead propose a decomposition into a \textquotedblleftsingle-particle\textquotedblright contribution that we denote v\textlesssup\textgreaterS\textless/sup\textgreater\textlesssub\textgreaterxc\textless/sub\textgreater, and a remainder. The single-particle contribution can be readily computed as an explicit orbital-functional, reduces to exchange in the Slater determinant case, and offers an alternative to the adiabatic approximation as a starting point for TDDFT approximations.}}, journal = {{Physical Chemistry Chemical Physics}}, volume = {18}, number = {31}, pages = {20976--20985}, }`

- D. J. Mowbray, A. Pérez Paz, G. Ruiz-Soria, M. Sauer, P. Lacovig, M. Dalmiglio, S. Lizzit, K. Yanagi, A. Goldoni, P. Ayala, T. Pichler, and A. Rubio, “Disentangling Vacancy Oxidation on Metallicity-Sorted Carbon Nanotubes,” The Journal of Physical Chemistry C, vol. Just Accepted Manuscript, 2016.

[Bibtex]`@article{escidoc:2325901, title = {{Disentangling Vacancy Oxidation on Metallicity-Sorted Carbon Nanotubes}}, author = {Mowbray, Duncan John and P\'erez Paz, Alejandro and Ruiz-Soria, Georgina and Sauer, Markus and Lacovig, Paolo and Dalmiglio, Matteo and Lizzit, Silvano and Yanagi, Kazuhiro and Goldoni, Andrea and Ayala, Paola and Pichler, Thomas and Rubio, Angel}, language = {eng}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.6b06163}, eprint = {1608.01424}, publisher = {American Chemical Society}, address = {Washington DC}, year = {2016}, abstract = {{Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O\textlesssub\textgreater2\textless/sub\textgreater and N\textlesssub\textgreater2\textless/sub\textgreater, which for low doses chemisorb only on defect sites or vacancies of the SWCNTs at the ppm level. However, very low doping has a major effect on the electronic properties and conductivity of the SWCNTs. Already at low O\textlesssub\textgreater2\textless/sub\textgreater doses (80 L), the X-ray photoelectron spectroscopy (XPS) O 1s signal becomes saturated, indicating nearly all the SWCNT\textquotesingles vacancies have been oxidized. As a result, probing vacancy oxidation on SWCNTs via XPS yields spectra with rather low signal-to-noise ratios, even for metallicity-sorted SWCNTs. We show that, even under these conditions, the first principles density functional theory calculated Kohn-Sham O 1s binding energies may be used to assign the XPS O 1s spectra for oxidized vacancies on SWCNTs into its individual components. This allows one to determine the specific functional groups or bonding environments measured. We find the XPS O 1s signal is mostly due to three O-containing functional groups on SWCNT vacancies: epoxy (C\textlesssub\textgreater2\textless/sub\textgreater\textgreaterO), carbonyl (C\textlesssub\textgreater2\textless/sub\textgreater\textgreaterC\textequalsO), and ketene (C\textequalsC\textequalsO), as ordered by abundance. Upon oxidation of nearly all the SWCNT\textquotesingles vacancies, the central peak\textquotesingles intensity for the metallic SWCNT sample is 60\textpercent greater than for the semiconducting SWCNT sample. This suggests a greater abundance of O-containing defect structures on the metallic SWCNT sample. For both metallic and semiconducting SWCNTs, we find O\textlesssub\textgreater2\textless/sub\textgreater does not contribute to the measured XPS O 1s spectra.}}, journal = {{The Journal of Physical Chemistry C}}, volume = {Just Accepted Manuscript}, }`

- A. Varas, P. Garc’ia-González, J. Feist, F. J. Garc’ia-Vidal, and A. Rubio, “Quantum plasmonics: from jellium models to ab initio calculations,” Nanophotonics, vol. Ahead of print, 2016.

[Bibtex]`@article{escidoc:2327940, title = {{Quantum plasmonics: from jellium models to ab initio calculations}}, author = {Varas, Alejandro and Garc\'\ia-Gonz\'alez, Pablo and Feist, Johannes and Garc\'\ia-Vidal, F. J. and Rubio, Angel}, language = {eng}, issn = {2192-8614}, doi = {10.1515/nanoph-2015-0141}, publisher = {de Gruyter}, address = {Berlin, Germany}, year = {2016}, abstract = {{Light-matter interaction in plasmonic nanostructures is often treated within the realm of classical optics. However, recent experimental findings show the need to go beyond the classical models to explain and predict the plasmonic response at the nanoscale. A prototypical system is a nanoparticle dimer, extensively studied using both classical and quantum prescriptions. However, only very recently, fully ab initio time-dependent density functional theory (TDDFT) calculations of the optical response of these dimers have been carried out. Here, we review the recent work on the impact of the atomic structure on the optical properties of such systems. We show that TDDFT can be an invaluable tool to simulate the time evolution of plasmonic modes, providing fundamental understanding into the underlying microscopical mechanisms.}}, journal = {{Nanophotonics}}, volume = {Ahead of print}, }`

- D. G. de Oteyza, A. P. Paz, Y. Chen, Z. Pedramrazi, A. Riss, S. Wickenburg, H. Tsai, F. R. Fischer, M. F. Crommie, and A. Rubio, “Non-Covalent Dimerization after Enediyne Cyclization on Au(111),” Journal of the American Chemical Society, vol. Just Accepted Manuscript, 2016.

[Bibtex]`@article{escidoc:2327551, title = {{Non-Covalent Dimerization after Enediyne Cyclization on Au(111)}}, author = {de Oteyza, Dimas G. and Paz, Alejandro P\'erez and Chen, Yen-Chia and Pedramrazi, Zahra and Riss, Alexander and Wickenburg, Sebastian and Tsai, Hsin-Zon and Fischer, Felix R. and Crommie, Michael F. and Rubio, Angel}, language = {eng}, issn = {0002-7863}, doi = {10.1021/jacs.6b05203}, publisher = {American Chemical Society}, address = {Washington, DC}, year = {2016}, abstract = {{We investigate the thermally-induced cyclization of 1,2-bis(2-phenylethynyl)benzene on Au(111) using scanning tunneling microscopy and computer simulations. Cyclization of sterically hindered enediynes is known to proceed via two competing mechanisms in solution: a classic C\textlesssup\textgreater1\textless/sup\textgreater\textendashC\textlesssup\textgreater6\textless/sup\textgreater or a C\textlesssup\textgreater1\textless/sup\textgreater\textendashC\textlesssup\textgreater5\textless/sup\textgreater cyclization pathway. On Au(111) we find that the C\textlesssup\textgreater1\textless/sup\textgreater\textendashC\textlesssup\textgreater5\textless/sup\textgreater cyclization is suppressed and that the C\textlesssup\textgreater1\textless/sup\textgreater\textendashC\textlesssup\textgreater6\textless/sup\textgreater cyclization yields a highly strained bicyclic olefin whose surface chemistry was hitherto unknown. The C\textlesssup\textgreater1\textless/sup\textgreater\textendashC\textlesssup\textgreater6\textless/sup\textgreater product self-assembles into discrete non-covalently bound dimers on the surface. The reaction mechanism and driving forces behind non-covalent association are discussed in light of density functional theory calculations.}}, journal = {{Journal of the American Chemical Society}}, volume = {Just Accepted Manuscript}, }`

- I. Kylänpää, F. Berardi, E. Räsänen, P. Garc’ia-González, C. A. Rozzi, and A. Rubio, “Stability of the Dirac cone in artificial graphene formed in quantum wells: a computational many-electron study,” New Journal of Physics, vol. 18, iss. 8, 2016.

[Bibtex]`@article{escidoc:2327549, title = {{Stability of the Dirac cone in artificial graphene formed in quantum wells: a computational many-electron study}}, author = {Kyl\"anp\"a\"a, Ilkka and Berardi, Fulvio and R\"as\"anen, Esa and Garc\'\ia-Gonz\'alez, Pablo and Rozzi, Carlo Andrea and Rubio, Angel}, language = {eng}, issn = {1367-2630}, doi = {10.1088/1367-2630/18/8/083014}, publisher = {IOP Publishing}, address = {Bristol}, year = {2016}, abstract = {{We carry out a comprehensive computational study on the stability of the Dirac cone in artificial graphene realized in nanopatterned quantum wells. Our real-space approach allows us to vary the size, shape, and positioning of the quantum dots in the hexagonal lattice. We compare the (noninteracting) single-particle calculations to density-functional studies within both local-density approximation and meta-generalized-gradient approximation. Furthermore, the density-functional results are compared against numerically precise path-integral quantum Monte Carlo calculations. As a whole, our results indicate high stability of the Dirac bands against external parameters, which is reassuring for further experimental investigations.}}, journal = {{New Journal of Physics}}, volume = {18}, number = {8}, eid = {083014}, }`

- L. Shi, P. Rohringer, K. Suenaga, Y. Niimi, J. Kotakoski, J. C. Meyer, H. Peterlik, M. Wanko, S. Cahangirov, A. Rubio, Z. J. Lapin, L. Novotny, P. Ayala, and T. Pichler, “Confined linear carbon chains as a route to bulk carbyne,” Nature Materials, vol. 15, iss. 6, pp. 634-639, 2016.

[Bibtex]`@article{escidoc:2261774, title = {{Confined linear carbon chains as a route to bulk carbyne}}, author = {Shi, Lei and Rohringer, Philip and Suenaga, Kazu and Niimi, Yoshiko and Kotakoski, Jani and Meyer, Jannik C. and Peterlik, Herwig and Wanko, Marius and Cahangirov, Seymur and Rubio, Angel and Lapin, Zachary J. and Novotny, Lukas and Ayala, Paola and Pichler, Thomas}, language = {eng}, issn = {1476-1122}, doi = {10.1038/nmat4617}, eprint = {1507.04896}, publisher = {Nature Pub. Group}, address = {London, UK}, year = {2016}, date = {2016-06}, abstract = {{Strong chemical activity and extreme instability in ambient conditions characterize carbyne, an infinite sp\textlesssup\textgreater1\textless/sup\textgreater hybridized carbon chain. As a result, much less has been explored about carbyne as compared to other carbon allotropes such as fullerenes, nanotubes and graphene. Although end-capping groups can be used to stabilize carbon chains, length limitations are still a barrier for production, and even more so for application. We report a route for the bulk production of long acetylenic linear carbon chains protected by thin double-walled carbon nanotubes. The synthesis of very long arrangements is confirmed by a combination of transmission electron microscopy, X-ray diraction and (near-field) near-resonance Raman spectroscopy. Our results establish a route for the bulk production of exceptionally long and stable chains composed of more than 6,000 carbon atoms, representing an elegant forerunner towards the final goal of carbyne\textquoterights bulk production.}}, journal = {{Nature Materials}}, volume = {15}, number = {6}, pages = {634--639}, }`

- L. Xian, A. P. Paz, E. Bianco, P. M. Ajayan, and A. Rubio, Square selenene and tellurene: novel group VI elemental 2D semi-Dirac materials and topological insulators, 2016.

[Bibtex]`@misc{escidoc:2316490, title = {{Square selenene and tellurene: novel group VI elemental 2D semi-Dirac materials and topological insulators}}, author = {Xian, Lede and Paz, Alejandro P\'erez and Bianco, Elisabeth and Ajayan, Pulickel M. and Rubio, Angel}, language = {eng}, eprint = {1607.01555}, year = {2016}, abstract = {{With first principles calculations, we predict a novel stable 2D layered structure for group VI elements Se and Te that we call square selenene and square tellurene, respectively. They have chair-like buckled structures similar to other layered materials such as silicene and germanene but with a square unit cell rather than hexagonal. This special structure gives rise to anisotropic band dispersions near the Fermi level that can be described by a generalized semi-Dirac Hamiltonian. We show that the considerably large band gap ($\sim$0.1 eV) opened by spin-orbit coupling makes square selenene and tellurene topological insulators, hosting non-trivial edge states. Therefore, square selenene and tellurene are promising materials for novel electronic and spintronic applications. Finally, we show that this new type of 2D elemental material can potentially be grown on proper substrates, such as a Au(100) surface.}}, }`

- I. Gierz, M. Chavez Cervantes, M. Mitrano, R. Tomar, H. Bromberger, H. Liu, S. Kaiser, M. A. Sentef, A. Stöhr, S. Link, U. Starke, C. Cacho, R. Chapman, E. Springate, F. Frasetto, L. Poletto, and A. Cavalleri, Enhanced electron-phonon coupling in a periodically distorted graphene lattice, 2016.

[Bibtex]`@misc{escidoc:2316466, title = {{Enhanced electron-phonon coupling in a periodically distorted graphene lattice}}, author = {Gierz, Isabella and Chavez Cervantes, Mariana and Mitrano, Matteo and Tomar, Raghu and Bromberger, Hubertus and Liu, Haiyun and Kaiser, Stefan and Sentef, Michael A. and St\"ohr, A. and Link, S. and Starke, U. and Cacho, C. and Chapman, R. and Springate, E. and Frasetto, F. and Poletto, L. and Cavalleri, Andrea}, language = {eng}, eprint = {1607.02314}, year = {2016}, abstract = {{Electron-phonon coupling influences most of the collective properties of solids, and directly determines the stability of emergent orders like superconductivity, charge and spin density waves. In this context, the ability to control electron-phonon coupling away from equilibrium may provide new ways to steer materials\textquotesingle functionalities, potentially at high speeds. Here we use time- and angle-resolved photoemission spectroscopy to measure the electron-phonon interaction in optically excited graphene. We find that the electron-phonon coupling constant, as extracted from the energy-dependent carrier relaxation, is unaffected by both interband excitation and by free carrier absorption in the mid-infrared. However, when the excitation is made resonant with the in-plane bond stretching phonon of bilayer graphene, the average electron-phonon coupling constant is markedly enhanced. The present experiment provides useful perspective for the enhancement of superconductivity observed in alkali-doped fullerites when a similar phonon mode was excited.}}, }`

- A. Riss, A. P. Paz, S. Wickenburg, H. Tsai, D. D. G. Oteyza, A. J. Bradley, M. M. Ugeda, P. Gorman, H. S. Jung, M. F. Crommie, A. Rubio, and F. R. Fischer, “Imaging single-molecule reaction intermediates stabilized by surface dissipation and entropy,” Nature Chemistry, vol. 8, iss. 7, pp. 678-683, 2016.

[Bibtex]`@article{escidoc:2273080, title = {{Imaging single-molecule reaction intermediates stabilized by surface dissipation and entropy}}, author = {Riss, Alexander and Paz, Alejandro P\'erez and Wickenburg, Sebastian and Tsai, Hsin-Zon and Oteyza, Dimas G. De and Bradley, Aaron J. and Ugeda, Miguel M. and Gorman, Patrick and Jung, Han Sae and Crommie, Michael F. and Rubio, Angel and Fischer, Felix R.}, language = {eng}, issn = {1755-4330}, doi = {10.1038/nchem.2506}, publisher = {Nature Publishing Group}, address = {London, UK}, year = {2016}, date = {2016-07}, abstract = {{Chemical transformations at the interface between solid/liquid or solid/gaseous phases of matter lie at the heart of key industrial-scale manufacturing processes. A comprehensive study of the molecular energetics and conformational dynamics that underlie these transformations is often limited to ensemble-averaging analytical techniques. Here we report the detailed investigation of a surface-catalysed cross-coupling and sequential cyclization cascade of 1,2-bis(2-ethynyl phenyl)ethyne on Ag(100). Using non-contact atomic force microscopy, we imaged the single-bond-resolved chemical structure of transient metastable intermediates. Theoretical simulations indicate that the kinetic stabilization of experimentally observable intermediates is determined not only by the potential-energy landscape, but also by selective energy dissipation to the substrate and entropic changes associated with key transformations along the reaction pathway. The microscopic insights gained here pave the way for the rational design and control of complex organic reactions at the surface of heterogeneous catalysts.}}, journal = {{Nature Chemistry}}, volume = {8}, number = {7}, pages = {678--683}, }`

- J. M. Garcia-Lastra, I. Boukahil, R. Qiao, A. Rubio, and F. J. Himpsel, “Electronic Structure of Low-Dimensional Carbon $\pi$-Systems,” The Journal of Physical Chemistry C, vol. 120, iss. 23, pp. 12362-12368, 2016.

[Bibtex]`@article{escidoc:2262357, title = {{Electronic Structure of Low-Dimensional Carbon $\pi$-Systems}}, author = {Garcia-Lastra, Juan Maria and Boukahil, Idris and Qiao, Ruimin and Rubio, Angel and Himpsel, Franz J.}, language = {eng}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.6b02530}, publisher = {American Chemical Society}, address = {Washington DC}, year = {2016}, date = {2016-06-16}, abstract = {{X-ray absorption spectroscopy (XAS) is combined with density functional theory (DFT) to determine the orbitals of one- and two-dimensional carbon $\pi$-systems (lycopene, beta-carotene, retinal, retinol, retinoic acid, coronene, triphenylene). Considerable fine structure is observed for the transition from the C 1s level to the lowest unoccupied molecular orbital (LUMO) and explained by DFT. The wave functions of the one-dimensional chain molecules display the node structure of a vibrating string. The XAS transition energy is decomposed into contributions from the C 1s core level, the $\pi$\textasteriskcentered final state, and the electron\textendashhole interaction. For the latter, we develop a simple model that accurately represents a full $\Delta$-self-consistent field ($\Delta$SCF) calculation. The distortion of the LUMO because of its interaction with the C 1s hole is investigated. These results illustrate the electronic states of prototypical $\pi$-bonded carbon structures with low-dimensional character, such as those used in molecular complexes for solar cells, confined graphene structures, and molecular wires.}}, journal = {{The Journal of Physical Chemistry C}}, volume = {120}, number = {23}, pages = {12362--12368}, }`

- W. Cao, P. Tang, S. Zhang, W. Duan, and A. Rubio, “Stable Dirac semimetal in the allotropes of group-IV elements,” Physical Review B, vol. 93, iss. 24, 2016.

[Bibtex]`@article{escidoc:2246002, title = {{Stable Dirac semimetal in the allotropes of group-IV elements}}, author = {Cao, Wendong and Tang, Peizhe and Zhang, Shou-Cheng and Duan, Wenhui and Rubio, Angel}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.241117}, eprint = {1601.05523}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2016}, date = {2016-06-15}, abstract = {{Three-dimensional topological Dirac semimetals represent a novel state of quantum matter with exotic electronic properties, in which a pair of Dirac points with a linear dispersion along all momentum directions exists in the bulk. Herein, by using first-principles calculations, we discover the metastable allotropes of Ge and Sn in the staggered layered dumbbell structure, named germancite and stancite, to be Dirac semimetals with a pair of Dirac points on their rotation axis. On the surface parallel to the rotation axis, a pair of topologically nontrivial Fermi arcs are observed and a Lifshitz transition is found by tuning the Fermi level. Furthermore, the quantum thin film of germancite is found to be an intrinsic quantum spin Hall insulator. These discoveries suggest novel physical properties and future applications of the metastable allotrope of Ge and Sn.}}, journal = {{Physical Review B}}, volume = {93}, number = {24}, eid = {241117(R)}, }`

- I. Theophilou, N. N. Lathiotakis, and N. Helbig, “Conditions for Describing Triplet States in Reduced Density Matrix Functional Theory,” Journal of Chemical Theory and Computation, vol. 12, iss. 6, pp. 2668-2678, 2016.

[Bibtex]`@article{escidoc:2306046, title = {{Conditions for Describing Triplet States in Reduced Density Matrix Functional Theory}}, author = {Theophilou, Iris and Lathiotakis, Nektarios N. and Helbig, Nicole}, language = {eng}, doi = {10.1021/acs.jctc.6b00257}, eprint = {1603.04634}, publisher = {American Chemical Society}, address = {Washington, D.C.}, year = {2016}, date = {2016-06-14}, abstract = {{We consider necessary conditions for the one-body reduced density matrix (1RDM) to correspond to a triplet wave function of a two-electron system. The conditions concern the occupation numbers and are different for the high spin projections, S\textlesssub\textgreaterz\textless/sub\textgreater \textequals $\pm$1, and the S\textlesssub\textgreaterz\textless/sub\textgreater \textequals 0 projection. Hence, they can be used to test if an approximate 1RDM functional yields the same energies for both projections. We employ these conditions in reduced density matrix functional theory calculations for the triplet excitations of two-electron systems. In addition, we propose that these conditions can be used in the calculation of triplet states of systems with more than two electrons by restricting the active space. We assess this procedure in calculations for a few atomic and molecular systems. We show that the quality of the optimal 1RDMs improves by applying the conditions in all the cases we studied.}}, journal = {{Journal of Chemical Theory and Computation}}, volume = {12}, number = {6}, pages = {2668--2678}, }`

- S. M. Anderson, N. Tancogne-Dejean, B. S. Mendoza, and V. Véniard, “Improved ab initio calculation of surface second-harmonic generation from Si(111)(1$\times$1):H,” Physical Review B, vol. 93, iss. 23, 2016.

[Bibtex]`@article{escidoc:2306423, title = {{Improved ab initio calculation of surface second-harmonic generation from Si(111)(1$\times$1):H}}, author = {Anderson, Sean M. and Tancogne-Dejean, Nicolas and Mendoza, Bernardo S. and V\'eniard, Val\'erie}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.235304}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2016}, date = {2016-06-15}, abstract = {{We carry out an improved ab initio calculation of surface second-harmonic generation (SSHG) from the Si(111)(1$\times$1):H surface. This calculation includes three new features in one formulation: (i) the scissors correction, (ii) the contribution of the nonlocal part of the pseudopotentials, and (iii) the inclusion of a cut function to extract the surface response, all within the independent particle approximation. We apply these improvements on the Si(111)(1$\times$1):H surface and compare with various experimental spectra from several different sources. We also revisit the three-layer model for the SSHG yield and demonstrate that it provides more accurate results over several, more common, two-layer models. We demonstrate the importance of using properly relaxed coordinates for the theoretical calculations. We conclude that this approach to the calculation of the second-harmonic spectra is versatile and accurate within this level of approximation. This well-characterized surface offers an excellent platform for comparison with theory and allows us to offer this study as an efficient benchmark for this type of calculation.}}, journal = {{Physical Review B}}, volume = {93}, number = {23}, eid = {235304}, }`

- R. Bertoni, C. Nicholson, L. Waldecker, H. Hübener, C. Monney, U. De Giovannini, M. Puppin, M. Hoesch, E. Springate, R. T. Chapman, C. Cacho, M. Wolf, A. Rubio, and R. Ernstorfer, Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2, 2016.

[Bibtex]`@misc{escidoc:2306171, title = {{Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2}}, author = {Bertoni, Roman and Nicholson, Christopher and Waldecker, Lutz and H\"ubener, Hannes and Monney, Claude and De Giovannini, Umberto and Puppin, Michele and Hoesch, Moritz and Springate, Emma and Chapman, Richard T. and Cacho, Cephise and Wolf, Martin and Rubio, Angel and Ernstorfer, Ralph}, language = {eng}, eprint = {1606.03218}, year = {2016}, abstract = {{Manipulation of spin and valley degrees of freedom is a key step towards realizing novel quantum technologies, for which atomically thin transition metal dichalcogenides (TMDCs) have been established as promising candidates. In monolayer TMDCs, the lack of inversion symmetry gives rise to a spin-valley correlation of the band structure allowing for valley-selective electronic excitation with circularly polarized light. Here we show that, even in centrosymmetric samples of 2H-WSe\textlesssub\textgreater2\textless/sub\textgreater, circularly polarized light can generate spin-, valley- and layer-polarized excited states in the conduction band. Employing time- and angle-resolved photoemission spectroscopy (trARPES) with spin-selective excitation, the dynamics of valley and layer pseudospins of the excited carriers are investigated. Complementary time-dependent density functional theory (TDDFT) calculations of the excited state populations reveal a strong circular dichroism of the spin-, valley- and layer-polarizations and a pronounced 2D character of the excited states in the K valleys. We observe scattering of carriers towards the global minimum of the conduction band on a sub-100 femtosecond timescale to states with three-dimensional character facilitating inter-layer charge transfer. Our results establish the optical control of coupled spin-, valley- and layer-polarized states in centrosymmetric materials and suggest the suitability of TMDC multilayer materials for valleytronic and spintronic device concepts.}}, }`

- J. Walkenhorst, U. De Giovannini, A. Castro, and A. Rubio, “Tailored pump-probe transient spectroscopy with time-dependent density-functional theory: controlling absorption spectra,” European Physical Journal B, vol. 89, iss. 5, 2016.

[Bibtex]`@article{escidoc:2243417, title = {{Tailored pump-probe transient spectroscopy with time-dependent density-functional theory: controlling absorption spectra}}, author = {Walkenhorst, Jessica and De Giovannini, Umberto and Castro, Alberto and Rubio, Angel}, language = {eng}, issn = {1434-6028}, doi = {10.1140/epjb/e2016-70064-0}, eprint = {1601.04544}, publisher = {Springer-Verlag Heidelberg}, address = {Heidelberg}, year = {2016}, date = {2016-05}, abstract = {{Recent advances in laser technology allow us to follow electronic motion at its natural time-scale with ultra-fast time resolution, leading the way towards attosecond physics experiments of extreme precision. In this work, we assess the use of tailored pumps in order to enhance (or reduce) some given features of the probe absorption (for example, absorption in the visible range of otherwise transparent samples). This type of manipulation of the system response could be helpful for its full characterization, since it would allow us to visualize transitions that are dark when using unshaped pulses. In order to investigate these possibilities, we perform first a theoretical analysis of the non-equilibrium response function in this context, aided by one simple numerical model of the hydrogen atom. Then, we proceed to investigate the feasibility of using time-dependent density-functional theory as a means to implement, theoretically, this absorption-optimization idea, for more complex atoms or molecules. We conclude that the proposed idea could in principle be brought to the laboratory: tailored pump pulses can excite systems into light-absorbing states. However, we also highlight the severe numerical and theoretical difficulties posed by the problem: large-scale non-equilibrium quantum dynamics are cumbersome, even with TDDFT, and the shortcomings of state-of-the-art TDDFT functionals may still be serious for these out-of-equilibrium situations.}}, journal = {{European Physical Journal B}}, volume = {89}, number = {5}, eid = {128}, }`

- C. Bacaksiz, S. Cahangirov, A. Rubio, R. T. Senger, F. M. Peeters, and H. Sahin, “Bilayer SnS2: Tunable stacking sequence by charging and loading pressure,” Physical Review B, vol. 93, iss. 12, 2016.

[Bibtex]`@article{escidoc:2256447, title = {{Bilayer SnS2: Tunable stacking sequence by charging and loading pressure}}, author = {Bacaksiz, C. and Cahangirov, S. and Rubio, Angel and Senger, R. T. and Peeters, F. M. and Sahin, H.}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.125403}, eprint = {1602.01824}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2016}, date = {2016-03-15}, abstract = {{Employing density functional theory\textendashbased methods, we investigate monolayer and bilayer structures of hexagonal SnS\textlesssub\textgreater2\textless/sub\textgreater, which is a recently synthesized monolayer metal dichalcogenide. Comparison of the 1H and 1T phases of monolayer SnS\textlesssub\textgreater2\textless/sub\textgreater confirms the ground state to be the 1T phase. In its bilayer structure we examine different stacking configurations of the two layers. It is found that the interlayer coupling in bilayer SnS\textlesssub\textgreater2\textless/sub\textgreater is weaker than that of typical transition-metal dichalcogenides so that alternative stacking orders have similar structural parameters and they are separated with low energy barriers. A possible signature of the stacking order in the SnS\textlesssub\textgreater2\textless/sub\textgreater bilayer has been sought in the calculated absorbance and reflectivity spectra. We also study the effects of the external electric field, charging, and loading pressure on the characteristic properties of bilayer SnS\textlesssub\textgreater2\textless/sub\textgreater. It is found that (i) the electric field increases the coupling between the layers at its preferred stacking order, so the barrier height increases, (ii) the bang gap value can be tuned by the external E field and under sufficient E field, the bilayer SnS\textlesssub\textgreater2\textless/sub\textgreater can become a semimetal, (iii) the most favorable stacking order can be switched by charging, and (iv) a loading pressure exceeding 3 GPa changes the stacking order. The E-field tunable band gap and easily tunable stacking sequence of SnS\textlesssub\textgreater2\textless/sub\textgreater layers make this 2D crystal structure a good candidate for field effect transistor and nanoscale lubricant applications.}}, journal = {{Physical Review B}}, volume = {93}, number = {12}, eid = {125403}, }`

- B. F. Milne, C. Kjaer, J. Houmoller, M. H. Stockett, Y. Toker, A. Rubio, and S. B. Nielsn, “On the Exciton Coupling between Two Chlorophyll Pigments in the Absence of a Protein Environment: Intrinsic Effects Revealed by Theory and Experiment,” Angewandte Chemie International Edition, vol. 55, iss. 21, pp. 6248-6251, 2016.

[Bibtex]`@article{escidoc:2261683, title = {{On the Exciton Coupling between Two Chlorophyll Pigments in the Absence of a Protein Environment: Intrinsic Effects Revealed by Theory and Experiment}}, author = {Milne, Bruce F. and Kj\aer, Christina and Houm\oller, J\orgen and Stockett, Mark H. and Toker, Yoni and Rubio, Angel and Nielsn, Steen Br\ondsted}, language = {eng}, issn = {1433-7851}, doi = {10.1002/anie.201601979}, publisher = {Wiley-VCH}, address = {Weinheim}, year = {2016}, date = {2016-05-17}, abstract = {{Exciton coupling between two or more chlorophyll (Chl) pigments is a key mechanism associated with the color tuning of photosynthetic proteins but it is difficult to disentangle this effect from shifts that are due to the protein microenvironment. Herein, we report the formation of the simplest coupled system, the Chl a dimer, tagged with a quaternary ammonium ion by electrospray ionization. Based on action spectroscopic studies in vacuo, the dimer complexes were found to absorb 50\textendash70 meV to the red of the monomers under the same conditions. First-principles calculations predict shifts that somewhat depend on the relative orientation of the two Chl units, namely 50 and 30 meV for structures where the Chl rings are stacked and unstacked, respectively. Our work demonstrates that Chl association alone can produce a large portion of the color shift observed in photosynthetic macromolecular assemblies.}}, journal = {{Angewandte Chemie International Edition}}, volume = {55}, number = {21}, pages = {6248--6251}, }`

- E. Torun, H. Sahin, S. Cahangirov, A. Rubio, and F. M. Peeters, “Anisotropic electronic, mechanical, and optical properties of monolayer WTe2,” Journal of Applied Physics, vol. 119, iss. 7, 2016.

[Bibtex]`@article{escidoc:2252907, title = {{Anisotropic electronic, mechanical, and optical properties of monolayer WTe2}}, author = {Torun, E. and Sahin, H. and Cahangirov, S. and Rubio, Angel and Peeters, F. M.}, language = {eng}, issn = {0021-8979}, doi = {10.1063/1.4942162}, publisher = {AIP Publishing}, address = {New York, NY}, year = {2016}, date = {2016-02-21}, abstract = {{Using first-principles calculations, we investigate the electronic, mechanical, and optical properties of monolayer WTe\textlesssub\textgreater2\textless/sub\textgreater. Atomic structure and ground state properties of monolayer WTe\textlesssub\textgreater2\textless/sub\textgreater (T\textlesssub\textgreaterd\textless/sub\textgreater phase) are anisotropic which are in contrast to similar monolayer crystals of transition metal dichalcogenides, such as MoS\textlesssub\textgreater2\textless/sub\textgreater, WS\textlesssub\textgreater2\textless/sub\textgreater, MoSe\textlesssub\textgreater2\textless/sub\textgreater, WSe\textlesssub\textgreater2\textless/sub\textgreater, and MoTe\textlesssub\textgreater2\textless/sub\textgreater, which crystallize in the H-phase. We find that the Poisson ratio and the in-plane stiffness is direction dependent due to the symmetry breaking induced by the dimerization of the W atoms along one of the lattice directions of the compound. Since the semimetallic behavior of the T\textlesssub\textgreaterd\textless/sub\textgreater phase originates from this W-W interaction (along the a crystallographic direction), tensile strain along the dimer direction leads to a semimetal to semiconductor transition after 1\textpercent strain. By solving the Bethe-Salpeter equation on top of single shot G\textlesssub\textgreater0\textless/sub\textgreaterW\textlesssub\textgreater0\textless/sub\textgreater calculations, we predict that the absorption spectrum of T\textlesssub\textgreaterd\textless/sub\textgreater-WTe\textlesssub\textgreater2\textless/sub\textgreater monolayer is strongly direction dependent and tunable by tensile strain.}}, journal = {{Journal of Applied Physics}}, volume = {119}, number = {7}, eid = {074307}, }`

- A. Putaja, F. G. Eich, T. Baldsiefen, and E. Räsänen, “Validity of power functionals for a homogeneous electron gas in reduced-density-matrix-functional theory,” Physical Review A, vol. 93, iss. 3, 2016.

[Bibtex]`@article{escidoc:2271072, title = {{Validity of power functionals for a homogeneous electron gas in reduced-density-matrix-functional theory}}, author = {Putaja, A. and Eich, Florian G. and Baldsiefen, T. and R\"as\"anen, E.}, language = {eng}, issn = {1050-2947}, doi = {10.1103/PhysRevA.93.032503}, eprint = {1602.03624}, publisher = {American Physical Society}, address = {New York, NY}, year = {2016}, date = {2016-03}, abstract = {{Physically valid and numerically efficient approximations for the exchange and correlation energy are critical for reduced-density-matrix-functional theory to become a widely used method in electronic structure calculations. Here we examine the physical limits of power functionals of the form f(n,n′) \textequals (nn′)\textlesssup\textgreater$\alpha$\textless/sup\textgreater for the scaling function in the exchange-correlation energy. To this end we obtain numerically the minimizing momentum distributions for the three- and two-dimensional homogeneous electron gas, respectively. In particular, we examine the limiting values for the power $\alpha$ to yield physically sound solutions that satisfy the Lieb-Oxford lower bound for the exchange-correlation energy and exclude pinned states with the condition n(k) \textless 1 for all wave vectors k. The results refine the constraints previously obtained from trial momentum distributions. We also compute the values for $\alpha$ that yield the exact correlation energy and its kinetic part for both the three- and two-dimensional electron gas. In both systems, narrow regimes of validity and accuracy are found at $\alpha$ $\gtrsim$ 0.6 and at r\textlesssub\textgreaters\textless/sub\textgreater $\gtrsim$ 10 for the density parameter, corresponding to relatively low densities.}}, journal = {{Physical Review A}}, volume = {93}, number = {3}, eid = {032503}, }`

- H. Hübener, M. A. Sentef, U. de Giovannini, A. F. Kemper, and A. Rubio, Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials, 2016.

[Bibtex]`@misc{escidoc:2266444, title = {{Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials}}, author = {H\"ubener, Hannes and Sentef, Michael A. and de Giovannini, Umberto and Kemper, Alexander F. and Rubio, Angel}, language = {eng}, eprint = {1604.03399}, year = {2016}, abstract = {{Tuning and stabilising topological states, such as Weyl semimetals, Dirac semimetals, or topological insulators, is emerging as one of the major topics in materials science. Periodic driving of many-body systems offers a platform to design Floquet states of matter with tunable electronic properties on ultrafast time scales. Here we show by first principles calculations how femtosecond laser pulses with circularly polarised light can be used to switch between Weyl semimetal, Dirac semimetal, and topological insulator states in a prototypical 3D Dirac material, Na3Bi. Our findings are general and apply to any 3D Dirac semimetal. We establish the concept of time-dependent bands and steering of Floquet-Weyl points (Floquet-WPs), and demonstrate how light can enhance topological protection against lattice perturbations. Our work has potential practical implications for the ultrafast switching of materials properties, like optical band gaps or anomalous magnetoresistance. Moreover, we introduce Floquet time-dependent density functional theory (Floquet-TDDFT) as a general and robust first principles method for predictive Floquet engineering of topological states of matter.}}, }`

- A. H. Larsen, U. De Giovannini, and A. Rubio, “Dynamical Processes in Open Quantum Systems from a TDDFT Perspective: Resonances and Electron Photoemission,” , N. Ferré, M. Filatov, and M. Huix-Rotllant, Eds., Cham, Switzerland: Springer International Publishing, 2016, vol. 368, pp. 219-271.

[Bibtex]`@incollection{escidoc:2188900, title = {{Dynamical Processes in Open Quantum Systems from a TDDFT Perspective: Resonances and Electron Photoemission}}, author = {Larsen, Ask Hjorth and De Giovannini, Umberto and Rubio, Angel}, language = {eng}, issn = {0340-1022}, isbn = {978-3-319-22080-2; 978-3-319-22081-9}, doi = {10.1007/128\textunderscore2014\textunderscore616}, publisher = {Springer International Publishing}, address = {Cham, Switzerland}, year = {2016}, date = {2016}, abstract = {{We present a review of different computational methods to describe time-dependent phenomena in open quantum systems and their extension to a density-functional framework. We focus the discussion on electron emission processes in atoms and molecules addressing excited-state lifetimes and dissipative processes. Initially we analyze the concept of an electronic resonance, a central concept in spectroscopy associated with a metastable state from which an electron eventually escapes (electronic lifetime). Resonances play a fundamental role in many time-dependent molecular phenomena but can be rationalized from a time-independent context in terms of scattering states. We introduce the method of complex scaling, which is used to capture resonant states as localized states in the spirit of usual bound-state methods, and work on its extension to static and time-dependent density-functional theory. In a time-dependent setting, complex scaling can be used to describe excitations in the continuum as well as wave packet dynamics leading to electron emission. This process can also be treated by using open boundary conditions which allow time-dependent simulations of emission processes without artificial reflections at the boundaries (i.e., borders of the simulation box). We compare in detail different schemes to implement open boundaries, namely transparent boundaries using Green functions, and absorbing boundaries in the form of complex absorbing potentials and mask functions. The last two are regularly used together with time-dependent density-functional theory to describe the electron emission dynamics of atoms and molecules. Finally, we discuss approaches to the calculation of energy and angle-resolved time-dependent pump\textendashprobe photoelectron spectroscopy of molecular systems.}}, series = {{Topics in Current Chemistry}}, editor = {Ferr\'e, Nicolas and Filatov, Michael and Huix-Rotllant, Miquel}, volume = {368}, pages = {219--271}, }`

- M. A. Sentef, A. F. Kemper, A. Georges, and C. Kollath, “Theory of light-enhanced phonon-mediated superconductivity,” Physical Review B, vol. 93, iss. 14, 2016.

[Bibtex]`@article{escidoc:2188951, title = {{Theory of light-enhanced phonon-mediated superconductivity}}, author = {Sentef, Michael A. and Kemper, A. F. and Georges, A. and Kollath, C.}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.144506}, eprint = {1505.07575}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2016}, date = {2016-04-01}, abstract = {{We investigate the dynamics of a phonon-mediated superconductor driven out of equilibrium. The electronic hopping amplitude is ramped down in time, resulting in an increased electronic density of states. The dynamics of the coupled electron-phonon model is investigated by solving Migdal-Eliashberg equations for the double-time Keldysh Green\textquotesingles functions. The increase of the density of states near the Fermi level leads to an enhancement of superconductivity when the system thermalizes to the new state at the same temperature. We provide a time- and momentum-resolved view on this thermalization process and show that it involves fast processes associated with single-particle scattering and much slower dynamics associated with the superconducting order parameter. The importance of electron-phonon coupling for the rapid enhancement and the efficient thermalization of superconductivity is demonstrated, and the results are compared to a BCS time-dependent mean-field approximation.}}, journal = {{Physical Review B}}, volume = {93}, number = {14}, eid = {144506}, }`

- M. Wanko, S. Cahangirov, L. Shi, P. Rohringer, Z. J. Lapin, L. Novotny, P. Ayala, T. Pichler, and A. Rubio, Polyyne Electronic and Vibrational Properties under Environmental Interactions, 2016.

[Bibtex]`@misc{escidoc:2263108, title = {{Polyyne Electronic and Vibrational Properties under Environmental Interactions}}, author = {Wanko, M. and Cahangirov, Seymur and Shi, Lei and Rohringer, Philip and Lapin, Zachary J. and Novotny, Lukas and Ayala, Paola and Pichler, Thomas and Rubio, Angel}, language = {eng}, eprint = {1604.00483}, year = {2016}, abstract = {{Recent advances allow growing linear carbon chains of up to thousands of atoms inside carbon nanotubes [L. Shi et al., (in press), DOI:10.1038/NMAT4617)]. This increases the interest in the unique properties of carbyne and finite polyynes and their response to different environments. Here, the experimental characterization still relies on empirical or bulk-physical models appropriate for specific chain lengths or environments. The physical understanding of the observed trends in terms of charge transfer, electron delocalization, and van-der-Waals interactions is widely missing. Here, we theoretically describe the main interactions between polyynes and graphene or carbon nanotubes to inspire future models that better account for experimental conditions.}}, }`

- M. Casadei, X. Ren, P. Rinke, A. Rubio, and M. Scheffler, “Density functional theory study of the $\alpha$\textminus$\gamma$ phase transition in cerium: Role of electron correlation and f-orbital localization,” Physical Review B, vol. 93, iss. 7, 2016.

[Bibtex]`@article{escidoc:2262135, title = {{Density functional theory study of the $\alpha$\textminus$\gamma$ phase transition in cerium: Role of electron correlation and f-orbital localization}}, author = {Casadei, Marco and Ren, Xinguo and Rinke, Patrick and Rubio, Angel and Scheffler, Matthias}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.075153}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2016}, abstract = {{The long standing problem of the $\alpha$\textminus$\gamma$ phase transition in cerium metal is approached by treating all electrons at the same quantum mechanical level, using both hybrid functionals (PBE0 and HSE06) and exact exchange plus correlation in the random-phase approximation (EX+cRPA). The exact-exchange contribution in PBE0 and HSE06 is crucial to produce two distinct solutions that can be associated with the $\alpha$ and $\gamma$ phases. An analysis of the band structure and the electron density reveals a localization and delocalization behavior of the f electrons in the $\gamma$ and $\alpha$ phases, respectively. However, a quantitative agreement with the extrapolated phase diagram to zero temperature is achieved only with EX+cRPA, based on the hybrid functional starting point. We predict that a pressure induced phase transition should exist at or close to T\textequals0K. By adding entropic contributions we determine the pressure-temperature phase diagram, which is in reasonable agreement with experiment.}}, journal = {{Physical Review B}}, volume = {93}, number = {7}, eid = {075153}, }`

- P. Cudazzo, E. Müller, C. Habenicht, M. Gatti, H. Berger, M. Knupfer, A. Rubio, and S. Huotari, Electron-energy-loss and time-dependent density functional theory study on the plasmon dispersion in 2H-NbS2, 2016.

[Bibtex]`@misc{escidoc:2261294, title = {{Electron-energy-loss and time-dependent density functional theory study on the plasmon dispersion in 2H-NbS2}}, author = {Cudazzo, Pierluigi and M\"uller, Eric and Habenicht, Carsten and Gatti, Matteo and Berger, Helmuth and Knupfer, Martin and Rubio, Angel and Huotari, Simo}, language = {eng}, eprint = {1603.03486}, year = {2016}, abstract = {{We examine the experimental and theoretical electron-energy loss spectra in 2H-Cu\textlesssub\textgreater0.2\textless/sub\textgreaterNbS\textlesssub\textgreater2\textless/sub\textgreater and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range q \textless 0.35 \AA\textlesssup\textgreater-1\textless/sup\textgreater. We argue that for a stoichiometric pure 2H-NbS\textlesssub\textgreater2\textless/sub\textgreater the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides.}}, }`

- M. Lubasch, J. I. Fuks, H. Appel, A. Rubio, I. J. Cirac, and M. C. Bañuls, Density functionals with the help of matrix product states, 2016.

[Bibtex]`@misc{escidoc:2261265, title = {{Density functionals with the help of matrix product states}}, author = {Lubasch, Michael and Fuks, Johanna I. and Appel, Heiko and Rubio, Angel and Cirac, J. Ignacio and Ba\~nuls, Mari Carmen}, language = {eng}, eprint = {1603.06565}, year = {2016}, abstract = {{In the context of strongly correlated fermions on a one-dimensional lattice, we propose to use matrix product states to construct approximate density functionals for density functional theory. Focussing on the exchange-correlation energy, we demonstrate how an efficient approximation can be found that includes and systematically improves beyond the local density approximation.}}, }`

- M. Casadei, X. Ren, P. Rinke, A. Rubio, and M. Scheffler, “Density functional theory study of the $\alpha$\textminus$\gamma$ phase transition in cerium: Role of electron correlation and f-orbital localization,” Physical Review B, vol. 93, iss. 7, 2016.

[Bibtex]`@article{escidoc:2259695, title = {{Density functional theory study of the $\alpha$\textminus$\gamma$ phase transition in cerium: Role of electron correlation and f-orbital localization}}, author = {Casadei, Marco and Ren, Xinguo and Rinke, Patrick and Rubio, Angel and Scheffler, Matthias}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.075153}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2016}, date = {2016-02-15}, abstract = {{The long standing problem of the $\alpha$\textminus$\gamma$ phase transition in cerium metal is approached by treating all electrons at the same quantum mechanical level, using both hybrid functionals (PBE0 and HSE06) and exact exchange plus correlation in the random-phase approximation (EX+cRPA). The exact-exchange contribution in PBE0 and HSE06 is crucial to produce two distinct solutions that can be associated with the $\alpha$ and $\gamma$ phases. An analysis of the band structure and the electron density reveals a localization and delocalization behavior of the f electrons in the $\gamma$ and $\alpha$ phases, respectively. However, a quantitative agreement with the extrapolated phase diagram to zero temperature is achieved only with EX+cRPA, based on the hybrid functional starting point. We predict that a pressure induced phase transition should exist at or close to T \textequals 0 K. By adding entropic contributions we determine the pressure-temperature phase diagram, which is in reasonable agreement with experiment.}}, journal = {{Physical Review B}}, volume = {93}, number = {7}, eid = {075153}, }`

- T. Brumme, M. Calandra, and F. Mauri, “Determination of scattering time and of valley occupation in transition-metal dichalcogenides doped by field effect,” Physical Review B, vol. 93, iss. 8, 2016.

[Bibtex]`@article{escidoc:2259575, title = {{Determination of scattering time and of valley occupation in transition-metal dichalcogenides doped by field effect}}, author = {Brumme, Thomas and Calandra, Matteo and Mauri, Francesco}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.081407}, eprint = {1602.00893}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2016}, date = {2016-02-15}, abstract = {{The transition-metal dichalcogenides have attracted a lot of attention as a possible stepping-stone toward atomically thin and flexible field-effect transistors. One key parameter to describe the charge transport is the time between two successive scattering events\textemdashthe transport scattering time. In a recent report, we have shown that it is possible to use density functional theory to obtain the band structure of two-dimensional semiconductors in the presence of field effect doping. Here, we report a simple method to extract the scattering time from the experimental conductivity and from the knowledge of the band structure. We apply our approach to monolayers and multilayers of MoS\textlesssub\textgreater2\textless/sub\textgreater, MoSe\textlesssub\textgreater2\textless/sub\textgreater, MoTe\textlesssub\textgreater2\textless/sub\textgreater, WS\textlesssub\textgreater2\textless/sub\textgreater, and WSe\textlesssub\textgreater2\textless/sub\textgreater in the presence of a gate. In WS\textlesssub\textgreater2\textless/sub\textgreater, for which accurate measurements of mobility have been published, we find that the scattering time is inversely proportional to the density of states at the Fermi level. Finally, we show that it is possible to identify the critical doping at which different valleys start to be occupied from the doping dependence of the conductivity.}}, journal = {{Physical Review B}}, volume = {93}, number = {8}, eid = {081407(R)}, }`

- L. N. Glanzmann, D. J. Mowbray, D. G. F. del Valle, F. Scotognella, G. Lanzani, and A. Rubio, “Photoinduced Absorption within Single-Walled Carbon Nanotube Systems,” The Journal of Physical Chemistry C, vol. 120, iss. 3, pp. 1926-1935, 2016.

[Bibtex]`@article{escidoc:2246938, title = {{Photoinduced Absorption within Single-Walled Carbon Nanotube Systems}}, author = {Glanzmann, Livia No\"emi and Mowbray, Duncan John and Valle, Diana Gisell Figueroa del and Scotognella, Francesco and Lanzani, Guglielmo and Rubio, Angel}, language = {eng}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.5b10025}, eprint = {1602.00010}, publisher = {American Chemical Society}, address = {Washington DC}, year = {2016}, date = {2016-01-28}, abstract = {{We study the photoabsorption properties of photoactive bulk polymer/fullerene/nanotube heterojunctions in the near-infrared region. By combining pump\textendashprobe spectroscopy and linear response time-dependent density functional theory within the random phase approximation (TDDFT-RPA) we elucidate the excited state dynamics of the E\textlesssub\textgreater11\textless/sub\textgreater transition within (6,5) and (7,5) single-walled carbon nanotubes (SWNTs) and combined with poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C\textlesssub\textgreater61\textless/sub\textgreater-butyric acid methyl ester (PCBM) in P3HT/PCBM/SWNT blended samples. We find the presence of a photoinduced absorption (PA) peak is related mainly to the width of the photobleach (PB) peak and the charge carrier density of the SWNT system. For mixed SWNT samples, the PB peak is too broad to observe the PA peak, whereas within P3HT/PCBM/SWNT blended samples P3HT acts as a hole acceptor, narrowing the PB peak by exciton delocalization, which reveals a PA peak. Our results suggest that the PA peak originates from a widening of the band gap in the presence of excited electrons and holes. These results have important implications for the development of new organic photovoltaic heterojunctions including SWNTs.}}, journal = {{The Journal of Physical Chemistry C}}, volume = {120}, number = {3}, pages = {1926--1935}, }`

- Y. Pavlyukh, J. Berakdar, and A. Rubio, Taming singularities of the diagrammatic many-body perturbation theory, 2016.

[Bibtex]`@misc{escidoc:2242985, title = {{Taming singularities of the diagrammatic many-body perturbation theory}}, author = {Pavlyukh, Yaroslav and Berakdar, Jamal and Rubio, Angel}, language = {eng}, eprint = {1601.04285}, year = {2016}, abstract = {{In a typical scenario the diagrammatic many-body perturbation theory generates asymptotic series. Despite non-convergence, the asymptotic expansions are useful when truncated to a finite number of terms. This is the reason for popularity of leading-order methods such as GW approximation in condensed matter, molecular and atomic physics. Emerging higher-order implementations suffer from the appearance of nonsimple poles in the frequency-dependent Green\textquotesingles functions and negative spectral densities making self-consistent determination of the electronic structure impossible. Here a method based on the Pad\'e approximation for overcomming these difficulties is proposed and applied to the Hamiltonian describing a core electron coupled to a single plasmonic excitation. By solving the model purely diagrammatically, expressing the self-energy in terms of combinatorics of chord diagrams, and regularizing the diverging perturbative expansions using the Pad\'e approximation the spectral function is determined self-consistently using 3111 diagrams up to the sixth order.}}, }`

- E. Baldini, L. Chiodo, A. Dom’inguez Garc’ia, M. Palummo, S. Moser, M. Yazdi, G. Auböck, B. P. P. Mallett, H. Berger, A. Magrez, C. Bernhard, M. Grioni, A. Rubio, and M. Chergui, Two-dimensional excitonic quasiparticles in a three-dimensional crystal: The case of anatase TiO2, 2016.

[Bibtex]`@misc{escidoc:2241547, title = {{Two-dimensional excitonic quasiparticles in a three-dimensional crystal: The case of anatase TiO2}}, author = {Baldini, Edoardo and Chiodo, Letizia and Dom\'\inguez Garc\'\ia, Adriel and Palummo, Maurizia and Moser, Simon and Yazdi, Meghdad and Aub\"ock, Gerald and Mallett, Benjamin P. P. and Berger, Helmuth and Magrez, Arnaud and Bernhard, Christian and Grioni, Marco and Rubio, Angel and Chergui, Majed}, language = {eng}, eprint = {1601.01244}, year = {2016}, abstract = {{Bound electronic excitations play a major role in the electrodynamics of insulators and are typically described by the concept of Wannier-Mott and Frenkel excitons. The former represent hydrogenic electron-hole pairs delocalized over several unit cells of a crystal and they occur in materials with high dielectric constant; the latter correspond to a correlated electron-hole pair localized on a single lattice site and they mostly prevail in molecular solids. Between these two extremes, an intermediate type of excitons exists, typically referred to as charge-transfer excitons. A prototypical system in which these rare quasiparticles have been theoretically predicted but never experimentally confirmed is the anatase polymorph of TiO\textlesssub\textgreater2\textless/sub\textgreater, which is one of the most promising material for light-energy conversion applications. Here, we combine angle-resolved photoemission and optical spectroscopies, along with ab initio state-of-the-art theoretical calculations, to demonstrate that the direct optical gap of anatase TiO\textlesssub\textgreater2\textless/sub\textgreater is dominated by a charge-transfer exciton band rising over the continuum of indirect interband transitions. In particular, we find that the lowest exciton possesses a two-dimensional nature and is characterized by a giant binding energy of \textasciitilde 300 meV. The universality of these findings is proven in highly defective samples used in light-energy conversion applications, by interrogating these systems out-of-equilibrium via ultrafast two-dimensional UV spectroscopy.}}, }`

### 2015

- A. Castro, A. Rubio, and E. K. U. Gross, “Enhancing and controlling single-atom high-harmonic generation spectra: a time-dependent density-functional scheme,” European Physical Journal B, vol. 88, iss. 8, 2015.

[Bibtex]`@article{escidoc:2180109, title = {{Enhancing and controlling single-atom high-harmonic generation spectra: a time-dependent density-functional scheme}}, author = {Castro, Alberto and Rubio, Angel and Gross, Eberhard K. U.}, language = {eng}, issn = {1434-6028}, doi = {10.1140/epjb/e2015-50889-7}, eprint = {1409.4070}, publisher = {Springer-Verlag Heidelberg}, address = {Heidelberg}, year = {2015}, date = {2015-08}, abstract = {{High harmonic generation (HHG) provides a flexible framework for the development of coherent light sources in the extreme-ultraviolet and soft X-ray regimes. However it suffers from low conversion efficiencies as the control of the HHG spectral and temporal characteristics requires manipulating electron trajectories on attosecond time scale. The phase matching mechanism has been employed to selectively enhance specific quantum paths leading to HHG. A few important fundamental questions remain open, among those how much of the enhancement can be achieved by the single-emitter and what is the role of correlations (or the electronic structure) in the selectivity and control of HHG generation. Here we address those questions by examining computationally the possibility of optimizing the HHG spectrum of isolated hydrogen and helium atoms by shaping the slowly varying envelope of a 800 nm, 200-cycles long laser pulse. The spectra are computed with a fully quantum mechanical description, by explicitly computing the time-dependent dipole moment of the systems using a time-dependent density-functional approach (or the single-electron Schr\"odinger equation for the case of H), on top of a one-dimensional model. The sought optimization corresponds to the selective enhancement of single harmonics, which we find to be significant. This selectivity is entirely due to the single atom response, and not to any propagation or phase-matching effect. Moreover, we see that the electronic correlation plays a role in the determining the degree of optimization that can be obtained.}}, journal = {{European Physical Journal B}}, volume = {88}, number = {8}, eid = {191}, }`

- J. Flick, M. Ruggenthaler, H. Appel, and A. Rubio, “Kohn\textendashSham approach to quantum electrodynamical density-functional theory: Exact time-dependent effective potentials in real space,” Proceedings of the National Academy of Sciences of the United States of America, vol. 112, iss. 50, pp. 15285-15290, 2015.

[Bibtex]`@article{escidoc:2231572, title = {{Kohn\textendashSham approach to quantum electrodynamical density-functional theory: Exact time-dependent effective potentials in real space}}, author = {Flick, Johannes and Ruggenthaler, Michael and Appel, Heiko and Rubio, Angel}, language = {eng}, issn = {0027-8424}, doi = {10.1073/pnas.1518224112}, eprint = {1509.01069}, publisher = {National Academy of Sciences}, address = {Washington, DC}, year = {2015}, date = {2015-12-15}, abstract = {{The density-functional approach to quantum electrodynamics extends traditional density-functional theory and opens the possibility to describe electron\textendashphoton interactions in terms of effective Kohn\textendashSham potentials. In this work, we numerically construct the exact electron\textendashphoton Kohn\textendashSham potentials for a prototype system that consists of a trapped electron coupled to a quantized electromagnetic mode in an optical high-Q cavity. Although the effective current that acts on the photons is known explicitly, the exact effective potential that describes the forces exerted by the photons on the electrons is obtained from a fixed-point inversion scheme. This procedure allows us to uncover important beyond-mean-field features of the effective potential that mark the breakdown of classical light\textendashmatter interactions. We observe peak and step structures in the effective potentials, which can be attributed solely to the quantum nature of light; i.e., they are real-space signatures of the photons. Our findings show how the ubiquitous dipole interaction with a classical electromagnetic field has to be modified in real space to take the quantum nature of the electromagnetic field fully into account.}}, journal = {{Proceedings of the National Academy of Sciences of the United States of America}}, volume = {112}, number = {50}, pages = {15285--15290}, }`

- U. De Giovannini, A. H. Larsen, and A. Rubio, “Modeling electron dynamics coupled to continuum states in finite volumes with absorbing boundaries,” European Physical Journal B, vol. 88, iss. 3, 2015.

[Bibtex]`@article{escidoc:2152184, title = {{Modeling electron dynamics coupled to continuum states in finite volumes with absorbing boundaries}}, author = {De Giovannini, Umberto and Larsen, Ask Hjorth and Rubio, Angel}, language = {eng}, issn = {1434-6028}, doi = {10.1140/epjb/e2015-50808-0}, eprint = {1409.1689}, publisher = {Springer-Verlag Heidelberg}, address = {Heidelberg}, year = {2015}, date = {2015-03}, abstract = {{Absorbing boundaries are frequently employed in real-time propagation of the Schr\"odinger equation to remove spurious reflections and efficiently emulate outgoing boundary conditions. These conditions are a fundamental ingredient for the calculation of observables involving infinitely extended continuum states in finite volumes. In the literature, several boundary absorbers have been proposed. They mostly fall into three main families: mask function absorbers, complex absorbing potentials, and exterior complex-scaled potentials. To date none of the proposed absorbers is perfect, and all present a certain degree of reflections. Characterization of such reflections is thus a critical task with strong implications for time-dependent simulations of atoms and molecules. We introduce a method to evaluate the reflection properties of a given absorber and present a comparison of selected samples for each family of absorbers. Further, we discuss the connections between members of each family and show how the same reflection curves can be obtained with very different absorption schemes.}}, journal = {{European Physical Journal B}}, volume = {88}, number = {3}, eid = {56}, }`

- I. Theophilou, N. N. Lathiotakis, N. I. Gidopoulos, A. Rubio, and N. Helbig, “Orbitals from local RDMFT: Are they Kohn-Sham or natural orbitals?,” The Journal of Chemical Physics, vol. 143, iss. 5, 2015.

[Bibtex]`@article{escidoc:2180991, title = {{Orbitals from local RDMFT: Are they Kohn-Sham or natural orbitals?}}, author = {Theophilou, Iris and Lathiotakis, Nektarios N. and Gidopoulos, Nikitas I. and Rubio, Angel and Helbig, Nicole}, language = {eng}, issn = {0021-9606}, doi = {10.1063/1.4927784}, eprint = {1505.00627}, publisher = {American Institute of Physics}, address = {Woodbury, N.Y.}, year = {2015}, date = {2015-08-07}, abstract = {{Recently, an approximate theoretical framework was introduced, called local reduced density matrix functional theory (local-RDMFT), where functionals of the one-body reduced density matrix (1-RDM) are minimized under the additional condition that the optimal orbitals satisfy a single electron Schr\"odinger equation with a local potential. In the present work, we focus on the character of these optimal orbitals. In particular, we compare orbitals obtained by local-RDMFT with those obtained with the full minimization (without the extra condition) by contrasting them against the exact NOs and orbitals from a density functional calculation using the local density approximation (LDA). We find that the orbitals from local-RMDFT are very close to LDA orbitals, contrary to those of the full minimization that resemble the exact NOs. Since local RDMFT preserves the good quality of the description of strong static correlation, this finding opens the way to a mixed density/density matrix scheme, where Kohn-Sham orbitals obtain fractional occupations from a minimization of the occupation numbers using 1-RDM functionals. This will allow for a description of strong correlation at a cost only minimally higher than a density functional calculation.}}, journal = {{The Journal of Chemical Physics}}, volume = {143}, number = {5}, eid = {054106}, }`

- N. Säkkinen, Y. Peng, H. Appel, and R. van Leeuwen, “Many-body Green\textquoterights function theory for electron-phonon interactions: The Kadanoff-Baym approach to spectral properties of the Holstein dimer,” The Journal of Chemical Physics, vol. 143, iss. 23, 2015.

[Bibtex]`@article{escidoc:2241594, title = {{Many-body Green\textquoterights function theory for electron-phonon interactions: The Kadanoff-Baym approach to spectral properties of the Holstein dimer}}, author = {S\"akkinen, Niko and Peng, Yang and Appel, Heiko and van Leeuwen, Robert}, language = {eng}, issn = {0021-9606}, doi = {10.1063/1.4936143}, eprint = {1507.04726}, publisher = {American Institute of Physics}, address = {Woodbury, N.Y.}, year = {2015}, date = {2015-12-21}, abstract = {{We present a Kadanoff-Baym formalism to study time-dependent phenomena for systems of interacting electrons and phonons in the framework of many-body perturbation theory. The formalism takes correctly into account effects of the initial preparation of an equilibrium state and allows for an explicit time-dependence of both the electronic and phononic degrees of freedom. The method is applied to investigate the charge neutral and non-neutral excitation spectra of a homogeneous, two-site, two-electron Holstein model. This is an extension of a previous study of the ground state properties in the Hartree (H), partially self-consistent Born (Gd) and fully self-consistent Born (GD) approximations published in S\"akkinen et al. [J. Chem. Phys. 143, 234101 (2015)]. Here, the homogeneous ground state solution is shown to become unstable for a sufficiently strong interaction while a symmetry-broken ground state solution is shown to be stable in the Hartree approximation. Signatures of this instability are observed for the partially self-consistent Born approximation but are not found for the fully self-consistent Born approximation. By understanding the stability properties, we are able to study the linear response regime by calculating the density-density response function by time-propagation. This amounts to a solution of the Bethe-Salpeter equation with a sophisticated kernel. The results indicate that none of the approximations is able to describe the response function during or beyond the bipolaronic crossover for the parameters investigated. Overall, we provide an extensive discussion on when the approximations are valid and how they fail to describe the studied exact properties of the chosen model system.}}, journal = {{The Journal of Chemical Physics}}, volume = {143}, number = {23}, eid = {234102}, }`

- C. Pellegrini, J. Flick, I. V. Tokatly, H. Appel, and A. Rubio, “Optimized Effective Potential for Quantum Electrodynamical Time-Dependent Density Functional Theory,” Physical Review Letters, vol. 115, iss. 9, 2015.

[Bibtex]`@article{escidoc:2190166, title = {{Optimized Effective Potential for Quantum Electrodynamical Time-Dependent Density Functional Theory}}, author = {Pellegrini, Camilla and Flick, Johannes and Tokatly, Ilya V. and Appel, Heiko and Rubio, Angel}, language = {eng}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.115.093001}, publisher = {American Physical Society}, address = {Woodbury, N.Y.}, year = {2015}, date = {2015-08-24}, abstract = {{We propose an orbital exchange-correlation functional for applying time-dependent density functional theory to many-electron systems coupled to cavity photons. The time nonlocal equation for the electron-photon optimized effective potential (OEP) is derived. In the static limit our OEP energy functional reduces to the Lamb shift of the ground state energy. We test the new approximation in the Rabi model. It is shown that the OEP (i) reproduces quantitatively the exact ground-state energy from the weak to the deep strong coupling regime and (ii) accurately captures the dynamics entering the ultrastrong coupling regime. The present formalism opens the path to a first-principles description of correlated electron-photon systems, bridging the gap between electronic structure methods and quantum optics for real material applications.}}, journal = {{Physical Review Letters}}, volume = {115}, number = {9}, eid = {093001}, }`

- M. Hellgren, F. Caruso, D. R. Rohr, X. Ren, A. Rubio, M. Scheffler, and P. Rinke, “Static correlation and electron localization in molecular dimers from the self-consistent RPA and GW approximation,” Physical Review B, vol. 91, iss. 16, 2015.

[Bibtex]`@article{escidoc:2139235, title = {{Static correlation and electron localization in molecular dimers from the self-consistent RPA and GW approximation}}, author = {Hellgren, Maria and Caruso, Fabio and Rohr, Daniel R. and Ren, Xinguo and Rubio, Angel and Scheffler, Matthias and Rinke, Patrick}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.91.165110}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2015}, date = {2015-04-07}, abstract = {{We investigate static correlation and delocalization errors in the self-consistent GW and random-phase approximation (RPA) by studying molecular dissociation of the H2 and LiH molecules. Although both approximations contain topologically identical diagrams, the nonlocality and frequency dependence of the GW self-energy crucially influence the different energy contributions to the total energy as compared to the use of a static local potential in the RPA. The latter leads to significantly larger correlation energies, which allow for a better description of static correlation at intermediate bond distances. The substantial error found in GW is further analyzed by comparing spin-restricted and spin-unrestricted calculations. At large but finite nuclear separation, their difference gives an estimate of the so-called fractional spin error normally determined only in the dissociation limit. Furthermore, a calculation of the dipole moment of the LiH molecule at dissociation reveals a large delocalization error in GW making the fractional charge error comparable to the RPA. The analyses are supplemented by explicit formulas for the GW Green\textquotesingles function and total energy of a simplified two-level model providing additional insights into the dissociation limit.}}, journal = {{Physical Review B}}, volume = {91}, number = {16}, eid = {165110}, }`

- G. Albareda, A. Abedi, I. Tavernelli, and A. Rubio, Universal Steps Guiding the Branching of the Nuclear Probability Density over Nonadiabatic Electronic Transitions, 2015.

[Bibtex]`@misc{escidoc:2240315, title = {{Universal Steps Guiding the Branching of the Nuclear Probability Density over Nonadiabatic Electronic Transitions}}, author = {Albareda, Guillermo and Abedi, Ali and Tavernelli, Ivano and Rubio, Angel}, language = {eng}, eprint = {1512.08531}, year = {2015}, abstract = {{In the conditional approach to molecular dynamics the electron-nuclear wavefunction is exactly decomposed into an ensemble of nuclear wavepackets governed by conditional time-dependent potential-energy surfaces (C-TDPESs) [G. Albareda, et al., Phys. Rev. Lett. 105, 123002 (2014)]. Employing a one-dimensional model system we show that for strong nonadiabatic couplings the C-TDPESs exhibit steps that bridge between piecewise adiabatic shapes. A detailed analysis of the steps sheds light into the ultimate nature of electron-nuclear correlations and its comparison with the discontinuities that emerge in the exact factorization of the molecular wavefunction brings us to claim their universality in theories without Born-Oppenheimer potential-energy surfaces.}}, }`

- B. M. M. Krishna, M. K. L. Man, S. Vinod, C. Chin, T. Harada, J. Taha-Tijerina, C. S. Tiwary, P. Nguyen, P. Chang, T. N. Narayanan, A. Rubio, P. M. Ajayan, S. Talapatra, and K. M. Dani, “Engineering Photophenomena in Large, 3D Structures Composed of Self-Assembled van der Waals Heterostructure Flakes,” Advanced Optical Materials, vol. 3, iss. 11, pp. 1551-1556, 2015.

[Bibtex]`@article{escidoc:2188920, title = {{Engineering Photophenomena in Large, 3D Structures Composed of Self-Assembled van der Waals Heterostructure Flakes}}, author = {Krishna, M. Bala Murali and Man, Michael K. L. and Vinod, Soumya and Chin, Catherine and Harada, Takaaki and Taha-Tijerina, Jaime and Tiwary, Chandra Sekhar and Nguyen, Patrick and Chang, Patricia and Narayanan, Tharangattu N. and Rubio, Angel and Ajayan, Pulickel M. and Talapatra, Saikat and Dani, Keshav M.}, language = {eng}, issn = {2195-1071}, doi = {10.1002/adom.201500296}, publisher = {WILEY-VCH Verlag GmbH \& Co. KGaA}, address = {Weinheim}, year = {2015}, date = {2015-11}, abstract = {{A photocapacitive, dissipationless, optoelectronic terahertz response is observed in robust 3D van der Waals heterostructured flakes of hexagonal boron nitride and graphene. The response is fundamentally distinct from that of the parent materials and can be tuned from insulating to photocapacitive to semiconducting by varying the composition. Such hybrid structures have broad implications for tunable optoelectronic materials and potential terahertz devices.}}, journal = {{Advanced Optical Materials}}, volume = {3}, number = {11}, pages = {1551--1556}, }`

- X. Andrade, D. A. Strubbe, U. De Giovannini, A. H. Larsen, M. J. T. Oliveira, J. Alberdi-Rodriguez, A. Varas, I. Theophilou, N. Helbig, M. Verstraete, L. Stella, F. Nogueira, A. Aspuru-Guzik, A. Castro, M. A. L. Marques, and A. Rubio, “Real-space grids and the Octopus code as tools for the development of new simulation approaches for electronic systems,” Physical Chemistry Chemical Physics, vol. 17, iss. 47, pp. 31371-31396, 2015.

[Bibtex]`@article{escidoc:2152173, title = {{Real-space grids and the Octopus code as tools for the development of new simulation approaches for electronic systems}}, author = {Andrade, Xavier and Strubbe, David A. and De Giovannini, Umberto and Larsen, Ask Hjorth and Oliveira, Micael J. T. and Alberdi-Rodriguez, Joseba and Varas, Alejandro and Theophilou, Iris and Helbig, Nicole and Verstraete, Matthieu and Stella, Lorenzo and Nogueira, Fernando and Aspuru-Guzik, Al\'an and Castro, Alberto and Marques, Miguel A. L. and Rubio, Angel}, language = {eng}, issn = {1463-9076}, doi = {10.1039/C5CP00351B}, eprint = {1501.05654}, publisher = {Royal Society of Chemistry}, address = {Cambridge, England}, year = {2015}, date = {2015-12-21}, abstract = {{Real-space grids are a powerful alternative for the simulation of electronic systems. One of the main advantages of the approach is the flexibility and simplicity of working directly in real space where the different fields are discretized on a grid, combined with competitive numerical performance and great potential for parallelization. These properties constitute a great advantage at the time of implementing and testing new physical models. Based on our experience with the Octopus code, in this article we discuss how the real-space approach has allowed for the recent development of new ideas for the simulation of electronic systems. Among these applications are approaches to calculate response properties, modeling of photoemission, optimal control of quantum systems, simulation of plasmonic systems, and the exact solution of the Schr\"odinger equation for low-dimensionality systems.}}, journal = {{Physical Chemistry Chemical Physics}}, volume = {17}, number = {47}, pages = {31371--31396}, }`

- J. Houmoller, M. Wanko, A. Rubio, and S. B. Nielsn, “Effect of a Single Water Molecule on the Electronic Absorption by o- and p-Nitrophenolate: A Shift to the Red or to the Blue?,” The Journal of Physical Chemistry A, vol. 119, iss. 47, pp. 11498-11503, 2015.

[Bibtex]`@article{escidoc:2227306, title = {{Effect of a Single Water Molecule on the Electronic Absorption by o- and p-Nitrophenolate: A Shift to the Red or to the Blue?}}, author = {Houm\oller, J\orgen and Wanko, Marius and Rubio, Angel and Nielsn, Steen Br\ondsted}, language = {eng}, issn = {1089-5639}, doi = {10.1021/acs.jpca.5b08634}, publisher = {American Chemical Society}, address = {Columbus, OH}, year = {2015}, date = {2015-11-25}, abstract = {{Many photoactive biomolecules are anions and exhibit $\pi$$\pi$\textasteriskcentered optical transitions but with a degree of charge transfer (CT) character determined by the local environment. The phenolate moiety is a common structural motif among biochromophores and luminophores, and nitrophenolates are good model systems because the nitro substituent allows for CT-like transitions. Here we report gas-phase absorption spectra of o- and p-nitrophenolate\mbox{$\cdot$}H\textlesssub\textgreater2\textless/sub\textgreaterO complexes to decipher the effect of just one H\textlesssub\textgreater2\textless/sub\textgreaterO and compare them with ab initio calculations of vertical excitation energies. The experimental band maximum is at 3.01 and 3.00 eV for ortho and para isomers, respectively, and is red-shifted by 0.10 and 0.13 eV relative to the bare ions, respectively. These shifts indicate that the transition has become more CT-like because of localization of negative charge on the phenolate oxygen, i.e., diminished delocalization of the negative excess charge. However, the transition bears less CT than that of m-nitrophenolate\mbox{$\cdot$}H\textlesssub\textgreater2\textless/sub\textgreaterO because this complex absorbs further to the red (2.56 eV). Our work emphasizes the importance of local perturbations: one water causes a larger shift than experienced in bulk for para isomer and almost the full shift for ortho isomer. Predicting microenvironmental effects in the boundary between CT and non-CT with high accuracy is nontrivial. However, in agreement with experiment, our calculations show a competition between the effects of electronic delocalization and electrostatic interaction with the solvent molecule. As a result, the excitation energy of ortho and para isomers is less sensitive to hydration than that of the meta isomer because donor and acceptor orbitals are only weakly coupled in the meta isomer.}}, journal = {{The Journal of Physical Chemistry A}}, volume = {119}, number = {47}, pages = {11498--11503}, }`

- M. Ruggenthaler, Ground-State Quantum-Electrodynamical Density-Functional Theory, 2015.

[Bibtex]`@misc{escidoc:2234824, title = {{Ground-State Quantum-Electrodynamical Density-Functional Theory}}, author = {Ruggenthaler, Michael}, language = {eng}, eprint = {1509.01417}, year = {2015}, abstract = {{In this work I show that the hybrid ground state of a quantum system, which consists of particles dipole-coupled to photons in a cavity, is uniquely determined by the charge density of the particles and the expectation value of the displacement operators of the cavity modes. This result extends ground-state density-functional theory to include photons and allows to determine all properties of the coupled quantum system by only knowing these two observables. The corresponding Kohn-Sham scheme shows how the photon field can be replaced by a classical electro-magnetic field and a non-linear coupling to the particles.}}, }`

- H. Zhang, Y. Miyamoto, X. Chengc, and A. Rubio, “Optical field terahertz amplitude modulation by graphene nanoribbons,” Nanoscale, vol. 7, iss. 45, pp. 19012-19017, 2015.

[Bibtex]`@article{escidoc:2225596, title = {{Optical field terahertz amplitude modulation by graphene nanoribbons}}, author = {Zhang, Hong and Miyamoto, Yoshiyuki and Chengc, Xinlu and Rubio, Angel}, language = {eng}, issn = {2040-3364}, doi = {10.1039/C5NR05889A}, publisher = {Royal Society of Chemistry}, address = {Cambridge, UK}, year = {2015}, date = {2015-12-07}, abstract = {{In this study, first-principles time-dependent density functional theory calculations were used to demonstrate the possibility to modulate the amplitude of the optical electric field (E-field) near a semiconducting graphene nanoribbon. A significant enhancement of the optical E-field was observed 3.34 \AA above the graphene nanoribbon sheet, with an amplitude modulation of approximately 100 fs, which corresponds to a frequency of 10 THz. In general, a six-fold E-field enhancement could be obtained, which means that the power of the obtained THz is about 36 times that of incident UV light. We suggest the use of semiconducting graphene nanoribbons for converting visible and UV light into a THz signal.}}, journal = {{Nanoscale}}, volume = {7}, number = {45}, pages = {19012--19017}, }`

- J. Jornet-Somoza, J. Alberdi-Rodriguez, B. F. Milne, X. Andrade, M. A. L. Marques, F. Nogueira, M. J. T. Oliveira, J. J. P. Stewart, and A. Rubio, “Insights into colour-tuning of chlorophyll optical response in green plants,” Physical Chemistry Chemical Physics, vol. 17, iss. 40, pp. 26599-26606, 2015.

[Bibtex]`@article{escidoc:2174860, title = {{Insights into colour-tuning of chlorophyll optical response in green plants}}, author = {Jornet-Somoza, Joaquim and Alberdi-Rodriguez, Joseba and Milne, Bruce F. and Andrade, Xavier and Marques, Miguel A. L. and Nogueira, Fernando and Oliveira, Micael J. T. and Stewart, James J. P. and Rubio, Angel}, language = {eng}, issn = {1463-9076}, doi = {10.1039/C5CP03392F}, publisher = {Royal Society of Chemistry}, address = {Cambridge, England}, year = {2015}, date = {2015-10-28}, abstract = {{First-principles calculations within the framework of real-space time-dependent density functional theory have been performed for the complete chlorophyll (Chl) network of the light-harvesting complex from green plants, LHC-II. A local-dipole analysis method developed for this work has made possible studies of the optical response of individual Chl molecules subject to the influence of the remainder of the chromophore network. The spectra calculated with our real-space TDDFT method agree with previous suggestions that weak interaction with the protein microenvironment should produce only minor changes in the absorption spectrum of Chl chromophores in LHC-II. In addition, relative shifting of Chl absorption energies leads the stromal and lumenal sides of LHC-II to absorb in slightly different parts of the visible spectrum providing greater coverage of available light frequencies. The site-specific alterations in Chl excitation energies support the existence of intrinsic energy transfer pathways within the LHC-II complex.}}, journal = {{Physical Chemistry Chemical Physics}}, volume = {17}, number = {40}, pages = {26599--26606}, }`

- M. H. Stockett, L. Musbat, C. Kjaer, J. Houmoller, Y. Toker, A. Rubio, B. F. Milne, and S. Brondsted Nielsn, “The Soret absorption band of isolated chlorophyll a and b tagged with quaternary ammonium ions,” Physical Chemistry Chemical Physics, vol. 17, iss. 39, pp. 25793-25798, 2015.

[Bibtex]`@article{escidoc:2175029, title = {{The Soret absorption band of isolated chlorophyll a and b tagged with quaternary ammonium ions}}, author = {Stockett, Mark H. and Musbat, Lihi and Kj\aer, Christina and Houm\oller, J\orgen and Toker, Yoni and Rubio, Angel and Milne, Bruce F. and Br\ondsted Nielsn, Steen}, language = {eng}, issn = {1463-9076}, doi = {10.1039/C5CP01513H}, publisher = {Royal Society of Chemistry}, address = {Cambridge, England}, year = {2015}, date = {2015-10-21}, abstract = {{We have performed gas-phase absorption spectroscopy in the Soret-band region of chlorophyll (Chl) a and b tagged by quaternary ammonium ions together with time-dependent density functional theory (TD-DFT) calculations. This band is the strongest in the visible region of metalloporphyrins and an important reporter on the microenvironment. The cationic charge tags were tetramethylammonium, tetrabutylammonium, and acetylcholine, and the dominant dissociation channel in all cases was breakage of the complex to give neutral Chl and the charge tag as determined by photoinduced dissociation mass spectroscopy. Two photons were required to induce fragmentation on the time scale of the experiment (microseconds). Action spectra were recorded where the yield of the tag as a function of excitation wavelength was sampled. These spectra are taken to represent the corresponding absorption spectra. In the case of Chl a we find that the tag hardly influences the band maximum which for all three tags is at 403 $\pm$ 5 nm. A smaller band with maximum at 365 $\pm$ 10 nm was also measured for all three complexes. The spectral quality is worse in the case of Chl b due to lower ion beam currents; however, there is clear evidence for the absorption being to the red of that of Chl a (most intense peak at 409 $\pm$ 5 nm) and also a more split band. Our results demonstrate that the change in the Soret-band spectrum when one peripheral substituent (CH\textlesssub\textgreater3\textless/sub\textgreater) is replaced by another (CHO) is an intrinsic effect. First principles TD-DFT calculations agree with our experiments, supporting the intrinsic nature of the difference between Chl a and b and also displaying minimal spectral changes when different charge tags are employed. The deviations between theory and experiment have allowed us to estimate that the Soret-band absorption maxima in vacuo for the neutral Chl a and Chl b should occur at 405 nm and 413 nm, respectively. Importantly, the Soret bands of the isolated species are significantly blueshifted compared to those of solvated Chl and Chl\textendashproteins. The protein microenvironment is certainly not innocent of perturbing the electronic structure of Chls.}}, journal = {{Physical Chemistry Chemical Physics}}, volume = {17}, number = {39}, pages = {25793--25798}, }`

- M. H. Stockett, J. Houmoller, C. Kjaer, B. F. Milne, L. Musbat, A. Rubio, Y. Toker, and S. B. Nielsn, “Action spectroscopy of chlorophyll and other coordination complexes,” , Toledo, Spain, 2015.

[Bibtex]`@inproceedings{escidoc:2214534, title = {{Action spectroscopy of chlorophyll and other coordination complexes}}, author = {Stockett, Mark H. and Houm\oller, J\orgen and Kj\aer, Christina and Milne, Bruce F. and Musbat, Lihi and Rubio, Angel and Toker, Yoni and Nielsn, Steen Br\ondsted}, language = {eng}, issn = {1742-6588}, doi = {10.1088/1742-6596/635/11/112015}, publisher = {IOP Publishing}, year = {2015}, abstract = {{Action spectroscopy provides key insights into the nature of electronic transitions of coordination complexes such as porhyrin-containing biochromophores like chlorophyll or transition metal complexes such as tris(bipyridine)ruthenium.}}, journal = {{Journal of Physics: Conference Series}}, volume = {635}, eid = {112015}, address = {Toledo, Spain}, }`

- C. Pellegrini, J. Flick, I. V. Tokatly, H. Appel, and A. Rubio, “Optimized Effective Potential for Quantum Electrodynamical Time-Dependent Density Functional Theory,” Physical Review Letters, vol. 115, iss. 9, 2015.

[Bibtex]`@article{escidoc:2191550, title = {{Optimized Effective Potential for Quantum Electrodynamical Time-Dependent Density Functional Theory}}, author = {Pellegrini, Camilla and Flick, Johannes and Tokatly, Ilya V. and Appel, Heiko and Rubio, Angel}, language = {eng}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.115.093001}, eprint = {1412.4530}, publisher = {American Physical Society}, address = {Woodbury, N.Y.}, year = {2015}, date = {2015-08-28}, abstract = {{We propose an orbital exchange-correlation functional for applying time-dependent density functional theory to many-electron systems coupled to cavity photons. The time nonlocal equation for the electron-photon optimized effective potential (OEP) is derived. In the static limit our OEP energy functional reduces to the Lamb shift of the ground state energy. We test the new approximation in the Rabi model. It is shown that the OEP (i) reproduces quantitatively the exact ground-state energy from the weak to the deep strong coupling regime and (ii) accurately captures the dynamics entering the ultrastrong coupling regime. The present formalism opens the path to a first-principles description of correlated electron-photon systems, bridging the gap between electronic structure methods and quantum optics for real material applications.}}, journal = {{Physical Review Letters}}, volume = {115}, number = {9}, eid = {093001}, }`

- J. D. Rameau, S. Freutel, M. A. Sentef, A. F. Kemper, J. K. Freericks, I. Avigo, M. Ligges, L. Rettig, Y. Yoshida, H. Eisaki, J. Schneeloch, R. D. Zhong, Z. J. Xu, G. D. Gu, P. D. Johnson, and U. Bovensiepen, Time-resolved Boson Emission in the Excitation Spectrum of Bi2Sr2CaCu2O8+$\delta$, 2015.

[Bibtex]`@misc{escidoc:2188954, title = {{Time-resolved Boson Emission in the Excitation Spectrum of Bi2Sr2CaCu2O8+$\delta$}}, author = {Rameau, J. D. and Freutel, S. and Sentef, Michael A. and Kemper, A. F. and Freericks, J. K. and Avigo, I. and Ligges, M. and Rettig, L. and Yoshida, Y. and Eisaki, H. and Schneeloch, J. and Zhong, R. D. and Xu, Z. J. and Gu, G. D. and Johnson, P. D. and Bovensiepen, U.}, language = {eng}, eprint = {1505.07055}, year = {2015}, abstract = {{Using femtosecond time- and angle-resolved photoemission spectroscopy we analyze the complex unoccupied electronic structure of optimally doped Bi\textlesssub\textgreater2\textless/sub\textgreaterSr\textlesssub\textgreater2\textless/sub\textgreaterCaCu\textlesssub\textgreater2\textless/sub\textgreaterO\textlesssub\textgreater8+$\delta$\textless/sub\textgreater after optical excitation. Electronic relaxation is characterized by a 250 fs relaxation time below and \textless20 fs above a single boson energy of \textasciitilde75 meV above the Fermi energy E\textlesssub\textgreaterF\textless/sub\textgreater which represents the unoccupied counterpart of the kink observed below E\textlesssub\textgreaterF\textless/sub\textgreater and T\textlesssub\textgreaterc\textless/sub\textgreater. With increasing optical excitation density this sharp energy-dependent step in the relaxation time is found to vanish, in good agreement with self-energy and time-dependent Keldysh Green\textquotesingles function calculations, due to redistribution of electrons within the relaxation phase space. These findings have implications regarding the origin of the primary nodal kink in the electronic structure and for the mechanism behind high T\textlesssub\textgreaterc\textless/sub\textgreater superconductivity in the cuprates.}}, }`

- H. Sun, D. J. Mowbray, A. Migani, J. Zhao, H. Petek, and A. Rubio, “Comparing quasiparticle H2O level alignment on anatase and rutile TiO2,” ACS Catalysis, vol. 5, iss. 7, pp. 4242-4254, 2015.

[Bibtex]`@article{escidoc:2157428, title = {{Comparing quasiparticle H2O level alignment on anatase and rutile TiO2}}, author = {Sun, Huijuan and Mowbray, Duncan J. and Migani, Annapaola and Zhao, Jin and Petek, Hrvoje and Rubio, Angel}, language = {eng}, doi = {10.1021/acscatal.5b00529}, eprint = {1506.04695}, publisher = {ACS}, address = {Washington, DC}, year = {2015}, date = {2015-07-02}, abstract = {{Knowledge of the molecular frontier levels\textquotesingle alignment in the ground state can be used to predict the photocatalytic activity of an interface. The position of the adsorbate\textquotesingles highest occupied molecular orbital (HOMO) levels relative to the substrate\textquotesingles valence band maximum (VBM) in the interface describes the favorability of photogenerated hole transfer from the VBM to the adsorbed molecule. This is a key quantity for assessing and comparing H\textlesssub\textgreater2\textless/sub\textgreaterO photooxidation activities on two prototypical photocatalytic TiO\textlesssub\textgreater2\textless/sub\textgreater surfaces: anatase (A)-TiO\textlesssub\textgreater2\textless/sub\textgreater(101) and rutile (R)-TiO\textlesssub\textgreater2\textless/sub\textgreater(110). Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) G\textlesssub\textgreater0\textless/sub\textgreaterW\textlesssub\textgreater0\textless/sub\textgreater calculations, we assess the relative photocatalytic activity of intact and dissociated H\textlesssub\textgreater2\textless/sub\textgreaterO on coordinately unsaturated (Ti\textlesssub\textgreatercus\textless/sub\textgreater) sites of idealized stoichiometric A-TiO\textlesssub\textgreater2\textless/sub\textgreater(101)/R-TiO\textlesssub\textgreater2\textless/sub\textgreater(110) and bridging O vacancies (O\textlesssub\textgreaterbr\textless/sub\textgreater\textlesssup\textgreatervac\textless/sup\textgreater) of defective A-TiO\textlesssub\textgreater2-x\textless/sub\textgreater(101)/R-TiO\textlesssub\textgreater2-x\textless/sub\textgreater(110) surfaces (x\textequals1/4,1/8) for various coverages. Such a many-body treatment is necessary to correctly describe the anisotropic screening of electron-electron interactions at a photocatalytic interface, and hence obtain accurate interfacial level alignments. The more favorable ground state HOMO level alignment for A-TiO\textlesssub\textgreater2\textless/sub\textgreater(101) may explain why the anatase polymorph shows higher photocatalytic activities than the rutile polymorph. Our results indicate that (1) hole trapping is more favored on A-TiO\textlesssub\textgreater2\textless/sub\textgreater(101) than R-TiO\textlesssub\textgreater2\textless/sub\textgreater(110) and (2) HO@Ti\textlesssub\textgreatercus\textless/sub\textgreater is more photocatalytically active than intact H\textlesssub\textgreater2\textless/sub\textgreaterO@Ti\textlesssub\textgreatercus\textless/sub\textgreater.}}, journal = {{ACS Catalysis}}, volume = {5}, number = {7}, pages = {4242--4254}, }`

- R. Biele, R. DtextquotesingleAgosta, and A. Rubio, “Time-Dependent Thermal Transport Theory,” Physical Review Letters, vol. 115, iss. 5, 2015.

[Bibtex]`@article{escidoc:2179952, title = {{Time-Dependent Thermal Transport Theory}}, author = {Biele, Robert and D\textquotesingleAgosta, Roberto and Rubio, Angel}, language = {eng}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.115.056801}, eprint = {1412.5765}, publisher = {American Physical Society}, address = {Woodbury, N.Y.}, year = {2015}, date = {2015-07-31}, abstract = {{Understanding thermal transport in nanoscale systems presents important challenges to both theory and experiment. In particular, the concept of local temperature at the nanoscale appears difficult to justify. Here, we propose a theoretical approach where we replace the temperature gradient with controllable external blackbody radiations. The theory recovers known physical results, for example, the linear relation between the thermal current and the temperature difference of two blackbodies. Furthermore, our theory is not limited to the linear regime and goes beyond accounting for nonlinear effects and transient phenomena. Since the present theory is general and can be adapted to describe both electron and phonon dynamics, it provides a first step toward a unified formalism for investigating thermal and electronic transport.}}, journal = {{Physical Review Letters}}, volume = {115}, number = {5}, eid = {056801}, }`

- J. Alberdi-Rodriguez, A. Rubio, M. Oliveira, A. Charalampidou, and D. Folias, Memory Optimization for the Octopus Scientific Code, 2015.

[Bibtex]`@misc{escidoc:2095302, title = {{Memory Optimization for the Octopus Scientific Code}}, author = {Alberdi-Rodriguez, J. and Rubio, A. and Oliveira, M. and Charalampidou, A. and Folias, D.}, language = {eng}, year = {2015}, abstract = {{Octopus is a software package for density-functional theory (DFT), and its time-dependent (TDDFT) variant. Linear Combination of the Atomic Orbitals (LCAO) is performed previous to the actual DFT run. LCAO is used to get an initial guess of densities, and therefore, to start with the Self Consistent Field (SCF) of the Ground-State (GS). System initialization and LCAO steps consume a large amount of memory and do not demonstrate good performance. In this study, extensive profiling has been performed, in order to identify large matrices and scaling behaviour of initialization and LCAO. Alternative implementations of LCAO in Octopus have been investigated in order to optimize memory usage and performance of LCAO approach. Use of ScaLAPACK library led to significant improvement of memory allocation and performance. Benchmark tests have been performed on MareNostrum III HPC system using various combinations of atomic systems\textquoteright sizes and numbers of CPU cores.}}, }`

- C. Schäfer, J. Flick, H. Appel, C. Pellegrini, and A. Rubio, Krieger-Li-Iafrate approximation to the optimized effective potential approach in density functional theory for quantum electrodynamics, 2015.

[Bibtex]`@misc{escidoc:2179846, title = {{Krieger-Li-Iafrate approximation to the optimized effective potential approach in density functional theory for quantum electrodynamics}}, author = {Sch\"afer, Christian and Flick, Johannes and Appel, Heiko and Pellegrini, Camilla and Rubio, Angel}, language = {eng}, year = {2015}, abstract = {{Many-body perturbation theory (MBPT) opens the possibility to construct approximations to every desired order of a \textquoterightweak\textquoteright interacting system. The drawback is a in general non-local interaction in space and time and it is therefore a demanding task to apply it to \textquoterightreal\textquoteright systems. The optimized effective potential (OEP), derived by inversion of the Sham-Schl\"uter equation, is a natural connection between local density-functional theory and MBPT. In principle, this variationally best local potential reduces the problem to solving a simple system of Kohn-Sham equations combined with the solution of the OEP integral equation. However, converging the full set of OEP equations is a quite challenging procedure and is in practice rarely tackled. The Krieger-Li-Iafrate (KLI) approximation reduces the integral equation to an analytically solvable one via a dominant orbital approximation. It performs usually quite well for electronic systems. In the present work, we extend the OEP and KLI approaches to the case of electron-photon interactions in quantum optics and quantum electrodynamics. Here an effective electronic interaction is transmitted via transversal photons. We present first static and time-dependent results for the OEP and KLI approximations of the Rabi model and compare with the exact configuration interaction solution and the corresponding exact Kohn-Sham potentials.}}, }`

- A. Varas, P. Zhang, J. Feist, P. Garc’ia-González, F. J. Garc’ia-Vidal, and A. Rubio, First-principles nanoplasmonics on metal-clusters dimers, 2015.

[Bibtex]`@misc{escidoc:2179837, title = {{First-principles nanoplasmonics on metal-clusters dimers}}, author = {Varas, Alejandro and Zhang, Pu and Feist, Johannes and Garc\'\ia-Gonz\'alez, Pablo and Garc\'\ia-Vidal, F. J. and Rubio, Angel}, language = {eng}, year = {2015}, abstract = {{Metal-cluster dimers are a prototypical scenario for quantum plasmonics. The well-known hybridization of localized surface plasmons (LSPs) is strongly affected by the appareance of light-induced currents between the nanoparticles, breacking down the predictions of classical optics. The main trends can be well understood using the jellium model and time-dependent density functional theory (TDDFT). However, atomic structure has to be consider to have results with enough predictive accuracy. In this work we provide further insights on the importance of the atomic structure, including effects due to the relative orientation of the atoms and relaxation of the atoms in the gap region.}}, }`

- L. N. Glanzmann, D. J. Mowbray, and A. Rubio, Exitons in P3HT/SWNT/PCBM Heterojunctions, 2015.

[Bibtex]`@misc{escidoc:2179745, title = {{Exitons in P3HT/SWNT/PCBM Heterojunctions}}, author = {Glanzmann, L. N. and Mowbray, D. J. and Rubio, Angel}, language = {eng}, year = {2015}, abstract = {{Semiconducting single walled carbon nanotubes (s-SWNT) are known for their high charge carrier transport. Thus, they are promising electron transporting materials for photovoltaic (PV) active layers of organic solar cells (OSCs). An increase in efficiency is found by adding s-SWNTs next to a donor (D) and acceptor (A) heterojunction such as P3HT and PCBM. Pump probe spectroscopy is used to follow the exciton dynamics through these active layers. However, such multi-component spectra are difficult to interpret. To disentangle the various processes, we use the triplet state of the system to model the evolution of the lowest energy s-SWNT excited state (E11). Specifically, using the electron (spin majority channel) and hole (spin minority channel) density distributions for the triplet state, we find where electrons and holes are transferred within the active layer. Further, applying a non-equilibrium Green\textquotesingles function (NEGF) approach, we obtain the rate of D to A charge transport.}}, }`

- J. N. Sanders, A. Pérez Paz, A. Aspuru-Guzik, and A. Rubio, Application of Compressed Sensing to Quantum Chemistry, 2015.

[Bibtex]`@misc{escidoc:2179750, title = {{Application of Compressed Sensing to Quantum Chemistry}}, author = {Sanders, Jacob N. and P\'erez Paz, Alejandro and Aspuru-Guzik, Al\'an and Rubio, Angel}, language = {eng}, year = {2015}, }`

- P. Cudazzo, F. Sottile, A. Rubio, and M. Gatti, “Exciton dispersion in molecular solids,” Journal of Physics: Condensed Matter, vol. 27, iss. 11, 2015.

[Bibtex]`@article{escidoc:2126793, title = {{Exciton dispersion in molecular solids}}, author = {Cudazzo, Pierluigi and Sottile, Francesco and Rubio, Angel and Gatti, Matteo}, language = {eng}, issn = {0953-8984}, doi = {10.1088/0953-8984/27/11/113204}, publisher = {IOP Publishing}, address = {Bristol, UK}, year = {2015}, date = {2015-03-25}, abstract = {{The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron\textendashhole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier\textendashMott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe\textendashSalpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron\textendashhole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.}}, journal = {{Journal of Physics: Condensed Matter}}, volume = {27}, number = {11}, eid = {113204}, }`

- B. M. K. Mariserla, M. K. L. Man, S. Vinod, C. Chin, T. Harada, J. Taha-Tijerina, C. S. Tiwary, P. Nguyen, P. Chang, T. N. Narayanan, A. Rubio, P. M. Ajayan, S. Talapatra, and K. M. Dani, “Emergent photophenomena in three dimensional van der Waals heterostructures,” in CLEO: QELS\textunderscoreFundamental Science 2015, San Jose, California, United States, 2015.

[Bibtex]`@inproceedings{escidoc:2175549, title = {{Emergent photophenomena in three dimensional van der Waals heterostructures}}, author = {Mariserla, Bala Murali Krishna and Man, Michael K. L. and Vinod, Soumya and Chin, Catherine and Harada, Takaaki and Taha-Tijerina, Jaime and Tiwary, Chandra Sekhar and Nguyen, Patrick and Chang, Patricia and Narayanan, Tharangattu N. and Rubio, Angel and Ajayan, Pulickel M. and Talapatra, Saikat and Dani, Keshav M.}, language = {eng}, isbn = {978-1-55752-968-8}, doi = {10.1364/CLEO\textunderscoreQELS.2015.FM3B.5}, publisher = {Optical Society of America}, year = {2015}, abstract = {{We report on the fabrication and observation of emergent opto-electronic phenomena in three dimensional, micron-sized van der Waals heterostructures self-assembled from atomic layers of graphene and hexagonal boron nitride in varying ratios.}}, booktitle = {{CLEO: QELS\textunderscoreFundamental Science 2015}}, eid = {FM3B.5}, address = {San Jose, California, United States}, }`

- M. Hellgren, F. Caruso, D. R. Rohr, X. Ren, A. Rubio, M. Scheffler, and P. Rinke, “Static correlation and electron localization in molecular dimers from the self-consistent RPA and GW approximation,” Physical Review B, vol. 91, iss. 16, 2015.

[Bibtex]`@article{escidoc:2152175, title = {{Static correlation and electron localization in molecular dimers from the self-consistent RPA and GW approximation}}, author = {Hellgren, Maria and Caruso, Fabio and Rohr, Daniel R. and Ren, Xinguo and Rubio, Angel and Scheffler, Matthias and Rinke, Patrick}, language = {eng}, issn = {1098-0121}, doi = {10.1103/PhysRevB.91.165110}, eprint = {1412.7507}, publisher = {American Physical Society}, address = {Woodbury, NY}, year = {2015}, date = {2015-04-15}, abstract = {{We investigate static correlation and delocalization errors in the self-consistent GW and random-phase approximation (RPA) by studying molecular dissociation of the H\textlesssub\textgreater2\textless/sub\textgreater and LiH molecules. Although both approximations contain topologically identical diagrams, the nonlocality and frequency dependence of the GW self-energy crucially influence the different energy contributions to the total energy as compared to the use of a static local potential in the RPA. The latter leads to significantly larger correlation energies, which allow for a better description of static correlation at intermediate bond distances. The substantial error found in GW is further analyzed by comparing spin-restricted and spin-unrestricted calculations. At large but finite nuclear separation, their difference gives an estimate of the so-called fractional spin error normally determined only in the dissociation limit. Furthermore, a calculation of the dipole moment of the LiH molecule at dissociation reveals a large delocalization error in GW making the fractional charge error comparable to the RPA. The analyses are supplemented by explicit formulas for the GW Green\textquotesingles function and total energy of a simplified two-level model providing additional insights into the dissociation limit.}}, journal = {{Physical Review B}}, volume = {91}, number = {16}, eid = {165110}, }`

- Y. Miyamoto, H. Zhang, T. Miyazaki, and A. Rubio, “Modifying the Interlayer Interaction in Layered Materials with an Intense IR Laser,” Physical Review Letters, vol. 114, iss. 11, 2015.

[Bibtex]`@article{escidoc:2127288, title = {{Modifying the Interlayer Interaction in Layered Materials with an Intense IR Laser}}, author = {Miyamoto, Yoshiyuki and Zhang, Hong and Miyazaki, Takehide and Rubio, Angel}, language = {eng}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.114.116102}, publisher = {American Physical Society}, address = {Woodbury, N.Y.}, year = {2015}, date = {2015-03-20}, abstract = {{We propose a transient interlayer compression in two-dimensional compound materials by using an intense IR laser resonant with the out-of-plane optical phonon mode (A\textlesssub\textgreater2u\textless/sub\textgreater mode). As a test case, we studied bilayer hexagonal boron nitride (h-BN), which is one of the compound layered materials. Excited state molecular dynamics calculations using time-dependent density functional theory show an 11.3\textpercent transient interlayer contraction of h-BN due to an interlayer dipole-dipole attraction of the laser-pumped A\textlesssub\textgreater2u\textless/sub\textgreater mode. These results are applicable to other layered compound materials. Such layered materials are a good material for nanospace chemistry, e.g., intercalating molecules and acting with them, and IR irradiation to contract the interlayer distance could provide a new route for chemical reactions under pressure. The duration of the contraction is at least 1 ps in the current simulation, which is observable by high-speed electron-beam diffraction measurements.}}, journal = {{Physical Review Letters}}, volume = {114}, number = {11}, eid = {116102}, }`

- A. Varas, P. Garc’ia-Gonza'{}lez, F. J. Garc’ia-Vidal, and A. Rubio, “Anisotropy Effects on the Plasmonic Response of Nanoparticle Dimers,” The Journal of Physical Chemistry Letters, vol. 6, iss. 10, pp. 1891-1898, 2015.

[Bibtex]`@article{escidoc:2152180, title = {{Anisotropy Effects on the Plasmonic Response of Nanoparticle Dimers}}, author = {Varas, Alejandro and Garc\'\ia-Gonza\'{}lez, Pablo and Garc\'\ia-Vidal, F. J. and Rubio, Angel}, language = {eng}, doi = {10.1021/acs.jpclett.5b00573}, publisher = {American Chemical Society}, address = {Washington, DC}, year = {2015}, date = {2015-05-21}, abstract = {{We present an ab initio study of the anisotropy and atomic relaxation effects on the optical properties of nanoparticle dimers. Special emphasis is placed on the hybridization process of localized surface plasmons, plasmon-mediated photoinduced currents, and electric-field enhancement in the dimer junction. We show that there is a critical range of separations between the clusters (0.1\textendash0.5 nm) in which the detailed atomic structure in the junction and the relative orientation of the nanoparticles have to be considered to obtain quantitative predictions for realistic nanoplasmonic devices. It is worth noting that this regime is characterized by the emergence of electron tunneling as a response to the driven electromagnetic field. The orientation of the particles not only modifies the attainable electric field enhancement but can lead to qualitative changes in the optical absorption spectrum of the system.}}, journal = {{The Journal of Physical Chemistry Letters}}, volume = {6}, number = {10}, pages = {1891--1898}, }`

- G. Albareda, J. M. Bofill, I. Tavernelli, F. Huarte-Larran~{}aga, F. Illas, and A. Rubio, “Conditional Born\textminusOppenheimer Dynamics: Quantum Dynamics Simulations for the Model Porphine,” The Journal of Physical Chemistry Letters, vol. 6, iss. 9, pp. 1529-1535, 2015.

[Bibtex]`@article{escidoc:2156060, title = {{Conditional Born\textminusOppenheimer Dynamics: Quantum Dynamics Simulations for the Model Porphine}}, author = {Albareda, Guillermo and Bofill, Josep Maria and Tavernelli, Ivano and Huarte-Larran\~{}aga, Fermin and Illas, Francesc and Rubio, Angel}, language = {eng}, doi = {10.1021/acs.jpclett.5b00422}, publisher = {American Chemical Society}, address = {Washington, DC}, year = {2015}, date = {2015-05-07}, abstract = {{We report a new theoretical approach to solve adiabatic quantum molecular dynamics halfway between wave function and trajectory-based methods. The evolution of a N-body nuclear wave function moving on a 3N-dimensional Born\textendashOppenheimer potential-energy hyper-surface is rewritten in terms of single-nuclei wave functions evolving nonunitarily on a 3-dimensional potential-energy surface that depends parametrically on the configuration of an ensemble of generally defined trajectories. The scheme is exact and, together with the use of trajectory-based statistical techniques, can be exploited to circumvent the calculation and storage of many-body quantities (e.g., wave function and potential-energy surface) whose size scales exponentially with the number of nuclear degrees of freedom. As a proof of concept, we present numerical simulations of a 2-dimensional model porphine where switching from concerted to sequential double proton transfer (and back) is induced quantum mechanically.}}, journal = {{The Journal of Physical Chemistry Letters}}, volume = {6}, number = {9}, pages = {1529--1535}, }`

- K. B. Ornsø, J. M. Garcia-Lastra, G. De La Torre, F. J. Himpsel, A. Rubio, and K. S. Thygesen, “Design of two-photon molecular tandem architectures for solar cells by ab initio theory,” Chemical Science, vol. 6, iss. 5, pp. 3018-3025, 2015.

[Bibtex]`@article{escidoc:2149595, title = {{Design of two-photon molecular tandem architectures for solar cells by ab initio theory}}, author = {\Orns\o, Kristian B. and Garcia-Lastra, Juan M. and De La Torre, Gema and Himpsel, F. J. and Rubio, Angel and Thygesen, Kristian S.}, language = {eng}, issn = {2041-6520}, doi = {10.1039/C4SC03835E}, publisher = {Royal Society of Chemistry}, address = {Cambridge, UK}, year = {2015}, date = {2015-05-01}, abstract = {{An extensive database of spectroscopic properties of molecules from ab initio calculations is used to design molecular complexes for use in tandem solar cells that convert two photons into a single electron\textendashhole pair, thereby increasing the output voltage while covering a wider spectral range. Three different architectures are considered: the first two involve a complex consisting of two dye molecules with appropriately matched frontier orbitals, connected by a molecular diode. Optimized combinations of dye molecules are determined by taking advantage of our computational database of the structural and energetic properties of several thousand porphyrin dyes. The third design is a molecular analogy of the intermediate band solar cell, and involves a single dye molecule with strong intersystem crossing to ensure a long lifetime of the intermediate state. Based on the calculated energy levels and molecular orbitals, energy diagrams are presented for the individual steps in the operation of such tandem solar cells. We find that theoretical open circuit voltages of up to 1.8 V can be achieved using these tandem designs. Questions about the practical implementation of prototypical devices, such as the synthesis of the tandem molecules and potential loss mechanisms, are addressed.}}, journal = {{Chemical Science}}, volume = {6}, number = {5}, pages = {3018--3025}, }`

- B. F. Milne, Y. Toker, A. Rubio, and S. B. Nielsn, “Unraveling the Intrinsic Color of Chlorophyll,” Angewandte Chemie International Edition, vol. 54, iss. 7, pp. 2170-2173, 2015.

[Bibtex]`@article{ANIE:ANIE201410899, title = {{Unraveling the Intrinsic Color of Chlorophyll}}, author = {Milne, Bruce F. and Toker, Yoni and Rubio, Angel and Nielsn, Steen Br\ondsted}, language = {eng}, doi = {10.1002/anie.201410899}, publisher = {Wiley-VCH Verlag GmbH \& Co. KGaA}, address = {Weinheim}, year = {2015}, date = {2015-02-09}, abstract = {{The exact color of light absorbed by chlorophyll (Chl) pigments, the light-harvesters in photosynthesis, is tuned by the protein microenvironment, but without knowledge of the intrinsic color of Chl it remains unclear how large this effect is. Experimental first absorption energies of Chl a and b isolated in vacuo and tagged with quaternary ammonium cations are reported. The energies are largely insensitive to details of the tag structure, a finding supported by first-principles calculations using time-dependent density functional theory. Absorption is significantly blue-shifted compared to that of Chl-containing proteins (by 30\textendash70 nm). A single red-shifting perturbation, such as axial ligation or the protein medium, is insufficient to account even for the smallest shift; the largest requires pigment\textendashpigment interactions.}}, journal = {{Angewandte Chemie International Edition}}, volume = {54}, number = {7}, pages = {2170--2173}, }`

### 2014

- M. E. Dávila, L. Xian, S. Cahangirov, A. Rubio, and G. Le Lay, “Germanene: a novel two-dimensional germanium allotrope akin to graphene and silicene,” New Journal of Physics, vol. 16, iss. 9, 2014.

[Bibtex]`@article{escidoc:2086947, title = {{Germanene: a novel two-dimensional germanium allotrope akin to graphene and silicene}}, author = {D\'avila, M. E. and Xian, L. and Cahangirov, S. and Rubio, Angel and Le Lay, G.}, language = {eng}, issn = {1367-2630}, doi = {10.1088/1367-2630/16/9/095002}, eprint = {1406.2488}, publisher = {IOP Publishing}, address = {Bristol}, year = {2014}, abstract = {{We have grown an atom-thin, ordered, two-dimensional multi-phase film in situ through germanium molecular beam epitaxy using a gold (111) surface as a substrate. Its growth is similar to the formation of silicene layers on silver (111) templates. One of the phases, forming large domains, as observed in scanning tunneling microscopy, shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced density functional theory calculations we can identify it as a $\surd$3 $\times$ $\surd$3 R(30\mbox{$^\circ$}) germanene layer in conjunction with a $\surd$7 $\times$ $\surd$7 R(19.1\mbox{$^\circ$}) Au(111) supercell, presenting compelling evidence of the synthesis of the germanium-based cousin of graphene on gold.}}, journal = {{New Journal of Physics}}, volume = {16}, number = {9}, eid = {095002}, }`

- D. Wegkamp, M. Herzog, L. Xian, M. Gatti, P. Cudazzo, C. L. McGahan, R. E. Marvel, R. F. Haglund, A. Rubio, M. Wolf, and J. Stähler, “Instantaneous Band Gap Collapse in Photoexcited Monoclinic VO2 due to Photocarrier Doping,” Physical Review Letters, vol. 113, iss. 21, 2014.

[Bibtex]`@article{escidoc:2086946, title = {{Instantaneous Band Gap Collapse in Photoexcited Monoclinic VO2 due to Photocarrier Doping}}, author = {Wegkamp, Daniel and Herzog, Marc and Xian, Lede and Gatti, Matteo and Cudazzo, Pierluigi and McGahan, Christina L. and Marvel, Robert E. and Haglund, Richard F. and Rubio, Angel and Wolf, Martin and St\"ahler, Julia}, language = {eng}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.113.216401}, eprint = {1408.3209}, publisher = {American Physical Society}, address = {Woodbury, N.Y.}, year = {2014}, date = {2014-11-21}, abstract = {{Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO\textlesssub\textgreater2\textless/sub\textgreater quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO\textlesssub\textgreater2\textless/sub\textgreater. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO\textlesssub\textgreater2\textless/sub\textgreater band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred.}}, journal = {{Physical Review Letters}}, volume = {113}, number = {21}, eid = {216401}, }`

- J. Solanpää, J. A. Budagosky, N. I. Shvetsov-Shilovski, A. Castro, A. Rubio, and E. Räsänen, “Optimal control of high-harmonic generation by intense few-cycle pulses,” Physical Review A, vol. 90, iss. 5, 2014.

[Bibtex]`@article{PhysRevA.90.053402, title = {{Optimal control of high-harmonic generation by intense few-cycle pulses}}, author = {Solanp\"a\"a, J. and Budagosky, J. A. and Shvetsov-Shilovski, N. I. and Castro, A. and Rubio, A. and R\"as\"anen, E.}, language = {eng}, issn = {1050-2947}, doi = {10.1103/PhysRevA.90.053402}, eprint = {1406.5893}, publisher = {American Physical Society}, address = {New York, NY}, year = {2014}, date = {2014-11}, abstract = {{At the core of attosecond science lies the ability to generate laser pulses of subfemtosecond duration. In tabletop devices the process relies on high-harmonic generation, where a major challenge is to obtain high yields and high cutoff energies required for the generation of attosecond pulses. We develop a computational method that can simultaneously resolve these issues by optimizing the driving pulses using quantum optimal control theory. Our target functional, an integral over the harmonic yield over a desired energy range, leads to a remarkable cutoff extension and yield enhancement for a one-dimensional model H atom. The physical enhancement process is shown to be twofold: the cutoff extension is of classical origin, whereas the yield enhancement arises from increased tunneling probability. The scheme is directly applicable to more realistic models and, within straightforward refinements, also to experimental verification.}}, journal = {{Physical Review A}}, volume = {90}, number = {5}, eid = {053402}, }`

- H. Appel, Light-matter Coupling in Density-functional Theory for Quantum Electrodynamics.

[Bibtex]`@misc{escidoc:2297173, title = {{Light-matter Coupling in Density-functional Theory for Quantum Electrodynamics}}, author = {Appel, Heiko}, }`