Understanding coupled transport of charge and heat impacts the development of sustainable energy sources. Thermoelectric phenomena, such as the Seebeck or Peltier effect, help convert waste heat back into useful electronic energy. In order to achieve a high efficiency of such processes dissipative effects, i.e., the loss of heat to the environment, need to be controlled. The aim of this project is to investigate how thermoelectric transport coefficients can be predicted based on the microscopic structure. This opens a host of fundamental questions, especially at very short length and time scales, where quantum mechanical effects become important.
Transport in Open Quantum Systems
Transport describes how separate subsystems exchange electronic charge and heat. This exchange is governed by the surface or interface between the subsystems. A typical transport setup consists of molecular break junctions, where a molecule connects two metallic leads. Since molecules are nanoscale objects they have to be described quantum-mechanically. At the same time the molecule is attached to the outside world via the metallic leads, i.e., it is an open quantum system.
Quite generally an open quantum system can interact with the environment in various ways, e.g., by radiative exchange of heat, energy transfer by molecular vibrations and/or phonons, and flow of electrons. In our group we investigate how these processes can be addressed in a unified framework based on nonequilibrium Green’s functions and Density-Functional Theories.
In recent years, with the advent of new powerful terahertz lasers, it has become feasible to selectively excite IR-active phonon modes in condensed matter leading to interesting effects such as optical enhancement of superconductivity or non-linear coupling between different phonon modes. Motivated by experiments in Andrea Cavalleri’s Department, we investigate the excitation of two phonon modes involving the same atoms which can lead to an effective rotation of them, thus breaking time-reversal symmetry.
In this emerging field of “Rotonics” we strife for a complete understanding of the changes in the electronic structure of different materials such as magnetic compounds. Using ab-initio and model calculations we want to gain a clear understanding of the microscopic origin of the coupling between different collective excitations such as phonons and magnons.
Thermalization in Semiconductors
Thermalization of hot carriers in semiconductors is a main factor determining the efficiency of electronic devices and solar cells. While it has been studied extensively by time-resolved optical techniques, theoretical studies have been limited to approximations that model the highly non-equilibrium states of the hot carriers and their dynamics empirically.
We take an approach of electron-ion dynamics based on Ehrenfest molecular dynamics coupled with time-dependent density functional theory. We connect the ion’s displacement after thermalization with the stress tensor and thermal expansion coefficient. This could lead to a first-principles description of light-induced structural phase transitions.