Electron Transport and Thermal Transport Simulation Using Ab Initio Methods.

摘要

In this project, the uniquely-developed TOhoku Mixed Basis Orbital ab initio calculation package (TOMBO) was used in simulations of electron and thermal transport in a small molecule organic-based circuit and in a GeSe supercell, respectively. For the small molecule, application of a bias lead to a non-equilibrium state for which Density Functional Theory (DFT) is not directly applicable. The simulation proved useful by modifying TOMBO by separating the whole system (circuit) into three parts: right and left semi- infinitive leads, and finite organic molecule. Since TOMBO uses two different types of basis sets (plane-waves and atomic orbitals) maximally localized Wannier function (Wannier90) was introduced to realize the separation. Since the two semi-infinitive leads are very close to their equilibrium state, non- equilibrium Green function (NEGF) was used to represent them by self-energy terms. Introduction of bias affected periodic boundary conditions, but were satisfied by introducing a potential drop in the vacuum. Simulation of thermal conductivity versus temperature was considered for a 6x6x6 k-mesh involving a crystalline, orthorhombic GeSe supercell. Its thermal conductivity tensor is diagonal, with three different values along the 3 principal axes. This work examined the third trace of that tensor and the contribution of the different branches. Acoustic branches usually contribute most to the thermal conductivity as their group velocity and lifetimes are largest. In this case, each of the acoustic modes contributes to about 20% of the total value. The first 3 branches contribute to almost half the total thermal conductivity.