Zandpack: A general tool for time-dependent transportsimulation of nanoelectronics

The auxiliary mode approach to time-dependent open quantum system calculations is implemented and refined to yield a feasible computational approach to simulate nanostructures far from equilibrium. It is done by a careful diagonalization of the electrode level-width function, and provides an efficient approach which can simulate large, open systems at the level of time-dependent density functional theory. The approach, as given in this work, is implemented in the new open-source code Zandpack. The framework is applied to three systems perturbed by the same THz electromagnetic field pulse-form: 1) A Hubbard model for hydrogen on graphene is used to calculate spin-currents, mutual information, spin-transitions, and a pump-probe setup. 2) An armchair graphene nanoribbon (AGNR) probed by a metal tip showing electrons excited from the valence band of the AGNR into the tip via electron-electron interactions. 3) A gold break-junction is modeled with various gap distances, and displays behavior that is more different from the adiabatic case as the gap widens. In the examples 2 and 3, we develop and use a general linearization scheme for time-dependent open system calculations, which utilizes the DFTB+or SIESTAcodes.针对含时开放量子体系计算的辅助模方法经实现与优化后，可得到一种可用于模拟远平衡态纳米结构的可行计算方案。该方法通过对电极能级展宽函数开展严谨对角化得以实现，能够高效模拟大规模开放体系，且适配含时密度泛函理论（Time-Dependent Density Functional Theory）尺度的计算需求。本研究提出的该方法已集成至全新开源代码Zandpack中。 本框架针对三种受同一太赫兹（THz）电磁脉冲场扰动的体系开展应用： 1) 采用石墨烯负载氢的哈伯德模型（Hubbard Model），计算自旋流、互信息、自旋跃迁过程，并搭建泵浦-探测实验构型； 2) 采用金属探针探测的扶手椅型石墨烯纳米带（Armchair Graphene Nanoribbon, AGNR）体系，该体系中电子通过电子-电子相互作用从其价带激发至金属探针中； 3) 针对不同间隙距离的金基断裂结（Gold Break-Junction）体系开展建模，随着间隙增大，该体系的行为与绝热（Adiabatic）情形的偏差愈发显著。 在示例2与示例3中，本研究开发并采用了一种适用于含时开放体系计算的通用线性化方案，该方案可调用DFTB+与SIESTA软件完成计算。