Liang-Yan Hsu's Group

We investigate time-dependent electron transport through a molecular junction in the adiabatic limit at the density-functional tight-binding level using the molecular dynamics-driven Liouville von Neumann (MD-DLvN) approach. When electron transport involves nuclear dynamics at finite temperature (∼70 K) within the NVE ensemble, we find that the steady-state current cannot be achieved even for a very short molecule (trans-fumaronitrile). Furthermore, to establish a relationship between electric current fluctuations and molecular vibrations, we analyze the similarities and differences between the current noise spectra and the MD power spectra. Our simulations show that not all normal modes can bring about current fluctuations. Furthermore, when a normal mode satisfies a particular symmetry, the normal mode can lead to frequency doubling of current fluctuations. This investigation offers new directions for studying electronic dynamics in a nonequilibrium open quantum system.

We propose a time-varying optimal window width (TVOWW) and an adaptive optimal window width selection schemes to optimize the performance of several nonlinear-type time-frequency analyses, including the reassignment method and its variations. A window rendering the most concentrated distribution in the time-frequency representation is regarded as the optimal window. The TVOWW selection scheme is particularly useful for signals that comprise fast-varying instantaneous frequencies and small spectral gaps. To demonstrate the efficacy of the method, in addition to analyzing synthetic signals, we study an atomic time-varying dipole moment driven by two-color mid-infrared laser fields in attosecond physics and near-threshold harmonics of a hydrogen atom in the strong laser field.