分子振動對電流雜訊影響之研究

Abstract

在本論文中，我們探討在單分子電子學中，分子振動對電子傳輸的 影響。為此，我們提出一個新的方法來計算複雜分子中，在受到分子 振動下，電子的量子輸送現象，這個方法是將分子動力學 (Molecular Dynamics) 與 Driven Liouville von Neumann approach 結合，因此我們將 之命名為 molecular dynamics-driven Liouville von Neumann method。 此研究中，我們發現，在有限溫度 (70 K)，當電子傳輸受到分子振 動的影響下，即使是非常短的分子 (trans-fumaronitrile) 也不能達到穩 態電流。此外，為了了解電流擾動與分子振動之間的關聯性，我們分 析了電流雜訊的譜密度和分子動力學的譜密度，兩者的相似與相異之 處。數值模擬的結果顯示並不是所有分子振動的簡正模 (Normal mode) 都會造成電流的擾動，而且當簡正模符合某種對稱性時，會造成二倍 頻的電流雜訊。

In this thesis, we propose a new computational method to simulate elec- tronic dynamics coupled to a number of normal modes in complicated molec- ular systems. The method incorporates molecular dynamics into the driven Liouville von Neumann (DLvN) approach, and we refer to our method as the molecular dynamics-driven Liouville von Neumann (MD-DLvN) method. We also investigate time-dependent electron transport through a molecu- lar junction in the adiabatic limit at the density-functional tight-binding level using the MD-DLvN method. When electron transport involves nuclear dy- namics 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 elec- tric 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 result in current fluctu- ations. Moreover, when a normal mode satisfies a particular symmetry, the normal mode can lead to frequency doubling of current fluctuations. This study provides new directions for studying electronic dynamics in a nonequi- librium open quantum system.