Theoretical study of charge transport in molecular junctions

本文藉由結合封閉時間路徑積分，非平衡葛林函數，與重整化微擾理論等方法，理論研究分子連接器中的電荷轉移過程。分子連接器的哈米爾頓子被映射並拆成多個獨立的單雜質安德森哈米爾頓子，此被作為第零階哈米爾頓子並被視為電荷能穿隧或佔據的通道或軌域，而不同通道間的作用力則被當作微擾項。偽粒子的概念被引入來描述動力學過程。各個軌域中的非平衡電子密度分佈可以藉由解偽粒子速率方程式來得到，並因此得知電流量。使用這個方法，我們可同時考慮到協調穿隧與庫倫障蔽的影響，來研究電荷轉移過程。此程序被用來研究庫倫作用力對數個位點的分子連接器系統。多個非線性電流響應，包括庫倫障蔽，負微分導電度，整流，電流滯留等，於測試計算中被識別與解釋。 除了簡單模型計算，我們亦應用此理論於真實系統，執行基於重整化微擾理論與帕里薩-帕-波卜哈米爾頓子的數值計算，且經由微擾處理第二波恩自能與ＧＷ自能來引入電子相關聯效應，進而對於使用苯環與丁二烯作為接橋之分子連接器的電流-電壓特徵進行研究與討論。Charge transport processes in molecular junctions are studied theoretically by combining closed-time path (CTP) formalism, nonequilibrium Green’s function (NEGF), and renormalized perturbation theory. The molecular-junction Hamiltonian is mapped and partitioned into a collection of independent single-impurity Anderson’s Hamiltonians, which are treated as the zeroth-order Hamiltonian and are seen as the channels or orbitals that charge tunneling through or occupying, and the interactions between the channels are treated as perturbation. The quasi-particle concept is introduced to describe the kinetic processes. The electron density distribution of each orbital out of equilibrium can be given by solving a quasi-particle rate equation, and then the electric current can be known. By this method, we can study the charge transport processes with both the coherent tunneling and Coulomb blockade being considered. The present formalism is applied to the study of Coulomb inter- action on charge transport processes of few sites systems. Various nonlinear current responses, included Coulomb blockade gap, negative differential resistance (NDR), rectification, and current hysteresis, are ascribed and explained in test calculations. Except the model calculation, we have also use the present theory to a real system. Numerical calculations based on renormalized perturbation theory and Pariser-Parr-Pople (PPP) Hamiltonian are performed, and the effects of electron correlations are included by perturbative treatments of second-Born self-energies and GW self-energies. The current-voltage characteristics of the molecular junctions using benzene and butadiene as bridges are studied and discussed