As a basic functional unit of molecular electronics, the structure of single molecule sandwiched between nano-electrodes has attracted a lot of interest in molecular science, in particular, its current-voltage (I-V) characteristic induced by an external field. Aiming at the molecular nano-junction which is composed of lead/molecule/lead, we use the method of extended master equation to compute the steady and transient current in the molecular nano-junction under the action of an externally applied electric field. The current can be adjusted by the external field,the relaxation in the molecule, the intra-molecular vibrational energy redistribution, etc. Owing to the strong electronic-vibrational coupling, the I-V curve has an inelastic characteristic in the molecular nano-junction and the stable current increases stepwise with the applied bias voltage increasing. The Franck-Condon blockage can be effectively removed by the external field. The molecular nano-junction being excited by different-width Gaussian pulses,the currents in the molecular nano-junction take different times to reach their steady state. The pulse width has a strong effect on the transient current enhancement. The transient current appears obviously for the 1 ps width pulse excitation. In this case the molecule is at a non-equilibrium state and the currents at both ends of the molecule are different. With the pulse width and the applied voltage increasing,the current through the molecular nano-junction tends to be balanced.%针对由金属电极/分子/金属电极组成的分子纳米结,应用扩展主方程的方法,考虑分子纳米结中影响其传输过程的外场、分子内的弛豫过程等因素研究了在外场作用下分子纳米结内的稳定电流和瞬间电流.由于分子内较强的电子-振动耦合,分子纳米结中的电流-电压曲线呈现台阶式非弹性特征.在不同的高斯型脉冲的激发作用下,分子纳米结中电流需要达到稳定的时间也不相同,脉冲宽度在1 ps 时瞬间电流现象明显,这时分子处于非平衡分布,分子两端的电流存在较大差异.随着脉冲宽度和外场偏压的增加,分子两端的电流趋于平衡.
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