Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, USA.
J Chem Phys. 2013 May 7;138(17):174111. doi: 10.1063/1.4802587.
The transport properties of a conduction junction model characterized by two mutually coupled channels that strongly differ in their couplings to the leads are investigated. Models of this type describe molecular redox junctions (where a level that is weakly coupled to the leads controls the molecular charge, while a strongly coupled one dominates the molecular conduction), and electron counting devices in which the current in a point contact is sensitive to the charging state of a nearby quantum dot. Here we consider the case where transport in the strongly coupled channel has to be described quantum mechanically (covering the full range between sequential tunneling and co-tunneling), while conduction through the weakly coupled channel is a sequential process that could by itself be described by a simple master equation. We compare the result of a full quantum calculation based on the pseudoparticle non-equilibrium Green function method to that obtained from an approximate mixed quantum-classical calculation, where correlations between the channels are taken into account through either the averaged rates or the averaged energy. We find, for the steady state current, that the approximation based on the averaged rates works well in most of the voltage regime, with marked deviations from the full quantum results only at the threshold for charging the weekly coupled level. These deviations are important for accurate description of the negative differential conduction behavior that often characterizes redox molecular junctions in the neighborhood of this threshold.
研究了由两个相互耦合的通道组成的传导结模型的输运性质，这两个通道在与引线的耦合方面有很大的不同。这种类型的模型描述了分子氧化还原结（其中一个与引线弱耦合的能级控制分子电荷，而一个强耦合的能级主导分子传导），以及电子计数器件，其中点接触的电流对附近量子点的充电状态敏感。在这里，我们考虑了在强耦合通道中传输必须用量子力学来描述的情况（覆盖了从顺序隧穿到共隧穿的整个范围），而通过弱耦合通道的传导是一个顺序过程，本身可以用简单的主方程来描述。我们将基于赝粒子非平衡格林函数方法的全量子计算结果与通过近似混合量子-经典计算得到的结果进行比较，其中通过平均速率或平均能量来考虑通道之间的相关性。对于稳态电流，我们发现基于平均速率的近似在大部分电压范围内都能很好地工作，只有在微弱耦合能级充电的阈值处才会与全量子结果有明显的偏差。这些偏差对于准确描述氧化还原分子结在这个阈值附近的负微分传导行为非常重要。
