Quantum Transport With Two Interacting Conduction Channels

TL;DR

This paper investigates quantum transport in a model with two interacting conduction channels, comparing full quantum calculations to approximate methods, and highlights the importance of correlations for accurate current predictions near threshold voltages.

Contribution

It introduces a detailed analysis of a two-channel transport model, comparing full quantum and mixed quantum-classical approaches, emphasizing the significance of correlations in molecular junctions.

Findings

Averaged rate approximation performs well across most voltages.

Significant deviations occur at the charging threshold of the weakly coupled level.

Correlations are crucial for accurately modeling negative differential conductance.

Abstract

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.