Effects of charge-dependent vibrational frequencies and anharmonicities in transport through molecules

TL;DR

This paper explores how charge-dependent vibrational frequencies and anharmonic potentials influence electronic transport in single-molecule devices, revealing complex vibrational features and negative differential conductance under certain conditions.

Contribution

It introduces a more realistic model of molecular vibrations by including charge dependence and anharmonicity, analyzing their effects on transport properties.

Findings

Vibrational steps split into multiple substeps under weak phonon relaxation.

Bias-dependent broadening of vibrational features in current-voltage characteristics.

Frequency differences can induce negative differential conductance in asymmetric molecules.

Abstract

As a step towards a more realistic modeling of vibrations in single-molecule devices, we investigate the effects of charge-dependent vibrational frequencies and anharmonic potentials on electronic transport. For weak phonon relaxation, we find that in both cases vibrational steps split into a multitude of substeps. This effectively leads to a bias-dependent broadening of vibrational features in current-voltage and conductance characteristics, which provides a fingerprint of nonequilibrium vibrations whenever other broadening mechanisms are secondary. In the case of an asymmetric molecule-lead coupling, we observe that frequency differences can also cause negative differential conductance.