Sharp negative differential resistance from vibrational mode softening in molecular junctions

TL;DR

This paper demonstrates that vibrational mode softening due to quadratic electron-vibration coupling can cause sharp negative differential resistance in molecular junctions, supported by theoretical analysis and experimental agreement.

Contribution

It introduces a minimal model showing how mode softening leads to NDR, highlighting the importance of quadratic electron-vibration coupling in molecular electronics.

Findings

Vibrational mode softening causes NDR at high bias.

Negative quadratic coupling coefficient induces sharp NDR.

Theoretical results match experimental measurements.

Abstract

We unravel the critical role of vibrational mode softening in single-molecule electronic devices at high bias. Our theoretical analysis is carried out with a minimal model for molecular junctions, with mode softening arising due to quadratic electron-vibration couplings, and by developing a mean-field approach. We discover that the negative sign of the quadratic electron-vibration coupling coefficient can realize at high voltage a sharp negative differential resistance (NDR) effect with a large peak-to-valley ratio. Calculated current-voltage characteristics, obtained based on ab initio parameters for a nitro-substituted oligo(phenylene ethynylene) junction, agree very well with measurements. Our results establish that vibrational mode softening is a crucial effect at high voltage, underlying NDR, a substantial diode effect, and the breakdown of current-carrying molecular junctions.