Vibrational Instabilities in Charge Transport through Molecular Nanojunctions: The Role of Anharmonic Nuclear Potentials

TL;DR

This paper investigates how anharmonic nuclear potentials influence vibrational instabilities and dissociation in molecular nanojunctions caused by nonconservative current-induced forces, extending previous harmonic models.

Contribution

It introduces a mixed quantum-classical approach to study anharmonic effects on vibrational instabilities in molecular junctions with multiple modes.

Findings

Anharmonicity affects vibrational instability thresholds.

Nonconservative forces can induce junction dissociation.

Steady-state current is influenced by anharmonic vibrational dynamics.

Abstract

The current-induced vibrational dynamics is a key factor determining the stability of molecular nanojunctions. Beyond conventional Joule heating, a different mechanism caused by nonconservative current-induced forces has been predicted for models with multiple vibrational modes, leading to vibrational instabilities already at low bias voltages. So far, this mechanism has only been investigated in models with harmonic nuclear potentials. Consequently, a natural question is whether this effect can also be observed in more realistic models containing anharmonic nuclear potentials, and, if so, whether it has a measurable impact on observables such as the junction dissociation probability. In this work, we apply a mixed quantum-classical approach based on electronic friction and Langevin dynamics to various anharmonic two-mode systems. By performing Langevin simulations of the vibrational dynamics, we investigate the influence of anharmonicity on instabilities arising from nonconservative forces and the corresponding dissociation dynamics of the junction, as well as steady-state observables, such as the electronic current.