Stochastic dynamics for a single vibrational mode in molecular junctions

TL;DR

This paper introduces a precise computational approach to model the stochastic dynamics of a classical oscillator in molecular junctions, revealing how dynamical fluctuations influence electronic transport and the validity of the adiabatic approximation.

Contribution

It develops an accurate scheme for oscillator dynamics in molecular junctions, including finite mass effects, and systematically analyzes the validity of the adiabatic approximation across different regimes.

Findings

Velocity distributions deviate from Gaussian at intermediate bias.

Dynamical effects enhance conduction away from electronic resonances.

Kinetic energy minima correlate with maxima in dynamical conductance.

Abstract

We propose a very accurate computational scheme for the dynamics of a classical oscillator coupled to a molecular junction driven by a finite bias, including the finite mass effect. We focus on two minimal models for the molecular junction: Anderson-Holstein (AH) and two-site Su-Schrieffer-Heeger (SSH) models. As concerns the oscillator dynamics, we are able to recover a Langevin equation confirming what found by other authors with different approaches and assessing that quantum effects come from the electronic subsystem only. Solving numerically the stochastic equation, we study the position and velocity distribution probabilities of the oscillator and the electronic transport properties at arbitrary values of electron-oscillator interaction, gate and bias voltages. The range of validity of the adiabatic approximation is established in a systematic way by analyzing the behaviour of the kinetic energy of the oscillator. Due to the dynamical fluctuations, at intermediate bias voltages, the velocity distributions deviate from a gaussian shape and the average kinetic energy shows a non monotonic behaviour. In this same regime of parameters, the dynamical effects favour the conduction far from electronic resonances where small currents are observed in the infinite mass approximation. These effects are enhanced in the two-site SSH model due to the presence of the intermolecular hopping t. Remarkably, for sufficiently large hopping with respect to tunneling on the molecule, small interaction strengths and at intermediate bias (non gaussian regime), we point out a correspondence between the minima of the kinetic energy and the maxima of the dynamical conductance.