Structure Phase Change Induced by Nonequilibrium Effects in Molecular Scale Junctions

TL;DR

This paper introduces a first-principles method to study how nonequilibrium effects from external biases induce structural phase changes in molecular junctions, revealing significant impacts on electronic and vibrational properties.

Contribution

It develops a novel computational approach combining Hershfield quantum statistics and density functional theory to analyze nonequilibrium forces and structural changes in molecular devices.

Findings

Bias voltages above 1.0 V induce structural phase changes.

Nonequilibrium effects significantly alter electronic and vibrational properties.

Structural changes are closely linked to transport characteristics.

Abstract

The interrelationship between a material's structure and its properties lies at the heart of materials-related research. Finding how the changes of one affect the other is of primary importance in theoretical and computational materials studies. In this work, based on Hershfield nonequilibrium quantum statistics and the mean-field approach with steady-state density functional theory, we derive a first-principles method to calculate nonequilibrium effects induced forces acting on atoms, enabling structure optimizations and molecular dynamics simulations for molecular junctions under external biases. By applying the method to a few molecular devices, we found that in general, the external bias can induce profound nonequilibrium effects on both electronic/transport properties and the geometric structure of these devices, and consequent changes in electronic properties and geometric structure are closely interrelated. Particularly, when the bias voltage is above 1.0 V, significant structure phase changes could occur, causing dramatic changes in I-V characteristics and vibrational spectra. These findings greatly broaden our understanding of quantum electronic devices and provide a new avenue for discovering novel transport phenomena at molecular scale.