Structure phase change induced by nonequilibrium effects in molecular-scale junctions

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 Letter, 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 forces acting on the atoms induced by nonequilibrium effects, 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 and 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 $I\text{\ensuremath{-}}V$ characteristics and vibrational spectra. These findings greatly broaden our understanding of quantum electronic devices and pave the way for discovering unconventional transport phenomena at the molecular scale.