Voltage-Tunable Nonequilibrium Dispersion Interactions

TL;DR

This paper develops a nonequilibrium Green's function theory to analyze how applied bias voltage influences dispersion interactions between nanostructures, revealing voltage-induced enhancements and possible repulsions.

Contribution

It introduces a general framework for nonequilibrium dispersion interactions, including a fluctuation-dissipation interpretation and analysis of voltage effects on interaction sign and magnitude.

Findings

Voltage can nearly tenfold enhance attraction between nanostructures.

Nonequilibrium conditions can induce repulsive dispersion interactions.

Population inversion can reverse the sign of the dispersion interaction.

Abstract

We develop a nonequilibrium Green's function theory for dispersion interactions between two nanostructures, each an open quantum system driven into a nonequilibrium steady state by an applied bias voltage. Starting from the two-particle nonequilibrium Green's function, we derive a general expression for the interaction energy in terms of the polarisation propagators of the individual systems. The interaction energy admits a physically transparent decomposition into charge noise and charge dissipation contributions, providing a fluctuation-dissipation interpretation that generalises the equilibrium London picture. Model calculations for coupled molecular junctions demonstrate that the applied voltage can enhance the attractive dispersion interaction by nearly an order of magnitude relative to equilibrium. In thermal equilibrium, the dispersion interaction is universally attractive, irrespective of the specific form of the nanostructure Hamiltonians or their coupling to reservoirs. Out of equilibrium, we introduce a generalised Kubo-Martin-Schwinger ratio that parametrises the departure from detailed balance. We show that, in contrast to equilibrium, nonequilibrium conditions can lead to a repulsive dispersion interaction. Finally, we discuss the conditions under which population inversion in the electronic leads can drive a sign reversal of the dispersion interaction.