Charge-carrier-induced frequency renormalization, damping and heating of vibrational modes in nanoscale junctions

TL;DR

This paper develops a nonequilibrium Green's functions theory to analyze how charge carriers affect vibrational modes in nanoscale junctions, revealing bias-dependent frequency shifts, damping, and heating effects.

Contribution

It introduces a new theoretical framework combining Green's functions and ab-initio calculations to explain vibrational effects in nanoscale electronic junctions.

Findings

Bias-dependent vibrational frequency shifts and damping observed.

Pronounced nonlinear vibrational heating at intermediate coupling.

Explanation of experimental Raman shifts and linewidths in OPV3 junctions.

Abstract

In nanoscale junctions the interaction between charge carriers and the local vibrations results in renormalization, damping and heating of the vibrational modes. We here formulate a nonequilibrium Green's functions based theory to describe such effects. Studying a generic junction model with an off-resonant electronic level, we find a strong bias dependence of the frequency renormalization and vibrational damping accompanied by pronounced nonlinear vibrational heating in junctions with intermediate values of the coupling to the leads. Combining our theory with ab-initio calculations we furthermore show that the bias dependence of the Raman shifts and linewidths observed experimentally in an OPV3 junction [D. Ward et al., Nature Nano. 6, 33 (2011)] may be explained by a combination of dynamic carrier screening and molecular charging.