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

In nanoscale junctions the interaction between charge carriers and the local vibrations results in renormalization, damping, and heating of the vibrational modes. Here we formulate a nonequilibrium Green's function 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 oligo(3)-phenylenevinylene (OPV3) junction may be explained by a combination of dynamic carrier screening and molecular charging.