Non-equilibrium transport through molecular junctions in the quantum regime

TL;DR

This paper investigates non-equilibrium electron transport in a quantum dot with vibrational modes, using a variational approach to analyze spectral functions and conductance across different regimes, revealing effects like polaron formation and negative differential conductance.

Contribution

It introduces a variational Lang-Firsov method to study electron-phonon interactions in quantum dots under non-equilibrium conditions, providing new insights into spectral and transport properties.

Findings

Polaron effects significantly alter inelastic tunneling signatures.

Polaron levels can follow lead chemical potential at high voltages.

Negative differential conductance may arise due to polaron formation.

Abstract

We consider a quantum dot, affected by a local vibrational mode and contacted to macroscopic leads, in the non-equilibrium steady-state regime. We apply a variational Lang-Firsov transformation and solve the equations of motion of the Green functions in the Kadanoff-Baym formalism up to second order in the interaction coefficients. The variational determination of the transformation parameter through minimization of the thermodynamic potential allows us to calculate the electron/polaron spectral function and conductance for adiabatic to anti-adiabatic phonon frequencies and weak to strong electron-phonon couplings. We investigate the qualitative impact of the quasi-particle renormalization on the inelastic electron tunneling spectroscopy signatures and discuss the possibility of a polaron induced negative differential conductance. In the high-voltage regime we find that the polaron level follows the lead chemical potential to enhance resonant transport.