Full counting statistics of vibrationally-assisted electronic conduction: transport and fluctuations of the thermoelectric efficiency

TL;DR

This paper analyzes charge and energy transport in electron-phonon coupled junctions, deriving a cumulant generating function to study thermoelectric efficiency and its fluctuations, revealing how vibrational mode properties influence transport statistics.

Contribution

It provides an exact second-order in electron-phonon interaction cumulant generating function for charge and energy transfer in molecular junctions, considering both harmonic and anharmonic vibrational modes.

Findings

Charge current and noise reveal junction properties at large bias.

Thermoelectric efficiency fluctuations depend on vibrational mode nature.

Macroscopic efficiency is unaffected by mode type.

Abstract

We study the statistical properties of charge and energy transport in electron conducting junctions with electron-phonon interactions, specifically, the thermoelectric efficiency and its fluctuations. The system comprises donor and acceptor electronic states, representing a two-site molecule or a double quantum dot system. Electron transfer between metals through the two molecular sites is coupled to a particular vibrational mode which is taken to be either harmonic or anharmonic- a truncated (two-state) spectrum. Considering these models we derive the cumulant generating function in steady state for charge and energy transfer, correct to second-order in the electron-phonon interaction, but exact to all orders in the metal-molecule coupling strength. This is achieved by using the non-equilibrium Green's function approach (harmonic mode) and a kinetic quantum master equation method (anharmonic mode). From the cumulant generating function we calculate the charge current and its noise and the large deviation function for the thermoelectric efficiency. We demonstrate that at large bias the charge current, differential conductance, and the current noise can identify energetic and structural properties of the junction. We further examine the operation of the junction as a thermoelectric engine and show that while the macroscopic thermoelectric efficiency is indifferent to the nature of the mode (harmonic or anharmonic), efficiency fluctuations do reflect this property.