Thermopower of Single-Molecule Devices

TL;DR

This paper studies the thermopower in single-molecule devices, revealing how it can probe electronic and vibrational excitations with minimal disturbance, especially under weak coupling conditions.

Contribution

It introduces a rate-equation approach including cotunneling to analyze thermopower, highlighting its effectiveness in spectroscopic measurements of molecular excitations.

Findings

Thermopower reveals electronic and phononic spectra.

Weak lead-molecule coupling enhances phonon features.

Thermopower measurement is less invasive than current-voltage methods.

Abstract

We investigate the thermopower of single molecules weakly coupled to metallic leads. We model the molecule in terms of the relevant electronic orbitals coupled to phonons corresponding to both internal vibrations and to oscillations of the molecule as a whole. The thermopower is computed by means of rate equations including both sequential-tunneling and cotunneling processes. Under certain conditions, the thermopower allows one to access the electronic and phononic excitation spectrum of the molecule in a linear-response measurement. In particular, we find that the phonon features are more pronounced for weak lead-molecule coupling. This way of measuring the excitation spectrum is less invasive than the more conventional current-voltage characteristic, which, by contrast, probes the system far from equilibrium.