Can the Seebeck Coefficient Identify Quantum Interference in Molecular Conduction?

Abstract

We look for manifestations of quantum interference effects in the Seebeck coefficient of a molecular junction, when the electronic conductance exhibits pronounced destructive interference features due to the presence of quasi-degenerate electronic states, which differ in their spatial symmetry. We perform our analysis by considering three separate limits for electron transport: coherent, fully dephased, and suffering inelastic scattering with molecular vibrations. We find that in the considered geometry, although the conductance displays strong signatures of the underlying transport mechanisms—destructive quantum interference features in the coherent case and thermal activation characteristics in the inelastic limit—the Seebeck coefficient conceals details of electron dynamics, whereas it robustly reveals information about the energy characteristics of the junction. It should be emphasized that in other geometries (e.g., when a transmission node develops close to the Fermi energy), the Seebeck coefficient may show different features, potentially exposing the underlying dynamics. We provide closed-form expressions for the electronic conductance and the thermopower of our system as a function of temperature, gate voltage, and hybridization energy in different transport limits, and we then exemplify our analysis on a specific conjugated molecule with quasi-degenerate orbitals of different spatial symmetry.