Experimental verification of reciprocity relations in quantum thermoelectric transport

TL;DR

This study experimentally verifies thermoelectric reciprocity relations in a quantum device, demonstrating their symmetry under certain conditions and their breakdown at higher thermal biases, which could impact thermoelectric efficiency.

Contribution

First experimental verification of thermoelectric reciprocity relations in a quantum mesoscopic device, showing their symmetry and controllable breakdown at finite thermal biases.

Findings

Reciprocity relations are symmetric under magnetic field reversal and contact exchange.

Breakdown of reciprocity relations occurs with increasing thermal bias.

Results suggest potential for enhancing thermoelectric performance by controlling reciprocity.

Abstract

Symmetry relations are manifestations of fundamental principles and constitute cornerstones of modern physics. An example are the Onsager relations between coefficients connecting thermodynamic fluxes and forces, central to transport theory and experiments. Initially formulated for classical systems, these reciprocity relations are also fulfilled in quantum conductors. Surprisingly, novel relations have been predicted specifically for thermoelectric transport. However, whereas these thermoelectric reciprocity relations have to date not been verified, they have been predicted to be sensitive to inelastic scattering, always present at finite temperature. The question whether the relations exist in practice is important for thermoelectricity: whereas their existence may simplify the theory of complex thermoelectric materials, their absence has been shown to enable, in principle, higher thermoelectric energy conversion efficiency for a given material quality. Here we experimentally verify the thermoelectric reciprocity relations in a four-terminal mesoscopic device where each terminal can be electrically and thermally biased, individually. The linear response thermoelectric coefficients are found to be symmetric under simultaneous reversal of magnetic field and exchange of injection and emission contacts. Intriguingly, we also observe the breakdown of the reciprocity relations as a function of increasing thermal bias. Our measurements thus clearly establish the existence of the thermoelectric reciprocity relations, as well as the possibility to control their breakdown with the potential to enhance thermoelectric performance