Diffusive non-reciprocity and thermal diode

TL;DR

This paper develops a theoretical framework to understand global reciprocity in diffusive heat processes, distinguishing it from local non-reciprocity and diode effects, and establishes conditions under which a thermal diode operates.

Contribution

It introduces the concept of global reciprocity for diffusive heat transfer and proves its equivalence to thermal diode behavior under certain conditions.

Findings

Defined global reciprocity for diffusive heat processes.

Proved equivalence between broken global reciprocity and thermal diode operation.

Discussed mechanisms for breaking diffusive reciprocity.

Abstract

Wave propagation and diffusion in linear materials preserve local reciprocity in terms of a symmetric Green's function. For wave propagations, the relation between the fields entering and leaving a system is more relevant than the detailed information about the fields inside it. In such cases, the global reciprocity of the scattering off a system through several ports is more important, which is defined as the symmetric transmission between the scattering channels. When a two-port system supports non-reciprocal (electromagnetic, acoustic) wave propagation, it is a (optical, phonon) diode directly following the definition. However, to date no concrete definition or discussion has been made on the global reciprocity of diffusive processes through a multiple-port system. It thus remains unclear what are the differences and relations between the three concepts, namely local non-reciprocity, global non-reciprocity, and diode effect in diffusion. Here, we provide theoretical analysis on the frequency-domain Green's function and define the global reciprocity of heat diffusion through a two-port system, which has a different setup from that of a thermal diode. We further prove the equivalence between a heat transfer system with broken steady-state global reciprocity and a thermal diode, assuming no temperature-dependent heat generation. The validities of some typical mechanisms in breaking the diffusive reciprocity and making a thermal diode have been discussed. Our results set a general background for future studies on symmetric and asymmetric diffusive processes.