Old wine in new bottles

TL;DR

This paper discusses the historical and modern significance of Onsager's reciprocity relations, highlighting their revival in spin caloritronics and their role in evaluating electronic devices.

Contribution

It reviews the enduring relevance of Onsager's relations and their recent application in the emerging field of spin caloritronics.

Findings

Onsager's relations are crucial for understanding local equilibrium.

They are instrumental in assessing the utility of spin caloritronic devices.

The relations have been revitalized in modern research fields.

Abstract

Onsager's reciprocity relations for the coefficients of transport equations are now 87 years old. Sometimes these relations are called the Fourth Law of Thermodynamics. Among others they provide an effective criterion for the existence of local equilibrium and of microscopic reversibility. Since the beginning of the century Onsager's relations have seen a revival in the field of spincaloritronics. There the relations are very helpful in judging the utility of modern devices for electronic data processing.