On A Proper Definition of Spin Current

TL;DR

This paper proposes a new, physically consistent definition of spin current that aligns with transport theory and vanishes in insulators, addressing limitations of the conventional approach.

Contribution

It introduces a proper, measurable spin current definition that overcomes the unphysical aspects of the conventional one in systems with spin-orbit coupling.

Findings

The new spin current definition is consistent with near-equilibrium transport theory.

It correctly vanishes in insulators with localized orbitals.

The theory has clear experimental implications.

Abstract

The conventional definition of spin current is incomplete and unphysical in describing spin transport in systems with spin-orbit coupling. A proper and measurable spin current is established in this study, which fits well into the standard framework of near-equilibrium transport theory and has the desirable property to vanish in insulators with localized orbitals. Experimental implications of our theory are discussed.