The role of gauge symmetry in spintronics

TL;DR

This paper uses a field theoretical approach to explain the non-conservation of spin current in spintronics, linking it to gauge invariance and symmetries of electromagnetism, and extends previous work by including dynamical electromagnetic fields.

Contribution

It formalizes the non-conservation of spin current in terms of gauge theory and Noether currents, providing a new interpretation in the context of spintronics and gauge invariance.

Findings

Non-conservation of spin current is linked to gauge invariance.

Broken continuity equation involves spin-transfer torque.

Including dynamical electromagnetic fields extends previous models.

Abstract

In this work we employ a field theoretical approach to explain the nature of the non-conserved spin current in spintronics. In particular, we consider the usual U(1) gauge theory for the electromagnetism at classical level in order to obtain the broken continuity equation involving the spin current and spin-transfer torque. Inspired in the recent work of A. Vernes, B. L. Gyorffy and P. Weinberger where they obtain such equation in terms of relativistic quantum mechanics, we formalize their result in terms of the well known currents of field theory such as the Bargmann-Wigner current and the chiral current. Thus, an interpretation of spintronics is provided in terms of Noether currents (conserved or not) and symmetries of the electromagnetism. In fact, the main result of the present work is that the non-conservation of the spin current is associated to the gauge invariance of physical observables where the breaking term is proportional to the chiral current. Moreover, we generalize their result by including the electromagnetic field as a dynamical field instead of an external one.