Theory of charge and spin pumping in atomic-scale spiral magnets

TL;DR

This paper investigates charge and spin pumping in atomic-scale spiral magnets using quantum models, revealing linear and quadratic dependencies on rotation frequency, Berry phase relations, and effects of disorder on transport properties.

Contribution

It introduces a microscopic quantum framework for spin and charge pumping in spiral magnets, connecting Berry phase concepts to quantized charge transport and analyzing non-adiabatic and disorder effects.

Findings

Charge current is linear in frequency and related to Berry phase.

Spin current scales quadratically with frequency.

Disorder can enhance spin current but suppress charge current.

Abstract

An Archimedean screw is a classical pump that exploits the equivalence of rotation and translation in helices. Similarly, a spin spiral texture can pump charge and spin by rotating at a frequency $\omega$. In the present paper, we study these pumping phenomena within a microscopic quantum model by both perturbation theory and numerical simulations. Inside the spiral region, the spin polarization and charge current are linear in $\omega$ whereas the spin current is $\omega^2$ for small $\omega$. We find that the charge current is related to the mixed momentum-phason Berry phase, which can be viewed as a novel approximate realization of a Thouless pump. It is nearly quantized in spirals with short pitch $\lambda$ but decays with $\lambda^{-1}$ for longer pitches, unlike true Thouless pumps or Archimedian screws. Moreover, we study the onset of non-adiabaticity (large $\omega$), the impact of attached non-magnetic or magnetic contacts, and the real-time evolution of the transport observables. Finally, we analyze the effects of disorders which, surprisingly, might enhance the spin current but suppress the charge current.