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

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.