Skyrmion quantum numbers and quantized pumping in two dimensional topological chiral magnets

TL;DR

This paper explores how inhomogeneous spin structures in 2D topological insulators can produce quantized charge and spin pumping, with skyrmions acting as fermionic particles with fractional quantum numbers in certain topological phases.

Contribution

It introduces a framework linking topological invariants in momentum and real space to quantized pumping and skyrmion quantum numbers in chiral magnetic insulators.

Findings

Quantized charge and spin pumping are governed by topological invariants.

Skyrmions can carry fractional electric charge and spin quantum numbers.

A new topological phase hosts skyrmions as spin-1/2 fermions with charge e.

Abstract

We investigate the general conditions to achieve the adiabatic charge and spin polarizations and quantized pumping in 2D magnetic insulators possessing inhomogeneous spin structures. In particular, we focus on the chiral ferrimagnetic insulators which are generated via spontaneous symmetry breaking from correlated two dimensional topological insulators. Adiabatic deformation of the inhomogeneous spin structure generates the spin gauge flux, which induces adiabatic charge and spin polarization currents. The unit pumped charge/spin are determined by the product of two topological invariants which are defined in momentum and real spaces, respectively. The same topological invariants determine the charge and spin quantum numbers of skyrmion textures. It is found that in noncentrosymmetric systems, a new topological phase, dubbed the topological chiral magnetic insulator, exists in which a skyrmion defect is a spin-1/2 fermion with electric charge $e$. Considering the adiabatic current responses of generic inhomogeneous systems, it is shown that the quantized topological response of chiral magnetic insulators is endowed with the second Chern number.