Quantum topological phase transitions in skyrmion crystals

TL;DR

This paper demonstrates the existence of topologically nontrivial magnon bands in quantum skyrmion crystals at zero magnetic field, revealing quantum topological phase transitions driven by anisotropy tuning.

Contribution

It uncovers quantum topological phase transitions in skyrmion crystals and shows topological magnons can exist without external magnetic fields, extending topological bosonic systems.

Findings

Nonzero Chern numbers in magnon bands indicating nontrivial topology

Observation of eight quantum topological phase transitions

Connection of phase transitions to band gap closures and reopenings

Abstract

Topological order is important in many aspects of condensed matter physics, and has been extended to bosonic systems. In this Letter we report on the nontrivial topology of the magnon bands in two distinct quantum skyrmion crystals appearing in zero external magnetic field. This is revealed by nonzero Chern numbers for some of the bands. As a bosonic analog of the quantum anomalous Hall effect, we show that topological magnons can appear in skyrmion crystals without explicitly breaking time-reversal symmetry with an external magnetic field. By tuning the value of the easy-axis anisotropy at zero temperature, we find eight quantum topological phase transitions signaled by discontinuous jumps in certain Chern numbers. We connect these quantum topological phase transitions to gaps closing and reopening between magnon bands.