# Quantum Brownian Motion of a Magnetic Skyrmion

## TL;DR

This paper develops a quantum microscopic theory for skyrmion motion in magnetic insulators, revealing finite zero-temperature damping, non-trivial fluctuation-dissipation relations, and effects of external drives on skyrmion dynamics.

## Contribution

It introduces a quantum framework for skyrmion Brownian motion, incorporating bath coupling, external drives, and generalized fluctuation-dissipation relations, advancing understanding of quantum skyrmion behavior.

## Key findings

- Finite damping persists at zero temperature.
- Quadratic thermal activation of mean square displacement.
- External drives generate additional dissipation and renormalization effects.

## Abstract

Within a microscopic theory, we study the quantum Brownian motion of a skyrmion in a magnetic insulator coupled to a bath of magnon-like quantum excitations. The intrinsic skyrmion-bath coupling gives rise to damping terms for the skyrmion center-of-mass, which remain finite down to zero temperature due to the quantum nature of the magnon bath. We show that the quantum version of the fluctuation-dissipation theorem acquires a non-trivial temperature dependence. As a consequence, the skyrmion mean square displacement is finite at zero temperature and has a fast thermal activation that scales quadratically with temperature, contrary to the linear increase predicted by the classical phenomenological theory. The effects of an external oscillating drive which couples directly on the magnon bath are investigated. We generalize the standard quantum theory of dissipation and we show explicitly that additional time-dependent dissipation terms are generated by the external drive. From these we emphasize a friction and a topological charge renormalization term, which are absent in the static limit. The skyrmion response function inherits the time periodicity of the driving field and it is thus enhanced and lowered over a driving cycle. Finally, we provide a generalized version of the nonequilibrium fluctuation-dissipation theorem valid for weakly driven baths.