University of Basel
Nonequilibrium quantum dynamics of skyrmions in magnetic insulators
We study the role of magnon excitations in the quantum propagation of a skyrmion in chiral magnetic insulators by generalizing the micromagnetic equations of motion. The magnons around the skyrmion give rise to a damping derived microscopically, which in some limit reduces to a skyrmion mass. We demonstrate that a skyrmion in a confined geometry behaves as a massive particle, a discovery with great impact on the technologically important case of linear tracks relevant for magnetic memory devices. An additional quantum mass term is predicted with an explicit temperature dependence which remains finite even at zero temperature. In the presence of time-dependent oscillating magnetic field gradients, the unavoidable coupling of the external field to the magnons gives rise to time-dependent dissipation for the skyrmion, with measurable consequences on the skyrmion’s path. These ac fields act as a net driving force on the skyrmion via its own intrinsic magnetic excitations. Finally we generalize the standard quantum theory of dissipation to include the effects of the driven bath on the skyrmion’s dynamics and we show explicitly that a new type of time-dependent dissipation is generated with a clear signature on the skyrmion's response function. We address the stochastic effects of the quantum driven bath on the skyrmion's propagation and provide a generalized version of the nonequilibrium fluctuation-dissipation relation for externally driven reservoirs.