Time-dependent spin-wave theory

TL;DR

This paper extends spin-wave theory to time-dependent quantum spin models, enabling analysis of rotating magnets and those in dynamic fields, with applications to magnetic state reorganization and experimental predictions.

Contribution

It introduces a generalized spin-wave expansion for time-dependent models, incorporating rotating reference frames and bosonization techniques.

Findings

Predicted a measurable dip in magnetization due to magnon Bose-Einstein condensation.

Applied the theory to yttrium-iron garnet, demonstrating its practical relevance.

Abstract

We generalize the spin-wave expansion in powers of the inverse spin to time-dependent quantum spin models describing rotating magnets or magnets in time-dependent external fields. We show that in these cases, the spin operators should be projected onto properly defined rotating reference frames before the spin components are bosonized using the Holstein-Primakoff transformation. As a first application of our approach, we calculate the reorganization of the magnetic state due to Bose-Einstein condensation of magnons in the magnetic insulator yttrium-iron garnet; we predict a characteristic dip in the magnetization which should be measurable in experiments.