Semiclassical dynamics of electrons in space-time crystal: Magnetization, polarization, and current response

TL;DR

This paper develops a semiclassical framework for analyzing electron dynamics in space-time crystals, revealing magnetization, polarization, and current responses, and linking these to Berry curvature and energy flow in non-equilibrium conditions.

Contribution

It introduces a semiclassical theory for Floquet-Bloch electrons in space-time crystals, including wave-packet dynamics, magnetization, polarization, and current response calculations.

Findings

Magnetization includes Berry curvature contributions.

Current response relates to the non-equilibrium energy flow.

The framework applies to systems with coupled space and time periodicity.

Abstract

A space-time crystal is defined as a quantum mechanical system with both spatial and temporal periodicity. Such a system can be described by the Floquet-Bloch (FB) theory. We first formulate a semiclassical theory by constructing a wave-packet through the superposition of the FB wave functions and derive the equations of motion of FB electrons subjected to slowly varying external fields (not to be confused with the fast-changing Floquet drive), revealing behaviors similar to ordinary Bloch electrons but with quantities modified in the Floquet context. Specifically, we study local magnetic moment due to the self-rotation of the wave-packet, a contribution to total magnetization from the Berry curvature in k-space, and the polarization of a fully occupied FB band. Based on the semiclassical theory, we can also show the fingerprint of the energy flow in such an energy-non-conserved system. We then discuss the density matrix of a FB system attached to a thermal bath, which allows us to investigate quantities involving many electrons in the non-interacting limit. As an application, we calculate the intrinsic current response in an oblique spacetime metal showing the non-equilibrium nature of the FB system. The current response can also be related to the acoustoelectric effect. Overall, we develop a systematic approach for studying space-time crystals and provide a powerful tool to explore the electronic properties of this exotic system with coupled space and time.