Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics with example of ultrafast-light-driven Weyl antiferromagnet Mn$_3$Sn

Abstract

The interaction of fs laser pulse with magnetic materials has been intensely studied for more than two decades in order to understand ultrafast demagnetization in single magnetic layers or THz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. However, in contrast to wellunderstood spin and charge pumping by dynamical magnetization in spintronic systems driven by microwaves or current injection, analogous processes in light-driven magnets and radiation emitted by them remain largely unexplained due to multiscale nature of the problem. Here we develop a multiscale quantum-classical formalism-where conduction electrons are described by quantum master equation of the Lindblad type; classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation; and incoming light is described by classical vector potential while outgoing electromagnetic radiation is computed using Jefimenko equations for retarded electric and magnetic fields-and apply it to a bilayer of antiferromagnetic Weyl semimetal Mn3Sn, hosting noncollinear local magnetization, and SO-coupled nonmagnetic material. Our QME+LLG+Jefimenko scheme makes it possible to understand how fs laser pulse generates directly spin and charge pumping and electromagnetic radiation by the latter, including both odd and even high harmonics (of the pulse center frequency) up to order n ≤ 7. The directly pumped spin current then exert spin torque on local magnetization whose dynamics, in turn, pumps additional spin and charge currents radiating in the THz range. By switching on and off LLG dynamics and SO couplings, we unravel which microscopic mechanism contribute the most to emitted THz radiation-charge pumping by local magnetization of Mn3Sn in the presence of its own SO coupling is far more important than standardly assumed (for other types of magnetic layers) spin pumping and subsequent spin-to-charge conversion within the neighboring nonmagnetic SO-coupled material.