Quantum-classical description of spin and charge pumping by dynamical noncolliner magnetic textures and the ensuing electromagnetic radiation in THz spintronics*

The interaction of femtosecond (fs) light pulses with magnetic materials has been intensely studied for more than two decades to understand ultrafast demagnetization in single magnetic layer or THz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. Despite long history, microscopic understanding of ultrafast-light-driven magnets is incomplete due to numerous competing effects and with virtually no study reporting calculation of output THz radiation. This talk presents a recently developed and versatile multiscale quantum-classical formalism where conduction electrons are described by quantum master equation [1] of the Lindblad type or by time-dependent nonequilibrium Green's functions [2-8]; 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 the Jefimenko equations for retarded electric and magnetic fields [1]. We illustrate it by application to ultrafast-light-driven bilayer of Weyl antiferromagnet Mn₃Sn with noncollinear local magnetization and SO coupling intrinsically present within Mn3Sn layer [1] which pumps charge current emitting THz radiation; as well as on magnetic-field driven annihilation of two topological solitons [8], realized as magnetic domain walls within a metallic ferromagnetic nanowire, which leads to spin wave burst observed experimentally [9], as well as our prediction [8] of additional spin pumping over ultrabroadband 0.03-27 THz frequency range and potentially electromagnetic radiation in the same range upon spin-to-charge conversion.