Ultrafast spintronics with geometric effects in nonadiabatic wave-packet dynamics

Band-geometric effects are of broad interest for transport phenomena in quantum materials. The wave-packet transport theory is a well-established framework that intuitively captures these effects, particularly well in the adiabatic regime. Motivated by intriguing possibilities of steering ultrafast electronic processes via band-geometric effects, we aim to extend this theory to the nonadiabatic and transient regime. This extension enables us to investigate macroscopic ways of manifesting microscopic band-geometric effects. Crucially, it highlights the special band-geometric manifestations arising from nonadiabatic driving not available to adiabatic driving. We point out three such manifestations in ultrafast processes. The first is the imprinting of band-geometric properties on the current rate after switching off the laser pulses. The second is the induction of intrinsic macroscopic spin polarization with an orientation not accessible by adiabatic processes. The third relates the microscopic, geometrically rooted intrinsic spin coherence to the spin-mediated parts of the macroscopic photocurrents. Through explicit calculations of an example with Rashba spin-orbit coupling, the spin-mediated part is shown to be discernible from the nonspin-mediated part by the anisotropy of the photocurrents. While our primary goal is to extend a previously established theoretical framework, we also present our own experimental data on SnSe, a material exhibiting apparent anisotropic spin-orbit coupling. We distill the key theoretical principles beyond the Rashba system and find consistent agreement with experiment, supporting the physical distinction between spin-mediated and nonspin-mediated components of the photocurrent.