Local Symmetry Breaking Drives Picosecond Spin Domain Formation in Polycrystalline Semiconducting Films

Photoinduced spin-charge interconversion in semiconductors with spin-orbit coupling could provide a route to optically addressable spintronics without the use of external magnetic fields. A central question is whether the resulting spin-associated charge currents are robust to structural disorder, which is inherent to polycrystalline semiconductors that are desirable for device applications. Using ultrafast circular polarization-resolved pump-probe microscopy with 15-fs time resolution on polycrystalline halide perovskite thin films, we observe the photoinduced ultrafast formation of spin-polarized positive and, unexpectedly, negative spin domains on the micron scale. By photoinducing a local spin polarized electronic population and imaging its transport on femtosecond timescales, we confirm that there are lateral local charge currents present. Further, the polarization of these photoinduced spin domains and lateral transport direction is switched upon switching the polarization of the pump helicity, confirming that the local spin currents are a result of local spin textures and spin-momentum locking. Micron scale variations in the intensity of optical second-harmonic generation and vertical piezoresponse suggest that the spatially varying spin textures that drive the spin domain formation arise through the presence of strong local inversion symmetry breaking via inter-grain structural disorder. We explore this hypothesis with a simple Monte Carlo model for spin transport and scattering in a spin texture disordered landscape and are able to capture the measured phenomenology. Taken together, we propose that this local symmetry breaking on the micron scale leads to spatially varying Rashba-like spin textures that drive spin-momentum locked currents, leading to local spin domain formation on picosecond timescales. Our results establish ultrafast spin domain formation in polycrystalline semiconductors as a new platform for optically addressable nanoscale spin-device physics.