Photocurrent streamline microscopy in micromagnetic heterostructure devices

Photoexcited electronic charge carriers in quantum materials are often collected at global contacts far away from the initial excitation, a process that is highly non-local. As described by the Shockley-Ramo theorem, this process involves an intricate spatial pattern of streamlines throughout the given device that depends sensitively on the configuration of current collecting contacts as well as the spatial non-uniformity and tensor structure of conductivity. Here we demonstrate a first-of-its-kind microscopy method to image such streamlines through ultrathin heterostructure devices composed of platinum on yttrium iron garnet (YIG). We do this by combining scanning photovoltage microscopy with a uniform rotating magnetic field to control the direction, as well as spatial position of, local photocurrent, which has enabled us to image these streamlines in a variety of geometries that include conventional Hall bar-type devices as well as unconventional devices with wing-shaped cutouts called electrofoils. In these, we show that the streamlines display geometry-specific contortion, compression, and expansion behavior near the cutouts in much the same way as tracers in a wind tunnel map the flow of air around aerodynamic airfoils. This affords a powerful tool to visualize and characterize charge flow in optoelectronic devices.