Visualizing photocurrent flow in anisotropic Weyl semimetals using NV magnetometry*

Photocurrent in Weyl semimetals characterizes their symmetry and topology. Here, we image the two-dimensional photocurrent flow inside the type-II Weyl semimetals WTe₂ and TaIrTe₄ using a new technique – photocurrent flow microscopy – based on high-sensitivity, ac magnetometry with nitrogen-vacancy (NV) centers in diamond. The directional flow patterns, which are aligned with the crystal axes, reveal that the photocurrent response originates from an overlooked symmetry breaking inside bulk, namely an anisotropic photothermoelectric effect (APTE). Combining photocurrent flow microscopy (PCFM) with conventional scanning photocurrent microscopy (SPCM) imaging, we directly visualize how the APTE stimulates the collectible photocurrent in devices at the sample’s interior and edges through the Shockley-Ramo theorem. Our results introduce a powerful probe for light-matter interactions in quantum materials and inspire novel photodetectors using bulk materials with thermoelectric anisotropy.