Visualizing bulk and edge photocurrent flow in anisotropic Weyl semimetals

TL;DR

This study directly images photocurrent flow in anisotropic Weyl semimetals, revealing a new anisotropic photothermoelectric effect that influences bulk photocurrent generation and collection.

Contribution

It introduces the anisotropic photothermoelectric effect (APTE) as a new mechanism for bulk photocurrent generation in Weyl semimetals, combining magnetic imaging with traditional microscopy.

Findings

Identification of the anisotropic photothermoelectric effect (APTE)

Visualization of photocurrent circulation around photoexcitation

Demonstration of long-range photocurrent influenced by APTE

Abstract

Materials that rectify light into current in their bulk are desired for optoelectronic applications. In inversion-breaking Weyl semimetals, bulk photocurrents may arise due to nonlinear optical processes that are enhanced near the Weyl nodes. However, the photoresponse of these materials is commonly studied by scanning photocurrent microscopy (SPCM), which convolves the effects of photocurrent generation and collection. Here, we directly image the photocurrent flow inside the type-II Weyl semimetals WTe2 and TaIrTe4 using high-sensitivity quantum magnetometry with nitrogen-vacancy center spins. We elucidate an unknown mechanism for bulk photocurrent generation termed the anisotropic photothermoelectric effect (APTE), where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation. Using simultaneous SPCM and magnetic imaging at the sample's interior and edges, we visualize how the APTE stimulates the long-range photocurrent collected in our Weyl semimetal devices through the Shockley-Ramo theorem. Our results highlight an overlooked, but widely relevant source of current flow and inspire novel photodetectors using homogeneous materials with anisotropy.