Materials that correct light in the current in their mass are preferred for optoelectronic applications. Bulk photocurrents can develop in Weyl semimetals with broken inversion symmetry due to intensified nonlinear optical processes close to the Weyl nodes. Nevertheless, scanning photocurrent microscopy, which obscures the effects of photocurrent generation and collection, is often used to study the photoresponse of these materials.

Boston College scientists have revealed a surprising new mechanism for converting light into electricity in Weyl semimetals using quantum sensors. They have shown that the spatial asymmetry within a single material can generate spontaneous photocurrents.

Scientists examined two materials: tungsten ditelluride and tantalum iridium tetratelluride. Both materials belong to the class of Weyl semimetals.

According to scientists, these materials would be an excellent choice for photocurrent generation because their crystal structure is inherently inversion-asymmetric. It means their crystal doesn’t map itself by reversing directions around a point.

Scientists in this study determined the reason behind the effectiveness of Weyl semimetals in converting light into electricity.

Previous measurements only estimate the amount of electricity coming out of a device. Much like creating a map of the swirling streams of water in the sink, scientists tried to visualize the flow of electricity in the device to better understand the origin of the photocurrents.

Graduate student Yu-Xuan Wang, lead author of the manuscript, said: “As part of the project, we developed a new technique using quantum magnetic field sensors, called nitrogen vacancy centers in diamond, to image the local magnetic field produced by the photocurrents and reconstruct the entire streamlines of the photocurrent. “

The team found that the electric current flowed in a quadruple vortex pattern around where the light hit the material. The team further visualized how the circulating current pattern is altered by the edges of the material and revealed that the precise angle of the edge determines whether the total photocurrent flowing out of the device is positive, negative or zero.

Brian Zhou, assistant professor of physics at Boston College, said: “These never-before-seen flow images enabled us to explain that the photocurrent generation mechanism is surprisingly due to an anisotropic photothermoelectric effect – that is, differences in how heat is converted to current along the different directions in the plane of the Weyl semi-metal.”

“Surprisingly, the appearance of anisotropic thermopower is not necessarily related to the inversion asymmetry displayed by Weyl semimetals, and may therefore be present in other classes of materials.”

“Our findings open a new direction for the search for other highly photosensitive materials. It shows the disruptive impact of quantum-enabled sensors on open questions in materials science.”

“Future projects will use the unique photocurrent microscope to understand the origins of photocurrents in other exotic materials and push the boundaries of detection sensitivity and spatial resolution.”

Magazine reference:

  1. Wang, YX., Zhang, XY., Li, C. et al. Visualization of bulk and edge photocurrent flow in anisotropic Weyl semimetals. Wet. Physically. (2023). DOI: 10.1038/s41567-022-01898-0