Materials that rectify light into the current in their bulk are preferred for optoelectronic applications. Bulk photocurrents may develop in Weyl semimetals with broken inversion symmetry due to intensified nonlinear optical processes close to the Weyl nodes. Nevertheless, scanning photocurrent microscopy, which muddles the effects of photocurrent generation and collecting, is frequently used to study the photoresponse of these materials.
Scientists from Boston College 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 a great choice for generating photocurrent because their crystal structure is inherently inversion asymmetric. It means their crystal does not map onto itself by reversing directions about a point.
Scientists in this study determined the reason behind the effectiveness of Weyl semimetals at converting light into electricity.
Previous measurements only estimated the amount of electricity coming out of a device. Like making a map of the swirling water currents in the sink, scientists attempted to visualize the flow of electricity within the device to understand the photocurrents’ origin better.
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 full streamlines of the photocurrent flow.”
The team found the electrical current flowed in a four-fold vortex pattern around where the light shined on the material. The team further visualized how the circulating flow pattern is modified 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.
Boston College Assistant Professor of Physics Brian Zhou said, “These never-before-seen flow images allowed us to explain that the photocurrent generation mechanism is surprisingly due to an anisotropic photothermoelectric effect – that is to say, differences in how heat is converted to current along the different in-plane directions of the Weyl semimetal.”
“Surprisingly, the appearance of anisotropic thermopower is not necessarily related to the inversion asymmetry displayed by Weyl semimetals, and hence, may be present in other classes of materials.”
“Our findings open a new direction for searching for other highly photoresponsive materials. It showcases the disruptive impact of quantum-enabled sensors on open questions in materials science.”
“Future projects will use the unique photocurrent flow microscope to understand the origins of photocurrents in other exotic materials and to push the limits in detection sensitivity and spatial resolution.”
- Wang, YX., Zhang, XY., Li, C. et al. Visualization of bulk and edge photocurrent flow in anisotropic Weyl semimetals. Nat. Phys. (2023). DOI: 10.1038/s41567-022-01898-0