A team led by Boston College has developed a new quantum sensing method to view and understand the source of photocurrent flux in Weyl semimetals. In an article recently published in the journal Nature PhysicsBrian Zhou, assistant professor of physics at Boston College, and colleagues have uncovered a surprising new method for converting light into electricity in Weyl semimetals using quantum sensors.

Many modern technologies such as cameras, fiber optic systems and solar panels rely on the conversion of light into electrical signals. However, in most materials, simply illuminating the surface does not produce electricity because there is no precise direction for the flow of electricity. To overcome these limitations and create new optoelectronic devices, researchers are studying the unique properties of electrons in Weyl semimetals.

“Most photovoltaic devices require two different materials to create an asymmetry in space,” said Zhou, who worked with eight BC colleagues and two researchers from Nanyang Technological University in Singapore. “Here, we show that spatial asymmetry within a single material – particularly asymmetry in thermoelectric transport properties – can spontaneously lead to photocurrents.”

The team investigated the materials tungsten ditelluride and tantalum iridium tetratelluride, both of which belong to the Weyl class of semimetals. The researchers suspect that these materials would be good candidates for photocurrent generation because their crystal structures are inherently inversion asymmetrical; that is, the crystal does not reflect itself by changing direction around a point.

Zhou’s research group sought to understand why Weyl semimetals efficiently convert light into electricity. Previous measurements could only measure the amount of electricity emanating from a device, such as measuring the amount of water flowing from a sink to a downpipe. To better understand the origin of the photocurrents, Zhou’s team tried to visualize the flow of electricity inside the device, similar to mapping swirling water currents in a sink.

“As part of the project, we developed a new technique using quantum sensors of the magnetic field, called nitrogen gap centers in diamond, to image the local magnetic field produced by the photocurrents and reconstruct all the photocurrent flow lines,” says graduate student Yu-. xuan. Wang, lead author of the paper, said:

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

“These never-before-seen flow images allowed us to explain that the photocurrent generation mechanism is surprisingly driven by the anisotropic photothermoelectric effect, that is, the difference in how heat is converted to current along different in-plane Weyl directions of semimetals,” said Zhou.

Surprisingly, the appearance of anisotropic thermoEMF is not necessarily associated with the inversion asymmetry exhibited by Weyl semimetals and may therefore be present in other material classes.

“Our findings open a new direction for finding other materials with high photosensitivity,” Zhou said. Said. “This demonstrates the devastating impact of quantum sensors on open questions in materials science.”

Future projects will use the unique photocurrent flow microscope to understand the origin of photocurrent in other exotic materials and push the limits of detection sensitivity and spatial resolution, Zhou said.