It sheds light on the superconductivity of the newly discovered kagome metals

Already used in computers and MRI machines, superconductors—materials that can transmit electricity without resistance—hold promise for the development of even more advanced technologies, such as levitation trains and quantum computers. However, how superconductivity works in many materials remains a mystery that limits its applications.

A new study published in Physics of Nature sheds light on AV superconductivity₃Sb₅, a recently discovered family of kagome metals. The research was led by Liang Wu of the School of Arts & Sciences and led by Yishuai Xu, a postdoctoral fellow in Wu’s lab, and graduate students Zhuoliang Ni and Qinwen Deng, in collaboration with researchers from the Weizmann Institute of Science and the University of California . Santa Barbara.

Since their discovery, superconductors with chemical formula AV₃Sb₅—where A refers to cesium, rubidium, or potassium—have generated enormous interest for their exotic properties. The joints feature a kagome lattice, an unusual atomic arrangement that resembles and is named after a Japanese basket weaving pattern of interlaced triangles that share corners. Kagome lattice materials have fascinated researchers for decades because they provide a window into quantum phenomena such as geometric frustration, topology, and strong correlations.

While previous research on AV₃Sb₅ discovered the coexistence of two different cooperative electronic states—charge-density wave order and superconductivity—the nature of the symmetry breaking accompanying these states was unclear. In physics, symmetry refers to a physical or mathematical characteristic of a system that remains unchanged under certain transformations. When a material goes from a high-temperature normal state to a low-temperature exotic state such as superconductivity, it undergoes symmetry breaking. Wu, whose lab develops and uses time-resolved and nonlinear optical techniques to study quantum materials, set out to elucidate the nature of symmetry breaking when AV₃Sb₅ enters the charge-density wave phase.

AV₃Sb₅ exhibits what the researchers call a “cascade” of symmetry-breaking phases. In other words, as the system cools, it begins to enter a state of symmetry breaking, with lower and lower temperatures leading to additional broken symmetries. “To use superconductors for applications, we need to understand them,” says Wu. “Because superconductivity develops at even lower temperatures, we first need to understand the charge-density wave phase.”

In its normal state, the AV₃Sb₅ consists of a hexagonal crystal structure, consisting of kagome lattices of vanadium (V) atoms coordinated by antimony (Sb) stacked on top of each other, with sheets of cesium, rubidium, or potassium between each V-Sb layer. The structure is sixfold rotationally symmetric. when rotated 60 degrees, it remains the same.

To find out if the AV₃Sb₅ maintains its sixfold symmetry in the charge-density wave phase, the researchers performed scanning birefringence measurements on all three members of the AV₃Sb₅ family. Birefringence, or birefringence, refers to an optical property exhibited by materials with crystallographically distinct axes, a principal axis and a nonequivalent axis. When light enters the material along the nonequivalent axis, it is split in two, with each ray being polarized and traveling at different speeds.

“In a kagome plane, the linear optical response should be the same in any direction, but it is not in AV₃Sb₅ because between the two kagome layers there is a relative shift,” says Wu, explaining that the birefringence measurements revealed the difference between two orthogonal in-plane directions and a phase shift between the two layers that reduces the materials’ sixfold rotational symmetry to double when they enter the charge density wave state. “This was not clear to the physics community before.”

Distinct axes are not the only explanation for the rotation of the plane of polarization of light. When linearly polarized light encounters a magnetic surface, it also changes, a phenomenon known as the magneto-optical Kerr effect. After separating the birefringent property by sending light along the principal axis to AV samples₃Sb₅, the researchers used a second optical technique to measure the onset of the Kerr effect. For all three metals, the experiments reveal that the Kerr effect starts in the charge-density wave state. This finding shows that the formation of charge-density waves breaks another symmetry, the time-reversal symmetry. The simplest way to break time-reversal symmetry—which holds that the laws of physics remain the same whether time is running forward or backward—is to use a permanent magnet, like the ones we put in a refrigerator, says Wu.

However, the Kerr effect is only observable at low temperatures with high resolution, indicating that kagome metals are essentially non-magnetic. “With these quantum materials,” says Wu, he and his colleagues see that the time-reversal symmetry “is not broken by a permanent magnet but by a circulating loop current.” To confirm the nature of the time-reversal symmetry breaking in the charge-density wave state, the researchers performed a third experiment in which they measured the circular dichroism, or unequal reflectivity, of left-handed and right-handed circularly polarized light. charge density wave phase. “We still need further work, but this finding really supports the possibility of loop currents,” the existence of which would indicate the unconventional nature of superconductivity in metals, Wu says.

In 2018, Congress passed the National Quantum Initiative Act, aimed at promoting quantum materials research and the development of quantum technology. Quantum materials include those with topological properties and those with correlation, such as AV kagome metals₃Sb₅. While Wu’s previous research focused on the former class and on antiferromagnets, he says the optical scanning technique he developed for those studies presented a “ready and versatile tool” for studying symmetry breaking in new kagome metals.

“All superconductors are interesting because they could potentially be used as a basis for quantum computing, but before we can use these new superconductors for quantum computing, we need to understand the nature of superconductivity,” says Wu.