Shedding light on the superconductivity of newly discovered kagome metals

Already used in computers and MRI machines, superconductors, materials capable of transmitting electricity without resistance, hold promise for the development of even more advanced technologies, such as aerotrains and quantum computing. Yet how superconductivity works in many materials remains a mystery that limits its applications.

A new study published in Natural Physics sheds light on the superconductivity of AV₃sb₅, a recently discovered family of kagome metals. The research was led by Liang Wu of the School of Arts and Sciences and conducted by Yishuai Xu, a postdoc in Wu’s lab, and graduate students Zhuoliang Ni and Qinwen Deng, in collaboration with researchers from the Weizmann Institute. of Sciences and the University of California, Sainte Barbe.

Since their discovery, superconductors with the chemical formula AV₃sb₅— where A stands for cesium, rubidium or potassium — have aroused immense interest for their exotic properties. The compounds feature a kagome lattice, an unusual atomic arrangement that resembles and takes its name from a Japanese basket-weave pattern of interlocking triangles sharing corners. Kagome’s 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 that accompanies these states is unclear. In physics, symmetry refers to a physical or mathematical characteristic of a system that remains unchanged under certain transformations. When a material changes from a normal high-temperature state to an exotic low-temperature state like superconductivity, it experiences symmetry breaking. Wu, whose lab develops and uses time-resolved and nonlinear optical techniques to study quantum materials, set out to clarify the nature of symmetry breaking when AV₃sb₅ enters the charge density wave phase.

OF₃sb₅ features what the researchers call a “cascade” of symmetry-broken phases. In other words, as the system cools, it begins to enter a state of symmetry breaking, with lower and lower temperatures leading to further broken symmetries. “In order 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, AV₃sb₅ consists of a hexagonal crystal structure, composed of kagome arrays 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 six times rotationally symmetric; when rotated 60 degrees, it remains the same.

To know if AV₃sb₅ retains its six-fold symmetry in the charge density wave phase, researchers performed scanning birefringence measurements on all three limbs of the AV₃sb₅ family. Birefringence, or double refraction, refers to an optical property exhibited by materials with crystallographically distinct axes, a principal axis, and a non-equivalent axis. When light enters the material along the non-equivalent axis, it splits into two, each ray being polarized and traveling at different speeds.

“In a kagome aircraft, the linear optical response should be the same in all directions, but they are not in AV₃sb₅ because between the two layers of kagome there is a relative shift,” says Wu, explaining that birefringence measurements have revealed the difference between two orthogonal directions in the plane and a phase shift between the two layers that reduces rotational symmetry six-fold. materials twice when they enter the charge-density wave state “It 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 property of birefringence by sending light along the principal axis in samples of AV₃sb₅, the researchers used a second optical technique to measure the onset of the Kerr effect. For all three metals, experiments reveal that the Kerr effect begins in the charge density wave state. This finding indicates that the formation of charge density waves breaks another symmetry, time reversal symmetry. The easiest way to break the time reversal symmetry – which holds that the laws of physics stay the same whether time goes forward or backward – is to use a permanent magnet, like the ones we put on a fridge, said Wu.

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

In 2018, Congress passed the National Quantum Initiative Act, with the goal of advancing quantum materials research and the development of quantum technology. Quantum materials include those with topological properties and those with correlation, such as kagome AV metals₃sb₅. While Wu’s previous research focused on the first category and antiferromagnets, he says the scanning optics technique he developed for these 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 the basis for quantum computers, but before using these new superconductors for quantum computing, we need to understand the nature of superconductivity,” says Wu.