Shedding light on the superconductivity of newly discovered Kagome metals

Shedding light on the superconductivity of newly discovered Kagome metals

Liang Wu is an assistant professor in the Department of Physics and Astronomy in the College of Arts and Sciences. Credit: University of Pennsylvania

Already used in computers and magnetic resonance imaging (MRI) machines, superconductors are materials that can conduct electricity without resistance, and they hold promise for the development of more advanced technologies, such as moving trains and quantum computing. However, how superconductivity works in many materials remains a mystery limiting its applications.

A new study was published in Nature Physics Sheds light on the superconductivity of compounds3swear5, a recently discovered family of Kagome minerals. The research was led by Liang Wu of the College of Arts and Sciences and conducted by Yishuai Xu, a postdoctoral researcher in Wu’s lab, and graduate students Chuoliang Ni and Kenwen Deng, in collaboration with researchers from the Weizmann Institute of Science and the University of California, Santa Barbara.

Since their discovery, superconductors with the chemical formula AV3swear5—where “A” stands for cesium, rubidium, or potassium, it has generated a great deal of interest in its peculiar properties. Compounds contain a kagome lattice, an unusual atomic arrangement that resembles and takes its name from a Japanese basket-weave pattern of interlocking triangles sharing a corner. Kagome mesh material has fascinated researchers for decades because it provides a window into quantum phenomena Such as geometric frustration, topology and strong correlations.

While the previous search for AV3swear5 Discover the coexistence of two different cooperatives electronic countries– Wave order of charge density and superconductivity – The nature of the symmetry breaking associated with these cases was unclear. In physics, symmetry refers to a physical or mathematical feature of a system that remains unchanged under certain transformations. When a substance transitions from a normal high-temperature state to an exotic low-temperature state such as superconductivity, it undergoes symmetry breaking. Wu, whose lab is developing and using nonlinear optical time-resolving techniques to study quantum materials, has demonstrated the nature of symmetry breaking when3swear5 It enters the charge density wave phase.

AV3swear5 They display what the researchers call a “chain” of symmetry breaking stages. In other words, when the system cools down, it begins to enter the symmetry-broken state, with low and low temperatures Which leads to additional broken symmetries. “In order to use superconductors for applications, we need to understand them,” Wu says. “Because superconductivity develops at lower temperatures, we need to understand the charge density wave phase first.”

In its normal state, AV3swear5 It 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 symmetrical; When rotated by 60 degrees, it remains the same.

To see if AV3swear5 It retains its six-fold symmetry in the phase of the charge density wave, and the researchers performed scanning refraction measurements on all three organs in the AV3swear5 family. Double birefringence, or double birefringence, refers to an optical property exhibited by materials with crystallographically distinct axes, a principal axis and a parabolic axis. When light enters the material along the non-equivalent axis, it splits in two, with each beam polarized and traveling at different speeds.

“In the kagome plane, the linear optical response should be the same along any direction, but it is not in the AV3swear5 Because between the two Kagome layers there is a relativistic shift,” says Wu, explaining that diffraction measurements revealed the difference between two orthogonal directions in the plane and a phase shift between the two layers reducing the six-fold rotational symmetry of materials to two when they enter a charge-density wave state. to the physics community before.”

Distinctive 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 magnetic-optical Kerr effect. After separating the refractive characteristic by sending light along the principal axis in the AV samples3swear5In the study, the researchers used a second optical technique to measure the onset of the Kerr effect. For all three metals, experiments revealed that the Kerr effect begins in the charge-density wave state. This result indicates that the formation of charge density waves breaks another symmetry, the time-reversal symmetry. The simplest way to break time reversal symmetry — which says the laws of physics stay the same whether time passes forward or backward — is to use permanent magnets, like the ones we keep in the refrigerator, Wu says.

However, the Kerr effect can only be observed at low temperatures and with high precision, indicating that Kagome minerals are not significantly magnetic. Using these quantum materials, Wu says, he and his collaborators theorized that time-reversible symmetry is “broken not by a permanent magnet but by a circular current.” To confirm the nature of time reflection symmetry Fractured in the case of a charge-density wave, the researchers conducted a third experiment in which they measured the circular dichroism, or asymmetric reflectivity of right-handed and left-handed circularly polarized light, of the phase of a charge-density wave. “More work is still needed, but this finding really supports the possibility of circulating toroidal currents,” Wu says, the presence of which may indicate the unconventional nature of superconductivity in metals.

In 2018, Congress passed the National Quantum Initiative Act, with the goal of promoting research into quantum materials and the development of quantum technology. Quantum materials include those with topological properties and those with correlation, such as Kagome metals AV3swear5. While Wu’s previous research has focused on the former and antimagnets, he says the scanning optics technology he developed for these studies provided a “ready and versatile tool” for the study. symmetry Breaking new Kagome minerals.

“All superconductors are interesting because they can 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,” Wu says.

more information:
Yishuai Xu et al, Three-state nematicity and Kerr magneto-optic effect on charge density waves in kagome superconductors, Nature Physics (2022). DOI: 10.1038 / s41567-022-01805-7

the quote: Shedding Light on the Superconductivity of Newly Discovered Kagome Metals (2022, Nov 7) Retrieved Nov 8, 2022 from

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