Electronic order correlates with kagome superconductivity

A kagome metal has an atomic lattice resembling the overlapping triangles and hexagons of traditional Japanese kagome basket-weaving. Recently, an alloy made of caesium, vanadium and antimony (CsV3Sb5, or CVS) has proved exciting because its electrons have so-called topological order, and because it exhibits superconductivity alongside charge-density waves (CDWs).

The relation of CDWs and superconductivity is a long-standing mystery in condensed-matter science. However, in CVS the situation is more intriguing, because the CDWs and superconductivity appear to be chiral – that is, they come in asymmetrical “left” and “right” patterns. It turns out that this chirality can also affect the flow of current, and is switchable with a magnetic field, suggesting applications in cutting-edge spintronic technology.

The question is how the electrons adopt this chiral ordering. Jochen Geck at TU Dresden and Matthieu Le Tacon at the Karlsruhe Institute of Technology in Germany and colleagues decided to find out, by performing high-resolution X-ray diffraction experiments on CSV while it was subjected to high pressures and low temperatures at the ESRF’s ID15B beamline, in collaboration with ID15B beamline scientist Gastón Garbarino. “Thanks to the high brilliance of the EBS, the refurbished beamline and the new detector, we were able to detect a very weak diffraction signal of the electronic order,” says Geck. “It was about five orders of magnitude weaker that the main Bragg peaks.”

The diffraction signals painted a clear correlation between CDWs and superconductivity. At ambient pressure and low temperatures, CSV exhibited a 2 × 2 CDW pattern, which was already known. When the pressure climbed to about 0.7 GPa, however, the 2 × 2 pattern faded away and a new CDW phase emerged – precisely when a peak in the superconducting transition temperature arose. Then at around 2 GPa the new CDW disappeared, and the superconducting transition temperature peaked again.

The results cannot prove that CDWs drive superconductivity. However, they do show that the two phenomena are closely interconnected, and suggest that CDW fluctuations have a role in enhancing superconductivity. “Our work is more about understanding the fundamental interactions driving electronic order in kagome lattices,” says Le Tacon. “Of course, once this has been achieved, we hope to pave the way for the design of quantum materials with tailored functionalities based on superconductivity, electronic order and switchable chirality.”

Meanwhile, there are implications for the theoretical understanding of CDW formation. Previously, physicists believed the waves appear when the energy levels of many electrons begin to match, or nest, with one another, but this mechanism appears to be inconsistent with the latest observed behaviour, which is highly pressure-dependent. “This standard model of electronic order seems not to work, and needs to be extended,” says Geck.

“In future measurements, we need to explore higher pressure ranges, and couple with other X-ray techniques to study the diffuse and inelastic X-ray scattering,” says Garbarino. “This will help us identify the CDW fluctuations themselves, and further explore their role in superconductivity.”

Reference:

F. Stier et al. Physical Review Letters, 4 December, 2024. https://doi.org/10.1103/PhysRevLett.133.236503

Text by Jon Cartwright

Top image: A kagome basket, with weaving similar to the atomic lattice of a kagome metal. Credits: Robert Izumi