Two publications explore unique charge orders in kagome superconductors

Superconductors can carry electricity with zero resistance, meaning no energy is lost as heat. So far, they only work at very low temperatures (or under high pressure), making them tricky to use in everyday life.

The scientific community is hopeful that in the future these materials could be used for electronic applications or transportation. In addition to superconductivity, charge order represents a key quantum state. Identifying systems that exhibit both phenomena, understanding their interplay, and mapping the parameter space to locate the optimal superconducting region remain central challenges in condensed matter physics.

“We need to understand the physics of superconductor candidates before we can design materials with the required properties”, explains Björn Wehinger, scientist at beamline ID27 and co-author of the publication.

Wehinger joined forces with a team at Paul Scherrer Institute, in Switzerland, led by Zurab Guguchia, to investigate the so-called kagome superconductors.

Kagome superconductors are important because they have a unique atomic structure that consists of layered corner-shared triangles (like a Japanese kagome basket weave) and host exotic quantum effects. They can be superconductive and at the same time show unconventional charge orders, and this makes them a promising platform to study novel quantum states.

Despite intense research on kagome superconductors, many fundamental questions remain—especially regarding the unconventional nature of their charge order and superconducting phases. These materials are rich in complexity, and to truly unravel their behavior, a broad and integrated approach is essential.

The team focused on the kagome superconductor LaRu₃Si₂, notable for the discovery—by the same group—of an exceptionally robust charge order that persists up to room temperature. Remarkably, LaRu₃Si₂ also exhibits the highest superconducting transition temperature among materials of this class.

“We wanted to investigate further because there is often an interplay between superconductivity and charge order: it is a kind of a competition, either the material becomes superconductor and the charge order disappears or vice versa”, explains Wehinger.

After detailed resistivity, muon spin rotation and magneto-transport experiments at PSI and MPI for Chemical Physics of Solids in Dresden, the scientists came to the ESRF’s ID27 beamline to track how the properties changed at different temperatures and pressures. In one four-day beamtime, they managed to get enough data for two publications in high impact factor journals.

The first finding was that, when the material is cooled down, one charge order that persists at room temperature coexists with a low temperature charge order and both remain when the material becomes superconducting. While the first charge order shows no magnetic response, the emergence of the second charge order comes along with a distinct magnetic response. Even more intriguingly, they identified a third, lower-temperature scale associated with time-reversal symmetry-breaking and a sign reversal of the Hall effect, pointing to an unconventional coupling between charge order, magnetism, and superconductivity. This work is featured in the Advanced Materials publication.

Interplay of charge order with superconductivity under pressure

In a second paper, this time published in Nature Communications, the researchers investigate further and apply pressure on the material to see how it reacts. High pressure is a very “clean” tool to apply forces without changing the chemistry of the materials, compared to the technique of “doping”, which is most commonly used in superconducting research but alters the material chemistry. “To understand the microscopic physics it is better to use one parameter, such as external pressure”, says Wehinger.

When applying a pressure of 12 GPa, the superconducting temperature is maximised (it increases from 7 to 9 Kelvin) the two charge orders still coexist, but with some interplay. Applying external pressure reduces the ordering temperature of the high temperature charge order drastically while the ordering temperature of the low-temperature time-reversal symmetry-breaking charge order remains about the same. Surprisingly, increasing pressure beyond a critical point suppresses the superconducting state, giving rise to a dome-shaped superconducting phase diagram —a hallmark of unconventional superconductivity. The superconducting transition temperature peaks at 9 K, the highest reported among kagome superconductors to date. The suppression of Tc is accompanied by a transformation of the charge order from an ordered to a disordered state. This is directly visible in x-ray diffraction, as diffuse scattering emerges out of the sharp charge order reflections. This indicate that superconductivity in LaRu₃Si₂ is closely linked to the charge-ordered state and the normal state electronic responses.

Zurab Guguchia, scientist at PSI and corresponding author of the publication, explains: “Our observations suggest that the spatial character of charge order—and its entanglement with the underlying kagome lattice, as well as the electronic and magnetic structures—is key to understanding superconductivity in this system. Our findings position LaRu₃Si₂ as a unique model system for kagome superconductivity, where charge order, magnetism, and superconductivity do not merely coexist, but actively shape one another. While a complete microscopic theory is still needed, our results offer strong motivation for future theoretical and experimental work—such as directional uniaxial stress tuning—to further manipulate the interplay between kagome Ru-site distortions, charge order, and superconductivity, with the potential to push Tc even higher.”

References:

Ma, K., Plokhikh, et al, Nat Commun 16, 6149 (2025). https://doi.org/10.1038/s41467-025-61383-z

C. Mielke III, et al, Advanced Materials, 23 July 2025. https://doi.org/10.1002/adma.202503065

Text by Montserrat Capellas Espuny

Top image: Top view of the atomic structure of LaRu3 Si2. The Ru atoms construct a kagome lattice, while the Si and La atoms form a honeycomb and triangular structure, respectively.