Cooper pairs exhibit a wave-like distribution in Kagome metals

A theory of superconductivity proposed by a team of physicists from Würzburg has been validated by an international experiment that showed that Cooper pairs exhibit a wave-like distribution in Kagome metals. This discovery will pave the way for new technological applications such as superconducting diodes.

For about fifteen years, Kagome materials, whose star-shaped structure is reminiscent of the pattern of Japanese basketwork, have fascinated researchers around the world. It is only since 2018 that scientists have been able to synthesize metal compounds with this structure in the laboratory.

Due to their unique crystal geometry, Kagome metals combine distinctive electronic, magnetic and superconducting properties, making them promising for future quantum technologies.

Professor Ronny Thomale from the Würzburg-Dresden Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matter, and chair of theoretical physics at the University of Würzburg (JMU), provided key insights into this class of materials with his first theoretical predictions.

Recent results published in Nature They suggest that these materials could lead to new electronic components, such as superconducting diodes.

Kagome’s Superconductor Is Shaking Up Science

In an article published on the preprint server arXiv On February 16, 2023, Professor Thomale’s team proposed that a unique type of superconductivity might manifest in Kagome metals, with Cooper pairs distributed in a wave-like manner across sublattices. Each “star point” contains a different number of Cooper pairs. This paper has now been published in Physical examination B.

Thomale’s theory was first confirmed by an international experiment, causing a worldwide scandal. This experiment contradicted the hypothesis that Kagome’s metals could only contain uniformly distributed Cooper pairs.

Cooper pairs, named after physicist Leon Cooper, are formed at very low temperatures by pairs of electrons and are essential for superconductivity. By acting collectively, they can create a quantum state and can also move through a Kagome superconductor without resistance.

“Initially, our research into Kagome metals such as potassium vanadium antimony (KV₃Sb₅) focused on the quantum effects of individual electrons, which, although not superconducting, can exhibit wave-like behavior in the material,” Thomale explains.

“After experimentally confirming our initial theory on electron behavior with the detection of charge density waves two years ago, we tried to find other quantum phenomena at ultra-low temperatures. This led to the discovery of the Kagome superconductor. However, global physics research on Kagome materials is still in its infancy,” notes Thomale.

Transmission of wave motion

“Quantum physics is well aware of the phenomenon of pair density waves, a special form of superconducting condensate. As we all know from cooking, when steam cools, it condenses and becomes liquid.

“Something similar happens in Kagome metals. At ultra-low temperatures around –193°C, electrons rearrange themselves and distribute themselves in waves in the material. This has been known since the discovery of charge density waves,” explains doctoral student Hendrik Hohmann, one of the main contributors to the theoretical work alongside his colleague Matteo Dürrnagel.

“When the temperature drops to -272° (near absolute zero), electrons group together in pairs. These Cooper pairs condense into a quantum fluid that also propagates in waves through the material, allowing resistance-free superconductivity. This wave distribution is therefore transmitted from electrons to Cooper pairs.”

Previous research on Kagome metals has demonstrated both superconductivity and the spatial distribution of Cooper pairs. The surprising new discovery is that these pairs can be distributed not only uniformly, but also in a wave-like pattern within atomic sublattices, a phenomenon called “sublattice-modulated superconductivity.”

Dürrnagel adds: “The presence of pair density waves in KV₃Sb₅ “Superconductivity is ultimately due to a wave-like electronic distribution at temperatures 80° above superconductivity. This combination of quantum effects holds considerable potential.”

ct.qmat researchers are now looking for Kagome metals in which Cooper pairs exhibit spatial modulation without charge density waves appearing before superconductivity. Promising candidates are already under investigation.

Nobel Prize-winning Josephson effect leads to breakthrough

The experiment, which pioneered the direct detection of Cooper pairs distributed in wave-like patterns in a Kagome metal, was developed by Jia-Xin Yin of the Southern University of Science and Technology in Shenzhen, China. It used a scanning tunneling microscope equipped with a superconducting tip capable of directly observing the Cooper pairs.

The design of this single-atom tip is based on the Nobel Prize-winning Josephson effect. A superconducting current passes between the microscope tip and the sample, allowing direct measurement of the Cooper pair distribution.

“The current results are a further step towards energy-efficient quantum devices. Although these effects are currently only observable at the atomic level, once Kagome superconductivity is achievable at the macroscopic scale, new superconducting devices will become feasible. And this is what motivates our fundamental research,” says Professor Thomale.

Perspectives

While the world’s longest superconducting cable has just been installed in Munich, intensive research is still being carried out into superconducting electronic components. The first superconducting diodes have already been developed in the laboratory, but they are based on a combination of different superconducting materials.

In contrast, Kagome’s unique superconductors, with their inherent spatial modulation of Cooper pairs, act as diodes themselves, offering exciting possibilities for superconducting electronics and lossless circuits.