Superconductivity theory proposed by a team of physicists confirmed in an international experiment: Cooper pairs show wave-like distribution in Kagome metals, paving the way for new technological advancements such as superconducting diodes.
For approximately fifteen years, Kagome materials—with their star-shaped design inspired by a traditional Japanese weaving pattern—have garnered attention from researchers worldwide. It wasn’t until 2018 that scientists succeeded in creating metallic compounds with this structure in laboratory settings. Owing to their exceptional crystal architecture, Kagome metals exhibit unique electronic, magnetic, and superconducting characteristics, making them highly promising for development in future quantum technologies. Professor Ronny Thomale, associated with the Würzburg-Dresden Cluster of Excellence ct.qmat — Complexity and Topology in Quantum Matter and holding the Chair of Theoretical Physics at the University of Würzburg (JMU), has played a pivotal role in this field with his early theoretical insights. Recent research published in Nature suggests that these materials have the potential to innovate electronic components, including superconducting diodes.
Kagome Superconductor Makes Waves in Science
In an online preprint published on February 16, 2023, Professor Thomale’s team hypothesized that a distinct kind of superconductivity could emerge in Kagome metals, where Cooper pairs are arranged in a wave-like pattern within the sublattices. Each “star point” in this structure contains varying numbers of Cooper pairs. This theory has now been directly validated for the first time through an international experiment, creating a significant stir in the scientific community. This challenges the previous understanding that Kagome metals could only support uniformly distributed Cooper pairs (or waveforms). Cooper pairs—named after physicist Leon Cooper—form at extremely low temperatures by pairs of electrons and are crucial for achieving superconductivity. When they act together, they can create a quantum state that allows them to travel through a Kagome superconductor without any resistance.
“Initially, our investigation into Kagome metals like potassium vanadium antimony (KV3Sb5) concentrated on the quantum behaviors of individual electrons, which, although they do not display superconductivity, can show wave-like characteristics within the material,” explains Thomale. “After we validated our initial theories on electron behavior by detecting charge density waves two years ago, we sought to uncover more quantum phenomena at ultra-low temperatures, leading us to discover the Kagome superconductor. However, the broader research community’s exploration of Kagome materials is still in its early stages,” Thomale remarks.
Transmitting Wave Motion
“The field of quantum physics is well-acquainted with the pair-density wave phenomenon—a unique form of superconducting condensate. Just as steam condenses into liquid when cooled while cooking, a similar process occurs in Kagome metals. At ultra-low temperatures around -193 degrees Celsius, electrons reorganize themselves and form waves within the material. This concept has been established since the detection of charge density waves,” explains doctoral candidate Hendrik Hohmann, a significant contributor to the theoretical framework alongside his peer Matteo Dürrnagel. “As the temperature decreases to -272 degrees (close to absolute zero), electrons pair up. These Cooper pairs then condense into a quantum fluid that also propagates in waves through the material, facilitating superconductivity without resistance. Therefore, this wave-like pattern is transferred from electrons to Cooper pairs.”
Previous studies on Kagome metals have evidenced both superconductivity and the arrangement of Cooper pairs in space. The remarkable new discovery is that these pairs can be organized not merely evenly but also in a wave-like formation within the atomic sublattices—a phenomenon referred to as “sublattice-modulated superconductivity.” Dürrnagel remarks: “The emergence of pair density waves in KV3Sb5 ultimately results from the wave-like electron distribution at temperatures 80 degrees higher than superconductivity. This combination of quantum effects has immense potential.”
The researchers at ct.qmat are actively investigating Kagome metals where Cooper pairs show spatial modulation before the occurrence of charge density waves, with promising candidates already under exploration.
Nobel Prize-Winning Josephson Effect Leads to Breakthrough
The experiment, notable for its direct observation of Cooper pairs configured in wave-like patterns within a Kagome metal, was developed by Jia-Xin Yin at the Southern University of Science and Technology in Shenzhen, China. This utilized a scanning tunneling microscope fitted with a superconducting tip, allowing for direct observation of Cooper pairs. The innovative design of this tip, which culminates in a singular atom, is based on the Nobel Prize-winning Josephson effect. This enables a superconducting current to flow between the microscope tip and the sample, allowing direct measurement of the Cooper pairs’ arrangement.
“Our current findings mark another significant step towards energy-efficient quantum devices. While these effects are presently observable only at the atomic level, once Kagome superconductivity is achieved on a larger scale, we will be able to develop novel superconducting components. This drives our fundamental research,” concludes Professor Thomale.
Future Prospects
Even as the world’s longest superconducting cable has been installed in Munich, extensive research continues into superconducting electronic components. Initial superconducting diodes have already been developed in laboratories, albeit using combinations of different superconducting materials. In contrast, the unique Kagome superconductors, with their inherent spatial modulation of Cooper pairs, function as diodes independently, offering exciting prospects for superconducting electronics and loss-free circuits.
