Progress has been made in the study of superconducting pairing of caged superconductors

Unconventional superconductivity is a frontier in condensed matter physics, and revealing the pairing symmetry of superconductivity and its pairing mechanism is one of the challenging topics. The recent discovery of caged superconductors has attracted attention due to the unique geometric characteristics of the cage lattice and the novel electronic properties that accompany it. It is found that the caged superconductor AV3Sb5 (A=K,Rb,Cs) exhibits rich correlation physical phenomena, such as possible unconventional superconductivity, novel charge density wave states, anomalous Hall effect and nematic order. Among them, although superconducting pairing has been studied a lot, the existence of nodes in its superconducting energy gap is controversial.

Recently, Wu Xianxin, associate researcher of the Institute of Theoretical Physics, Chinese Academy of Sciences, together with Dr. Zhong Yigui and Professor Kozo Okazaki of the University of Tokyo, Professor Shi Xun and researcher Wang Zhiwei of the School of Physics of Beijing Institute of Technology, Professor Yin Jiaxin of Southern University of Science and Technology, and Zurab Guguchia experimental team of Professor Zurab Guguchia of the Paul Scheller Institute in Switzerland cooperated to explore the extremely low temperature superconducting energy gap of the CsV3Sb5 series superconductors, and directly revealed the nodeless superconducting energy gap for the first time. Furthermore, the precise regulation of superconducting states and charge density wave states by meticulous chemical substitution is studied. Comparing the regions with no charge density sequence in the phase diagram, it was found that the superconducting energy gaps were all nodeless and isotropic. At the same time, after the charge density wave state is suppressed, the muon scattering experiment shows that the superconducting state destroys the time inversion symmetry. These experimental evidence suggests that caged superconductors may have S+IS or D+ID pairings that disrupt time inversion. Among them, chiral D-wave pairing has been proposed in previous research on the theory of cage lattice and Wu Xianxin’s theoretical work. These experimental results clarify the controversy about whether the superconducting energy gap of caged superconductors has nodes, and lay a solid foundation for further finalizing their pairing symmetry and revealing their pairing mechanism.

Robust nodeless superconducting pairing in the CsV3Sb5 architecture. a-b, schematic diagram of superconducting energy gap in momentum space; c. Phase diagram of chemical doping.

The research was published in Nature. Researchers from the Institute of Physics of the Chinese Academy of Sciences and the Kavli Institute of Theoretical Science of the University of Chinese Academy of Sciences participated in the research.

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