Progress in the research of cage structure superconductors丨 Nature

The research team composed of Chen Xianhui, Wu Tao and Wang Zhenyu, hefei National Research Center for Physical Sciences at the Microscale of the University of Science and Technology of China, School of Physics, and Key Laboratory of Physics of Strongly Coupled Quantum Materials of the Chinese Academy of Sciences, has made important progress in the study of superconductors of the cage structure (kagome). In the cage-order superconductor CsV3Sb5, the team observed that the charge density sequence evolved at low temperatures into the electron vascular phase described by the three state Potts model. The discovery of this facade provides important experimental evidence for understanding the abnormal competition between charge density waves and superconductivity in cage-structured superconductors, and provides a new research direction for further study of intertwined orders in associated electronic systems that are closely related to unconventional superconductivity. On February 9, the research results were published online in Nature in the form of an Approved Article Preview, titled Charge-density-wave-driven electronic nematicity in a kagome superconductor.

The electron phase is an electron ordered state produced by the spontaneous breaking of the rotational symmetry of the degree of freedom of electrons, which is widely present in electronic systems such as high-temperature superconductors and quantum Hall insulators. There is a close connection between the electron homing phase and high-temperature superconductivity, which is considered to be an interleaved sequence associated with high-temperature superconductivity, and is an important scientific issue and research hotspot in the theoretical research of high-temperature superconductivity. Exploring superconducting material systems with new structures, so as to further explore the correlation between superconductivity and various interwoven orders, is an important research direction in the current field, and one of the most concerned systems is two-dimensional cage structures. Theory predicts that near van Hove singularities doping, two-dimensional cage-order systems can exhibit novel superconductivity and rich electron ordered states, but there has long been a lack of suitable material systems to achieve their associative physics. In recent years, the discovery of the csV3Sb5 of the cage superconductor has provided a new research system for the exploration of this direction. Previous studies by the superconductivity research team at the University of Science and Technology of China have revealed the charge density wave state of the in-plane triple-Q in the system [Physical Review X, 11, 031026 (2021)] and the abnormal competitive relationship between charge density waves and superconductivity under pressure [Nature Communications, 12, 3645 (2021)].

On the basis of the above research, the research team fully combined the three experimental techniques of scanning tunneling microscopy, nuclear magnetic resonance and elastic resistance to conduct a detailed study on the evolution of charge density wave states in CsV3Sb5. Studies have shown that before the system enters the superconducting state, the triple modulation charge density wave state will further evolve into a thermodynamically stable electron facies, and the transition temperature is determined to be around 35 Kelvin. This electron homing phase is different from the electron homing previously observed in high-temperature superconductors: the electron vaguoliary phase in the high-temperature superconductor is an Ising type of vaguoliary phase with Z2 symmetry; while the electron matisine phase found in cage superconducting CsV3Sb5 has Z3 symmetry, which is theoretically described by the three state Potts model, and is therefore also known as the "Potts" matigraph phase. Interestingly, this new type of electron-oriented phase has also recently been observed in bilayer angled graphene systems.

These findings reveal a novel electron facade in the cage-structured superconductor and provide experimental evidence for understanding the competition between superconductivity and charge density waves in such systems. Previous scanning tunnel spectroscopy studies have shown that there may be pair density wave states (PDW) formed by the interweaving of superconductivity and charge density wave sequences in CsV3Sb5 systems. The electron sequence found above the transition temperature of superconductivity can be understood as a PDW-related interleaving sequence, which provides important clues and ideas for understanding PDW in high-temperature superconductors. How to understand the formation mechanism of superconductivity and its interleaving sequence in the superconductor of the cage structure still needs further experimental and theoretical research.

The research work is supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, Anhui Province and the Innovation Team Project of the University of Science and Technology of China.

Physical diagram of electron sequence and superconductivity caused by triple modulated charge density waves in a cage structure superconductor

Source: University of Science and Technology of China