Source: China University of Science and Technology News
A research team composed of Xianhui Chen, Tao Wu and Zhenyu Wang from the National Research Center for Microscale Matter Science, the School of Physics, and the Key Laboratory of Strongly Coupled Quantum Materials Physics of the Chinese Academy of Sciences has recently made important progress in the research of kagome superconductors. The team observed that the charge density wave sequence in CsV3Sb5 superconductor evolves into the electron nematic phase described by the three state Potts model at low temperature. The discovery of this nematic phase not only provides important experimental evidence for understanding the anomalous competition between charge density wave and superconductivity in caged superconductors, but also provides important experimental evidence for understanding the abnormal competition between charge density wave and superconductivity in caged superconductors. It also provides a new research direction for further study of intertwined orders which are closely related to unconventional superconductivity in correlated electron systems. The results of the study were published February 9 in the journal Nature under the title "Charge-density-wave-driven electronic nematicity in a kagome superconductor" in the journal "Accelerated. Article Preview "is published online.

Electron nematic phase is a kind of electron ordered state produced by spontaneous breaking of rotational symmetry of electron degrees of freedom, which is widely found in high temperature superconductors, quantum Hall insulators and other electronic systems. There is a close relationship between electron nematic phase and high-temperature superconductivity, which is considered as a kind of interweaving sequence associated with high-temperature superconductivity, and is an important scientific problem and research hotspot in the theoretical research of high-temperature superconductivity. It is an important research direction to explore new superconducting material systems with new structures and to further study the relationship between superconductivity and various interweaving sequences. One of the systems receiving much attention is the two-dimensional cage structure. Theory predicts that the 2-D cage system may show novel superconductivity and abundant electron ordered states near van Hove singularities, but for a long time there was a lack of suitable material systems to realize their associated physics. In recent years, the discovery of CsV3Sb5 provides a new research system for the exploration of this direction. The superconductivity research team of the University of Science and Technology of China has successfully revealed the charge density wave state of the in-plane triple modulation (triple-Q) in this system (Phys. Rev.X 11, 031026 (2021)). And the anomalous competition between charge density waves and superconductivity under pressure (Nat.Commun. 12, 3645 (2021)).

On this basis, the research team fully combined the three experimental techniques of scanning tunneling microscopy, nuclear magnetic resonance and elastic resistance, and conducted a detailed study of the evolution of charge density wave states in CsV3Sb5. They found that the triple modulated charge density wave state further evolved into a thermodynamically stable electron nematic phase before entering the superconducting state, and determined that the transition temperature was around 35 Kelvin. This electron nematic phase is different from the electron nematic phase previously observed in high-temperature superconductors: the electron nematic phase in high-temperature superconductors is an Ising type nematic phase with Z2 symmetry; The electron nematic phase found in CsV3Sb5 has Z3 symmetry and is theoretically described by the three state Potts model, so it is also called "Potts" nematic phase. Interestingly, this novel electron nematic phase has also recently been observed in bilayer angular graphene systems.

These findings not only reveal a new type of electron nematic phase in cage structure superconductors, but also 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 a Pair density wave state (PDW) in CsV3Sb5 system, which is formed by the interweaving of superconductivity and charge density wave sequences. The electron nematic order found above the superconducting transition temperature can be understood as an interweaving order related to PDW, which also provides important clues and ideas for understanding PDW in high-temperature superconductors. Further experimental and theoretical studies are needed to understand the formation mechanism of superconductivity and interlacing sequence in cage structure superconductors.

Nie Linpeng and Sun Kuhnlu, PhD students from the National Research Center for Microscale Matter Science in Hefei, and Ma Wanru, PhD students from the School of Physics, are co-first authors of the paper, and Professors Chen Xianhui, Wu Tao and Wang Zhenyu are co-corresponding authors. The related work has been supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, the Guiding Project of Anhui Province and the University Innovation Team Project.

Paper link：https://www.nature.com/articles/s41586-022-04493-8

(National Research Center for Microscale Matter Science, School of Physics, Department of Scientific Research, Hefei)