Nat. Phys. Academician Xianhui Chen's team makes important progress in superconductor research
Recently, Academician Xianhui Chen and Professor Zhenyu Wang at the School of Physics, University of Science and Technology of China (USTC) and the Key Laboratory of Strongly Coupled Quantum Materials Physics, Chinese Academy of Sciences (CAS) have made important progress in the study of electron nematic phases of kagome superconductors.
Using scanning tunneling microscopy, the experimental team observed the microscopic information in the kagome superconductor CsV3-xTixSb5. The relevant findings not only provide important experimental clues for understanding the evolutionary physics of cagome superconductors, but also provide new directions for studying the correlation effects of electrons in the actual lattice system.
On May 11, the research results were published as "Unidirectional electron-phonon coupling in the nematic state of a kagome superconductor". The paper was published in the journal Nature Physics under the title "Unidirectional electron-phonon coupling in the nematic state of a kagome superconductor". Ping Wu, a Ph.D. student in the School of Physics at CSU, and Yubing Tu from Anhui University are the co-first authors of the article, and Prof. Zhenyu Wang and Xianhui Chen are the co-corresponding authors.
Electron nematicity is a phenomenon prevalent in related quantum fluids, including high-temperature superconductors (HTS) and quantum Hall systems, where rotational symmetry is spontaneously broken by electron degrees of freedom. Today, electron nematicity is widely observed in systems such as copper- and iron-based high-temperature superconductors, quantum Hall liquids, and graphene molar superlattices. Its driving mechanism and the relationship with other symmetry-breaking phases (especially superconductivity) have become a key focus of modern condensed matter physics.
The exploration of superconducting material systems with new structures, and thus the further study of the association of superconductivity with various symplectic orders, is an important current research direction in the field, and one class of systems that has received much attention is the two-dimensional cage structure. Theory predicts that near van Hove singularities (van Hove singularities) doping, two-dimensional cage mesh systems can exhibit novel superconductivity and abundant electronically ordered states, but for a long time there is a lack of suitable material systems to realize their correlation physics.
In recent years, the discovery of the cage mesh superconductor AV₃Sb₅(A = K, Rb, Cs) has provided an important platform to study the symmetry breaking of electronic states in the triangular lattice. In a previous study of the cage eye superconducting CsV₃Sb₅ system, Xianhui Chen's team successfully revealed the in-plane triple modulated charge density wave state (Phys. Rev. X11, 031026 (2021)), observed and elucidated the evolution of the charge density wave under pressure and its anomalous competition with the superconducting state (Nat. Commun. 12, 3645 (2021); Nature 611, 682-687 (2022)), and the discovery that this charge density wave can evolve a novel class of electron nematic phases at low temperatures (Nature 604, 59-64 (2022)).
Physical diagram of the electron nematic sequence and superconductivity due to triple modulated charge density waves in cage-like structured superconductors
This time, Prof. Zhenyu Wang and others from Academician Xianhui Chen's team have systematically investigated the electron nematic phase and superconducting states in the titanium-doped caged-mirror superconductor Cs(V, Ti)₃Sb₅ using quasiparticle coherent scattering imaging. The technique uses the measurement of coherent scattering of the electron (quasiparticle) wave function to obtain the microscopic electronic structure within a single domain.
The experimental results show that the electron energy band corresponding to the caged lattice shows a significant rotational symmetry breaking after entering the electron nematic phase. A comprehensive analysis of the electron dispersion and quasiparticle lifetime reveals that the electro-phonon coupling effect leads to a unidirectional electron self-energy correction in the energy band of the cage-eye lattice, which results in a strong second-degree symmetry characteristic of the electron dispersion and electron low-energy dynamics.
Coherent scattering images of quasiparticles in cage-lattice superconductivity with unidirectional electro-phonon coupling on the cage-lattice energy band
Coherent scattering measurements also reflect coherent information about the electron wave function. The team found that above the electron nematic phase temperature, the interference pattern corresponding to the cage mesh lattice energy band disappears rapidly and the coherence of the electron wave function becomes significantly worse.
-- This indicates that in this system, the electron nematic phase is an electron coherent state formed by the combined effect of electron correlation and phonons. When the coherence temperature increases, the superconducting transition temperature decreases significantly: indicating that there is a competitive relationship between these two electronically ordered states.
Visualization of unidirectional electron-mode coupling
The above results reveal the microscopic features of the electronic structure in the nematic phase of the caged superconductor and its competition with the superconducting state. In addition to theoretical modeling, the research team says that "future experiments on samples using different techniques are necessary to unveil more details of the nematic state in this kagome superconductor and to understand how this breakthrough can help create new quantum states."
Reference links:
[1] http://quantummatter.ustc.edu.cn/index.php/Show/index/id/273
[2]https://www.nature.com/articles/s41567-023-02031-5
[3] https://mp.weixin.qq.com/s/nuc7paXpKrZtXNn_WNGrDQ
