Recently, Prof. Yao Yugui, Prof. Shi Xun and Researcher Wang Zhiwei from the School of Physics, BIT, along with Dr. Zhong Yigui and Prof. Kozo Okazaki from the University of Tokyo, Researcher Wu Xianxin from the Institute of Theoretical Physics, Chinese Academy of Sciences, Prof. Yin Jiaxin from Southern University of Science and Technology and Prof. Zurab Guguchia from the Paul Scherrer Institute collaborated to publish a paper entitled Nodeless Electron Pairing in CsV3Sb5-derived Kagome Superconductors inNature, revealing the robust nodeless superconducting energy gap in cage superconductors, providing a key experimental basis for studying their superconducting pairing mechanism.

Superconductivity is an important frontier field in condensed matter physics. In its century long research process, elucidating the microscopic mechanism of superconductivity and exploring room temperature and atmospheric pressure superconductors have been the unremitting pursuit of researchers. Due to the unique geometric characteristics of cage lattice and its accompanying novel electronic properties, cage superconductors discovered in recent years have become another new platform for exploring superconducting states in complex electronic sequences. In order to clarify the superconductivity and its cooperation or competition with multiphase, a key but still lack of consensus problem is to determine the superconductivity gap symmetry.

In this research, the researchers used a high-resolution angle resolved photoelectron spectrometer to carry out accurate spectral research on the CsV3Sb5 series superconductors at extremely low temperatures, repeatedly measured and carefully analyzed the superconducting gap structures of all Fermi surface from the momentum space, and directly revealed the superconducting gap without nodes for the first time (Fig. 1), eliminating the continuing controversy on this issue in the academic community.

Furthermore, the researchers controlled the replacement of some vanadium atoms with larger niobium or tantalum atoms of the same family through careful chemical replacement, so as to precisely control the superconducting order and charge density wave order (Fig. 2). By comparing the regions with and without charge density wave order in the phase diagram, it is found that the superconducting energy gap is node free and isotropic (Fig. 3). Such a robust energy gap structure greatly narrows the candidate range for superconducting pairing symmetry. At the same time, combined with muon scattering experiments, it is found that the symmetry is inverted by the breaking time of the superconducting state, pointing to the possibility of d+id wave superconducting pairing.

This research has been supported by projects such as the National Key Rresearch and Dvelopment Program, the National Natural Science Foundation of China, and the Young Teacher Academic Initiation Program of BIT.

Fig.1 High precision spectral characteristics of Cs(V0.86Ta0.14)3Sb5 superconducting energy gap (left) and its momentum dependence (right)

Fig. 2 Superconducting Phase Diagram of Element Replacement in CsV3Sb5 System

Fig. 3 Robust Nodeless Superconductive Pairing in the CsV3Sb5 System

Paper link: https://www.nature.com/articles/s41586-023-05907-x