Our group and collaborators have revealed a new superconducting mechanism of spin-orbit-parity coupling in the thin layer two-dimensional layered single crystal superconductor 2M-WS2. On November 14, 2022, the relevant research findings were published online in the journal Nature Physics under the title “Spin-Orbit-Parity Coupled Superconductivity in Atomically Thin 2M-WS2”, and a review titled “News & Views” was also published simultaneously. The first unit of the paper is the Department of Physics at Fudan University. Corresponding authors include Professor Faxian Xiu from the Department of Physics at Fudan University, Professor Jintuan Luo from the Hong Kong University of Science and Technology, and Researchers Fuqiang Huang and Jinshan Yang from the Shanghai Institute of Silicates, Chinese Academy of Sciences. The co first authors are postdoctoral fellow Enze Zhang (now Research Fellow at the National University of Singapore) from the Faxian Xiu’s research group, doctoral student Yingming Xie from the Jintuan Luo’s research group at the Hong Kong University of Science and Technology, postdoctoral fellow Yuqiang Fang from the Fuqiang Huang’s research group at the Shanghai Institute of Silicates, Chinese Academy of Sciences, and researcher Jinglei Zhang from the Strong Magnetic Field Center, Chinese Academy of Sciences.
In recent years, two-dimensional layered single crystal superconductors have become a hot research topic internationally. Compared to traditional amorphous and polycrystalline superconducting thin films, two-dimensional layered single crystal superconductors, due to their extremely high single crystal quality, can maintain the superconducting state to the original cell layer thickness at the nanoscale, providing a good research platform for people to understand and detect the novel characteristics of intrinsic two-dimensional superconductivity in samples. A very important research direction in two-dimensional layered single crystal superconductors is to study their superconducting behavior under strong in-plane critical fields, and to find superconductors that can resist large external magnetic fields. This type of research is of great significance for both basic physics research and applied research on superconductivity.
The superconductivity of spin-orbit-parity coupling was first predicted by Professor Jintuan Luo from the Hong Kong University of Science and Technology (Phys. Rev. Lett. 125.107001, (2020)). It refers to a two-dimensional centrosymmetric superconductor with topological band flipping as shown in Figure a, where a topological band gap opens near the topological band flipping region with opposite parity. In this case, the traditional spin orbit coupling (SOC) term that only includes spin and momentum is prohibited due to the spatial inversion symmetry of the system, but the spatial inversion symmetry allows the parity of the spin, momentum, and electronic states of the system to couple together near the topological band flipping region, known as spin orbit parity coupling, This spin orbit parity coupling leads to novel superconducting states in the system near the topological band flipping region, known as spin-orbit-parity coupling superconductivity, which has a huge (exceeding the Pauli limit) and anisotropic in-plane critical magnetic field.
In order to search for spin orbit parity coupling superconductivity, our group and collaborators studied a new type of layered single crystal superconductor 2M-WS2. This emerging layered single crystal superconductor, due to its topological band flipping characteristics, is an experimental platform for detecting spin orbit parity coupling superconductivity ideas. Our group prepared a 2M-WS2 thin layer electric transport device using mechanical stripping method. Through measurement, it was found that 2M-WS2 with a thickness of about 4nm has a superconducting critical temperature of about 7.6K, and the superconductivity is at a clean limit. Magnetic transport also proves that, unlike bulk materials, the superconductivity in the thin layer 2M-WS2 exhibits two-dimensional superconductivity properties. For example, the I-V relationship curve of the device exhibits a power phenomenon, and the upper critical magnetic field of the device's small angle range at the outside to inside angle follows the two-dimensional Tinkham formula (Figure b).
Figure. (a) Schematic diagram of the energy band of thin layer 2M-WS2. (b) The evolution law of the upper critical field of the outward to inward rotation angle in the thin layer 2M-WS2. Illustration, Evolution Law of the Upper Critical Field with a Small Angle Range of Outside to Inside Turning Angle. (c) Dual Degeneration Characteristics of the Upper Critical Field in the Plane of 2M-WS2 Thin Films. (d) Anisotropic behavior of normalized resistance of thin layer 2M-WS2 under low temperature and strong magnetic field. (e) Evolution of Differential Conductivity of Thin Layer 2M-WS2 under Magnetic Field (9T) in Different Directions. (f) Anisotropic behavior of thin layer 2M-WS2 superconducting energy gap under strong in-plane magnetic field.
Based on the two-dimensional superconductivity in thin layer 2M-WS2, we studied the behavior of in-plane critical magnetic field and the evolution of superconducting energy gap in thin layer 2M-WS2 under low temperature and strong magnetic field. Research has found that below the superconducting transition temperature, the in-plane critical magnetic field of the thin layer 2M-WS2 greatly exceeds the Pauli limit and exhibits strong double degenerate anisotropy (Figure c, d). Consistent with this (Figure e, f), differential conductivity measurements indicate that its superconducting energy gap can exist in an in-plane magnetic field far greater than the Pauli limit, and it also exhibits strong double degenerate anisotropy. The average field theory calculations of the theoretical collaborators also indicate that the thin layer 2M-WS2 exhibits asymmetric spin orbit parity coupling due to its topological band flipping characteristics. This coupling can effectively pin the spin states near the topological band crossing and anisotropic renormalize the influence of external Zeeman fields, leading to the novel phenomenon of superconductivity observed in the above experiments under strong in-plane magnetic fields. The above research also indicates that the superconducting mechanism in the thin layer 2M-WS2 is spin orbit parity coupling superconductivity.
This research achievement is the first to experimentally discover spin orbit parity coupled superconductivity. It has been revealed that in two-dimensional centrosymmetric superconductors with topological band flipping characteristics, due to the influence of spin-orbit-parity coupling, the upper critical magnetic field not only can greatly exceed the Pauli limit, but also has strong in-plane anisotropy. It is of great significance for a deeper understanding of the singular superconducting behavior in two-dimensional centrosymmetric superconductors with topological band flipping characteristics. At the same time, the unique physical properties exhibited by the new layered single crystal superconductor 2M-WS2 in research also indicate that it has good research value in exploring high-order topological superconductivity and new devices.
Paper link: https://www.nature.com/articles/s41567-022-01812-8
