image: Figure 1. Electronic stripe modulations in 2M-WS2.
Credit: ©Science China Press
Topological materials are capable of hosting exotic Majorana bound states (MBSs) that obey non-Abelian statistics required for topological quantum computing. These states can be generated via the combination of electronic topology and superconductivity. Recently, there has been a burgeoning interest in the exotic electronic order in intrinsic topological superconductors. Given that MBSs in Fu-Kane superconductors reside on the surface, modifications of the surface electronic properties can be used to control the position and distribution of MBSs. Furthermore, it is of fundamental interest to study the interplay between surface electronic order and MBSs. Transition metal dichalcogenides have attracted significant research interest due to their novel quantum properties. Notably, 2M-WS2 features a topologically nontrivial band structure with a superconducting transition temperature of 8.8 K, which has been demonstrated to host MBSs in the vortex cores (Nat Phys 2019; 15: 1046-51). This observation suggests that 2M-WS2 is an intrinsic Fu-Kane topological superconductor, thereby providing an optimal platform for investigating the interplay between surface charge order and Majorana bound states.
Recently, Prof. Wei Li and Prof. Qi-Kun Xue from the State Key Laboratory of Low-Dimensional Quantum Physics at Tsinghua University, in collaboration with Prof. Fuqiang Huang from Shanghai Institute of Ceramics, Chinese Academy of Science, Prof. Peizhe Tang from Beihang University, Prof. Taylor L. Hughes and Dr. Xiao-Qi Sun from University of Illinois at Urbana-Champaign conducted a systematic study on the stripe charge order and its interaction with Majorana bound states in 2M-WS2. By utilizing low-temperature scanning tunneling microscopy (STM) technique, they revealed the coexistence of a striped surface charge order coexisting with superconductivity in 2M-WS2. Its impact on the topological surface states and MBSs was further investigated, providing critical insights for the future application of Majorana quantum qubits. This work has been published in National Science Review, titled “Stripe charge order and its interaction with Majorana bound states in 2M-WS2 topological superconductor.”
The researchers identified and thoroughly characterized the stripe charge order in 2M-WS2. As shown in Fig. 1(a-f), these stripes break the rotational and translational symmetries of the lattice and exhibit an incommensurate spatial period of 1.31 nm with slight fluctuations in different regions of the sample. Low-energy-scale dI/dV spectra taken at 400 mK exhibit U-shaped superconducting gaps, indicating the nodeless superconducting pairing in 2M-WS2 [Fig. 1(g)]. In addition, the superconducting coherence peaks show spatially periodic modulations. The periodic modulations of the coherence peaks are closely related to the corresponding locations where the spectra are acquired [Fig. 1(g), (h)], demonstrating the intimate correlation between superconductivity and stripe charge order. To further investigate the interaction between the stripe charge order and Majorana bound states, an external magnetic field was applied perpendicular to the sample surface. It was found that MBS is absent in the vortices in the region with stripe order. This is in contrast to adjacent underlaying layers without charge order where vortex-bound MBSs are observed [Fig. 2, 3]. These experiment results demonstrate that the surface stripe order does not destroy the bulk topology, but it can effectively modify the spatial distribution of MBSs, i.e., it pushes them downward away from the 2M-WS2 surface, which has been verified by the theoretical simulations [Fig. 4]. These findings demonstrate that the interplay of charge order and topological superconductivity can potentially be used to tune the positions of MBS, providing a novel approach for the spatial manipulation of MBSs.