Revealing Higher-Order Topological Bulk-boundary Correspondence in Bismuth Crystal with Spin-helical Hinge State Loop and Proximity Superconductivity

Abstract: Topological materials are typically characterized by gapless boundary states originated from nontrivial bulk band topology, known as topological bulk-boundary correspondence. Recently, this fundamental concept has been generalized in higher-order topological insulators (HOTIs). E.g., a second-order three-dimensional (3D) TI hosts one-dimensional (1D) topological hinge states winding around the crystal. However, a complete verification of higher-order topology is still lacking as it requires probing all the crystal boundaries. Here we studied a promising candidate of second-order TI, bismuth (Bi), in the form of mesoscopic crystals grown on superconducting V3Si. Using low-temperature scanning tunneling microscopy, we directly observed dispersive 1D states on various hinges of the crystal. Upon introducing magnetic scatterers, new scattering channels emerged selectively on certain hinges, revealing their spin-helical nature. Combining first-principle calculation and global symmetry analysis, we find these hinge states are topological and formed a closed loop encircling the crystal. This provides direct evidence on the higher-order topology in Bi. Moreover, proximity superconductivity is observed in the topological hinge states, enabling HOTI as a promising platform for realizing topological superconductivity and Majorana quasiparticles.