Revealing higher-order topology through loop of spin-helical hinge states in Bi nanocrystal with proximity superconductivity

Higher-order topological insulators (HOTI) are newly proposed topological materials which host gapless boundary states in a codimension of order index. E.g., a 2nd order time-reversal invariant 3D TI will possess 1D boundary states residing in a series of hinges encircling the crystal. These hinge states are spin-helical which resembles the edge state of a quantum spin hall insulator. Experimentally, verifying a 3D HOTI would require systematical investigation on all the boundaries of a 3D crystal, which is challenging. Here we studied the element of Bismuth (Bi), a promising candidate of 2nd order TI, in the form of nanocrystals. The nanocrystals were fabricated on superconducting V3Si (111) substrate. Via scanning tunneling microscopy/spectroscopy (STM/S) measurement, we revealed dispersive 1D states on various hinges of the crystal, which are consistent with first-principle band calculation. After introducing ferromagnetic clusters, we found new scattering channels opened at certain hinges, which suggest they are spin-helical. Remarkably, these spin-helical states on different hinges formed a closed loop surrounding the nanocrystal, further supporting their topological origin. Thus our study provided direct evidence on the existence of HOTI state in nature. Moreover, we detected proximity-induced superconductivity in the hinge states, which enables HOTI as a novel platform for generating Majorana quasi-particles.