Topological Kondo insulator nanowire as a probe tip for spin-sensitive imaging*

Spin-momentum locking in topological surface states (TSS) gives rise to exotic spin-dependent phenomenon in materials like the quantum spin Hall effect, Edelstein effect, etc. Achieving these in experiments have been challenging with garden-variety topological insulators due to the coexistence of highly conducting bulk states and the TSS being buried deep in the bulk bands. However, topological Kondo insulators (TKI) are pertinent for such studies as the TSS are well isolated from the bulk by the Kondo hybridization gap and are usually pinned at the Fermi energy. Nanomaterials of topological insulators furnish enhanced surface properties and allow versatility in spintronics applications. Here, we report the fabrication and use of nanowires of a TKI as probe tips for scanning tunneling microscopy and spectroscopy (STM/S). We exploit the presence of spin-momentum locked TSS at the Fermi energy in this TKI to achieve spin-resolved tunneling. We demonstrate that the direction of spin-polarization is tied to the tunneling direction, a unique feature of TSS-dominated tunneling where the group velocity is locked to the spin direction. Our findings initiate a novel form of STM-based spectroscopy for probing spin degrees of freedom in quantum materials using topological phenomena.