High-resolution spin-polarized scanning tunneling spectroscopy using a functionalized superconducting tip

Detecting spin states of electrons at the atomic scale has been at the heart of progress in condensed matter physics. Spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/STS) has provided important insights into understanding the nature of various spin-dependent phenomena, owing to its capability to visualize energy- and spin-resolved local density-of-states with atomic resolution. This review provides an overview of recent progress in SP-STS using functionalized superconducting tips, focusing on two approaches: conventional superconducting tips and Yu-Shiba-Rusinov (YSR) tips, which are formed by placing a single magnetic atom at the apex of a superconducting tip. Due to their nearly full spin polarization, both types allow for precise detection of the sample's spin polarization. These advanced techniques will be powerful probes for pursuing emergent quantum phenomena that demand ultra-high spin sensitivity, such as the spin polarization of Majorana zero modes around vortex cores in topological superconductors.