Tunneling Andreev Reflection - new quantitative microscopy of superconductors with atomic resolution

Andreev reflection across a point contact (PCAR) is an established method to probe superconductivity, and, crucially, the order parameter symmetry. PCAR relies on the so-called directional contacts, which inject quasiparticles into superconductors with well-defined momentum but necessarily poor spatial resolution. Here we will present our technique of Tunneling Andreev Reflection - which detects Andreev reflection across atomic-scale tunnel junction [1]. Similar to PCAR, TAR exhibits direct sensitivity to the superconducting order parameter in both conventional and unconventional superconductors [2]. For example, TAR unambiguously confirmed the sign-changing order parameter in FeSe, and further revealed suppression of superconductivity along the nematic boundaries [2]. However, due to atomic resolution TAR lacks momentum resolution - the opposite of PCAR - so that the observables and their interpretation are fundamentally distinct from PCAR. We will discuss our present understanding of this technique, relevant methods of data and the specific effects of band structure on TAR. These developments pave way to robust characterization of unconventional superconductivity on the atomic-scale, including multiband, topological and inhomogeneous superconductivity, complementing tunneling spectroscopy and quasiparticle imaging in search for exotic quantum materials.