Near contact tunneling spectroscopy - a surprisingly versatile probe of quantum materials on the atomic scale

Pairing symmetry continues to be a frontier topic in light of emergence of new superconducting materials and their prospects in quantum information. Recently we introduced a new technique, Tunneling Andreev Reflection (TAR) [1,2] - which detects Andreev reflection via excess tunneling barrier height, partcularly at near-contact separation between metal and superconductor. In a scanning tunneling microscope, TAR enables a direct of probe pairing symmetry, magnetism, and topological properties with atomic resolution. For example, we used TAR to confirm the sign-changing order parameter in paradigmatic FeSe, reveal suppression of superconductivity along the nematic twin boundaries [2], and uniquely probe higher-order Andreev processes that are hidden in point contact spectroscopy [1]. However, to achieve atomic-scale resolution, TAR makes a necessary trade-off in the loss of momentum resolution, necessitating rethinking of the origins of specific tunneling Andreev spectra. Here, based on detailed tight-binding modeling with support from model experiments, we will reveal the basic mechanisms by which TAR spectra connect to the properties of the superconductor. The key ingredients of these spectra can be rationalized by considering contributions from: (1) competition between Andreev reflection and quasiparticle tunneling; (2) electronic proximity effects as a function of increasing coupling strength; (3) energy-dependence of Andreev tunneling, particularly for sign-changing and nodal order parameters; and (4) specific details of the band structure. From this analysis, tunneling Andreev spectra will in general reflect both intrinsic properties of the superconductor as well as those tunneling at large coupling strength - providing a wealth of information to characterize complicated materials and expanding the ability of tunneling spectroscopy to search for exotic quantum materials. 1. Nano Letters 2022 22 , 4042-4048; 2. Nano Letters, 2023 23 , 8310-8318; 3. Nano Letters 2023 23, 2822-2830.