Scanning d-wave Andreev Reflection Spectroscopy of Superconducting Transition-Metal Chalcogenides

At the interface of metal/d-wave superconductor junctions, quasiparticles can constructively interfere via consecutive Andreev and normal reflections to form zero-energy resonant states on non-principal axis surfaces [1]. By using a superconducting YBa₂Cu₃O_7-δ single crystal as a scanning probe, we exploit these d-wave resonances to achieve atomic-scale Andreev reflection spectroscopy with inherent sensitivity to spin polarization. In this study, we apply our scanning d-wave Andreev reflection technique down to 300 mK and up to 0.3 T to probe impurities and superconducting vortices in the chalcogenide superconductors 2H-NbSe₂, 2H-TaS₂ and Fe(Te,Se). We observe a variety of low energy spectral peaks in our measurements, which are analyzed using a generalized BTK model for d-wave Andreev reflection [2]. These results are discussed in the context of recent studies and signatures of spin-polarized vortex bound states [3].

[1] C. R. Hu, Phys. Rev. Lett. 72 (1994).

[2] S. Kashiwaya and Y. Tanaka, Rev. Prog. Phys. 63 (2000).

[3] H.-H. Sun et al., Phys. Rev. Lett. 116 (2016).