Scanning d-wave Andreev Reflection Spectroscopy of Vortex Bound States in Superconducting Chalcogenides

Andreev reflection is an electron transmission process that converts quasiparticles to Cooper pairs near metal-superconductor interfaces. In the case of d-wave superconductors, quasiparticles can constructively interfere via consecutive Andreev and normal reflections for very-high junction impedances, to form zero-energy resonant states on non-principal axis surfaces [1]. 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, our scanning Andreev reflection technique is applied to probe vortex cores in the chalcogenide superconductors 2H-NbSe₂, 2H-TaS₂ and Fe(Se,Te). Our measurements are made down to 300 mK and up to 0.3 T, and show a variety of low energy spectral peaks in the vortex cores. These results are analyzed using the generalized BTK model [2] for d-wave Andreev reflection and in terms of signatures associated with topological superconductivity and 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)