Scanning d-wave Andreev reflection probe of 2H-NbSe₂

2H-NbSe₂ has been heavily studied by scanning tunneling spectroscopy (STS) for both its multiband superconductivity and coexisting charge density waves. Recent STS advances using s-wave superconducting tips [1] have revealed pair density-wave order in its superconducting state [2]. Recent theoretical studies have also predicted ferromagnetic instability leading to singlet-triplet mixing, due to spin-orbit coupling and broken inversion symmetry [3]. We present STS measurements of 2H-NbSe₂ using a d-wave superconducting tip, a novel technique that exploits resonant d-wave Andreev reflection (AR) to achieve atomic-scale resolution with potential sensitivity to spin-polarization. Our scanning d-wave AR data were taken in the normal, superconducting and mixed state of 2H-NbSe₂, to probe bound states within vortex cores or near impurities. We discuss our results in light of the recent studies, and also address the possibility for high-impedance proximity coupling across s-wave/insulator/d-wave junctions [4].

[1]M.Randeria et al, PRB93 (2016)

[2]X.Liu et al, Science372 (2021)

[3]D.Wickramaratne et al, PRX10 (2020)

[4]Sinha and Ng, PRL80 (1998)