Hybridization between surface flat bands and bulk bands in the topological nodal-line semimetal Sn$_{0.15}$NbSe$_{1.75}$ probed via soft-point-contact spectroscopy

Abstract

We report a detailed study of soft-point-contact spectroscopy of the superconducting topological nodal-line semimetal Sn$_{0.15}$NbSe$_{1.75}$ with the superconducting transition temperature $T_{c}=9.5$ K. In the normal state, we observe prominent asymmetric double peaks in the differential conductance $dI/dV$. The asymmetric $dI/dV$ curves are attributed to Fano resonance, quantum interference between two distinct tunneling paths of transmitting electrons into flat energy bands and dispersive bands. A phenomenological double Fano resonance model reveals the hybridization between these bands below the hybridization temperature $T_{\mathrm{hyb}}=23$ K. This hybridization drives an opening of a pseudogap below a characteristic temperature $T_{\mathrm{PG}}=6.8$ K. In the superconducting state, we observe an unusual upper critical field that increases linearly with decreasing temperatures from $0.4T_{c}$ to $0.01T_{c}$, suggestive of a possible exotic superconducting state. Our results suggest the presence of surface flat energy bands that stem from nontrivial topological nature of nodal lines in the bulk band structure and the hybridization between the surface flat bands and bulk bands in Sn$_{0.15}$NbSe$_{1.75}$.