Low-energy excitations and non-BCS superconductivity in Nbx−Bi2Se3

When certain elemental metals such as Cu, Sr, and Nb are intercalated between the layers of ${\mathrm{Bi}}₂{\mathrm{Se}}₃$, a topological insulator, the intercalated systems superconduct with critical temperatures around 3 K. Naturally, in all these cases the possibility of topological superconductivity was suggested and explored. However, in cases of Cu and Sr intercalated systems, the low-temperature scanning tunneling microscopy (STM) experiments revealed fully formed gaps where no signature of low-energy states, a requisite for topological superconductivity, was found. Here, through STM spectroscopy down to 400 mK we show that in ${\mathrm{Nb}}_x\text{\ensuremath{-}}{\mathrm{Bi}}₂{\mathrm{Se}}₃$ ($x=0.25$), the spectra deviate from a BCS-like behavior, and the tunneling conductance at low bias is large. Our observations are consistent with the idea that the order parameter of ${\mathrm{Nb}}_x\text{\ensuremath{-}}{\mathrm{Bi}}₂{\mathrm{Se}}₃$ is nodal. Therefore, our results conclude that compared with other members of the family, ${\mathrm{Nb}}_x\text{\ensuremath{-}}{\mathrm{Bi}}₂{\mathrm{Se}}₃$ has a stronger possibility of being a topological superconductor.