We investigate superconductivity in the candidate topological superconductor ${\mathrm{Nb}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ from $x=0$ to 1. We observe superconductivity for all $x>0$ with a superconducting transition temperature $T_C\sim$ 3 K similar to that reported in Cu- and Sr-intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Structurally, for small $x$ we find introduction of Nb modifies the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ separation while for larger $x$ the system is driven to the BiSe structure. While all compounds are found to be superconducting, two distinct behaviors for the upper critical field $H_{c2}$ are observed. From photoemission spectroscopy we find that the enhanced $H_{c2}$ corresponds to compounds with a low surface Dirac point energy ($-0.67$ eV) relative to the Fermi level; in absolute scale this value is lower than previously reported superconductors in this materials family. We comment on the utility of this system for the study of topological superconductivity in a new structure type combining a doping (${\mathrm{Bi}}_2$ type) layer and layers with topologically protected gapless state (${\mathrm{Bi}}_2{\mathrm{Se}}_3$ type).

• Received 2 June 2016
• Revised 12 November 2016

DOI:https://doi.org/10.1103/PhysRevB.95.180503