Large variation in superconducting transition temperature in the NbxBi2−xSe3 system

Samples synthesized as Nb substituted onto Bi sites from the nominal composition of NbxBi2−xSe3 resulted in superconducting critical temperatures (Tc's) in transport measurements reaching 5.6 K and 5.4 K, for x = 0.2 and 0.25, respectively. All, significantly higher than reported in earlier studies of compositions aiming at Nb intercalation NbxBi2Se3. Bulk susceptibility showed lower responses for powders with a large variety, including Tc's ranging from 1.8 K to 4.5 K. Magnetotransport experiments were conducted with angular dependence at the High Field Magnet Laboratory (HFML). Clear Shubnikov–de Haas oscillations were observed in all substituted samples, and a thorough study of the angular dependence of the Shubnikov–de Haas frequencies is presented together with estimates of the Fermi energy (EF). All samples contain the phases of Bi2Se3 and the misfit compound (BiSe)1.1NbSe2. However, only some areas of the crystal boules contain the third phase of BiSe, and the calculated niobium contents from the nominal composition and the results from the phase analysis show large variations. The results obtained on these substituted samples are compared to the more studied Nb-doped Bi2Se3 stoichiometry, thereby giving further insight into this system currently under high interest.