Structural and superconducting properties of non-stoichiometric Nb1+Se2

The composition-dependent crystal and defect structures, and superconducting properties of Nb1+xSe2 (0 ≤ x ≤ 0.09) polycrystalline samples were revealed by x-ray diffraction, electrical resistivity, magnetization and specific-heat measurements. Stacking-disordered structures were characterized by gradually broadened and weakened (h0l) reflections: disordered 2 Ha structure for x ≤ 0.04 and 2Hb + 3 R mixed-layer structure for x ≥ 0.05. Superconducting transition temperature (Tc) is 7.3 K in pristine NbSe2, which decreases with increasing the content of interstitial-Nb (Nbint), down to below 2 K in x = 0.09. The lowered Tc originates from the decreased electron-phonon coupling and density of states at the Fermi level EF. Upper critical field μ0Hc2 of all samples evidently exceed the Pauli limit except for pristine NbSe2. μ0Hc2, irreversible field μ0Hirr and critical current density Jc are all raised in slight-Nbint samples compared with NbSe2. The enhancements are due to the increase in charge carrier scattering and vortex pinning by Nbint impurity and stacking disorders. These results indicate that minor nonmagnetic impurity doping could improve the current-carrying capacity under the premise of small impact on Tc, which thus is a promising strategy to develop high-field superconducting magnets.