Enhancement of the upper critical field in the cubic Laves-phase superconductor HfV2 by Nb doping

Abstract

We report the effect of Nb doping on the upper critical field of the cubic Laves-phase superconductor HfV2 studied in a series of HfV2−x Nbx samples with 0 ≤ x ≤ 0.3 under pulsed fields up to 30 T. The undoped HfV2 undergoes a martensitic structural transition around 110 K, and becomes superconducting below Tc = 9.4 K. Upon Nb doping, while the structural transition is suppressed for x ≥ 0.1, a maximum in Tc of 10.1 K and zero-temperature upper critical field Bc2(0) of 22.4 T is found at x = 0.2, which is ascribed to an increase of the density of states at the Fermi level. For all samples, the temperature dependence of Bc2 can be well described by the Werthamer–Helfand–Hohenberg theory that takes into account both the spin paramagnetic effect and spin orbit scattering. In addition, a comparison is made between the Bc2 behavior of HfV2−x Nbx and those of Nb–Ti and Nb3Sn.