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

We report the effect of Nb doping on the upper critical field of the cubic Laves-phase superconductor HfV₂ studied in a series of HfV_2-xNb_x 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 T_c = 9.4 K. Upon Nb doping, while the structural transition is suppressed for x ≥ 0.1, a maximum in T_c of 10.1 K and zero-temperature upper critical field B_c2(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 B_c2 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 B_c2 behavior of HfV_2-xNb_x and those of Nb-Ti and Nb₃Sn.