Enhancement of the upper critical field in the cubic Laves-phase superconductor HfV$_{2}$ by Nb doping

TL;DR

Nb doping in HfV2 enhances the upper critical field and superconducting transition temperature, with the maximum critical field observed at x=0.2, attributed to increased density of states at the Fermi level.

Contribution

This study demonstrates how Nb doping suppresses structural transitions and enhances superconducting properties in HfV2, providing insights into tuning critical fields in cubic Laves-phase superconductors.

Findings

Maximum T_c of 10.1 K at x=0.2

Upper critical field B_c2(0) of 22.4 T at x=0.2

B_c2(T) fits WHH theory including spin effects

Abstract

We report the effect of Nb doping on the upper critical field of the cubic Laves-phase superconductor HfV$_{2}$ studied in a series of HfV$_{2-x}$Nb$_{x}$ samples with 0 $\leq$ $x$ $\leq$ 0.3 under pulsed fields up to 30 T. The undoped HfV$_{2}$ undergoes a martensitic structural transition around 110 K, and becomes superconducting below $T_{\rm c}$ = 9.4 K. Upon Nb doping, while the structural transition is suppressed for $x$ $\geq$ 0.1, a maximum in $T_{\rm c}$ of 10.1 K and zero-temperature upper critical field $B_{\rm c2}$(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 $B_{\rm c2}$ can be well described by the Werthamer-Helfand-Hohenberg (WHH) theory that takes into account both the spin paramagnetic effect and spin orbit scattering. In addition, a comparison is made between the $B_{\rm c2}$ behavior of HfV$_{2-x}$Nb$_{x}$ and those of Nb-Ti and Nb$_{3}$Sn.