Evidence for Isotropic s-Wave Superconductivity in High-Entropy Alloys

High-entropy alloys (HEA) form through the random arrangement of five or more chemical elements on a crystalline lattice. Despite the significant amount of resulting compositional disorder, a subset of HEAs enters a superconducting state below critical temperatures $T_c<10$ K. The superconducting properties of the known HEAs seem to suffice a Bardeen-Cooper-Schrieffer (BCS) description, but little is known about their superconducting order parameter and the microscopic role of disorder. We report on magnetic susceptibility measurements on films of the superconducting HEA (TaNb)$_{1-x}$(ZrHfTi)$_x$ for characterizing the lower and upper critical fields $H_{c1}(T)$ and $H_{c2}(T)$, respectively as a function of temperature $T$. Our resulting analysis of the Ginzburg-Landau coherence length and penetration depth demonstrates that HEAs of this type are single-band isotropic s-wave superconductors in the dirty limit. Despite a significant difference in the elemental composition between the $x=0.35$ and $x=0.71$ films, we find that the observed $T_c$ variations cannot be explained by disorder effects.