  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.
