Superconductivity in hexagonal Nb-Mo-Ru-Rh-Pd high-entropy alloys

We report the superconducting properties of new hexagonal Nb${}_{10+2x}$Mo${}_{35-x}$Ru${}_{35-x}$Rh${}_{10}$Pd${}_{10}$ high-entropy alloys (HEAs) (0 $\le$ $x$ $\le$ 5). With increasing $x$, the superconducting transition temperature $T_{\mathrm{c}}$ shows a maximum of 6.19 K at $x$ = 2.5, while the zero-temperature upper critical field $B_{\mathrm{c}2}$(0) increases monotonically, reaching 8.3 T at $x$ = 5. For all $x$ values, the specific heat jump deviates from the Bardeen-Cooper-Schreiffer behavior. In addition, we show that $T_{\rm c}$ of these HEAs is not determined mainly by the density of states at the Fermi level and would be enhanced by lowering the valence electron concentration.