Preparation and Characterization of High-Entropy Alloy (TaNb)$_{1-x}$(ZrHfTi)$_x$ Superconducting Films

Abstract: We report on the preparation and the physical properties of superconducting (TaNb)_1-x(ZrHfTi)_x high-entropy alloy films. The films were prepared by means of magnetron sputtering at room temperature, with x ranging from 0 to 1 with an average thickness of 600 - 950 nm. All films crystallize in a pseudo body-centered cubic (BCC) structure. For samples with x < 0.65, the normal-state properties are metallic, while for x > 0.65 the films are weakly insulating. The transition from metallic to weakly insulating occurs right at the near-equimolar stoichiometry. We find all films, except for x = 0 or 1, to become superconducting at low temperatures, and we interpret their superconducting properties within the Bardeen-Cooper-Schrieffer (BCS) framework. The highest transition temperature T_c = 6.9 K of the solid solution is observed for x = 0.43. The highest upper-critical field B_c2(0) = 11.05 T is found for the near-equimolar ratio x = 0.65, where the mixing entropy is the largest. The superconducting parameters derived for all the films from transport measurements are found to be close to those that are reported for amorphous superconductors. Our results indicate that these films of high-entropy alloys are promising candidates for superconducting device fabrication.