Superconductivity in medium- and high-entropy alloy thin films: Impact of thickness and external pressure

We have prepared and investigated superconducting Nb67Hf11Ti11Zr11 and Nb35Ta35Hf10Ti10Zr10 medium- and high-entropy alloys in form of thin films with thicknesses of 600, 100, and 30 nm, and compared their properties with bulk counterparts. We show that the superconducting transition temperature Tc as well as the upper critical magnetic field Bc2(0) decrease with decreasing thickness. Application of hydrostatic pressure up to 33 kbar on the 600-nm Nb35Ta35Hf10Ti10Zr10 film shows a decrease of Tc with pressure, which differs from that observed on bulk sample. However, no clear Tc dependence was observed if pressure was applied on the 100-nm film. This result is most likely related to increasing disorder (tendency to structure amorphization) in thinner films. Moreover, we performed point-contact spectroscopy measurements on the 600-nm Nb67Hf11Ti11Zr11 and Nb35Ta35Hf10Ti10Zr10 films and were able to observe directly the temperature development of the superconducting energy gap Δ(T) and determine the superconducting coupling strength 2Δ/kBTc=3.54 and 2Δ/kBTc=4.21, respectively, which is consistent with that of conventional s-wave phonon-mediated Bardeen-Cooper-Schrieffer superconductors.