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

Abstract

We have prepared and investigated superconducting ${\mathrm{Nb}}_{67}{\mathrm{Hf}}_{11}{\mathrm{Ti}}_{11}{\mathrm{Zr}}_{11}$ and ${\mathrm{Nb}}_{35}{\mathrm{Ta}}_{35}{\mathrm{Hf}}_{10}{\mathrm{Ti}}_{10}{\mathrm{Zr}}_{10}$ 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 ${T}_{c}$ as well as the upper critical magnetic field ${B}_{c2}(0)$ decrease with decreasing thickness. Application of hydrostatic pressure up to 33 kbar on the 600-nm ${\mathrm{Nb}}_{35}{\mathrm{Ta}}_{35}{\mathrm{Hf}}_{10}{\mathrm{Ti}}_{10}{\mathrm{Zr}}_{10}$ film shows a decrease of ${T}_{c}$ with pressure, which differs from that observed on bulk sample. However, no clear ${T}_{c}$ 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 ${\mathrm{Nb}}_{67}{\mathrm{Hf}}_{11}{\mathrm{Ti}}_{11}{\mathrm{Zr}}_{11}$ and ${\mathrm{Nb}}_{35}{\mathrm{Ta}}_{35}{\mathrm{Hf}}_{10}{\mathrm{Ti}}_{10}{\mathrm{Zr}}_{10}$ films and were able to observe directly the temperature development of the superconducting energy gap $\mathrm{\ensuremath{\Delta}}(T)$ and determine the superconducting coupling strength $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.54$ and $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=4.21$, respectively, which is consistent with that of conventional $s$-wave phonon-mediated Bardeen-Cooper-Schrieffer superconductors.