Superconductivity in medium- and high-entropy alloy thin films: Impact of thickness and external pressure
G. Pristáš, Július Bačkai, Matúš Orendáč, S. Gabáni, Filip Košuth, M. Kuzmiak, P. Szabó, E. Gažo, Robert Franz, Sabrina Hirn, Georg C. Gruber, Christian Mitterer, Serhii Vorobiov, К. Flachbart
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.