Enhancement of critical current density and critical magnetic field of superconducting medium entropy alloy Nb2/5Hf1/5Zr1/5Ti1/5

We investigated the superconducting properties of medium entropy alloys (MEA) Nb2/5Hf1/5Zr1/5Ti1/5, synthesized by arc melting (AM) and powder metallurgical spark plasma sintering (SPS) methods. The samples exhibit superconducting transitions at Tc=7.99 K (AM) and 6.63 K (SPS) from the electrical resistivity measurements. The upper critical field at zero-temperature limit μoHc2(0) gives a relatively high value of 14.63 T (AM) and 11.68 T (SPS). The isothermal magnetic hysteresis of the SPS-MEA shows significantly enhanced vortex pinning, resulting in the enhancement of critical current density (Jc= 39,000 A/cm2) compared with the AM-MEA (Jc= 5,500 A/cm2) at 2 K (about seven times enhancement). The SPS-MEA shows a significant suppression of vortex avalanches compared with the previously reported SPS-HEA Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 superconductor. The SPS-MEA showed the secondary fishtail effect at a high magnetic field. There was a sizable specific heat jump ΔC(Tc)/γTc (3.3 for AM and 3.9 for SPS), significantly higher than the value of BCS prediction (1.43). Superconducting properties of MEA suggest the intermediate coupling of the electron-phonon coupling constant λ∼1 (λel−ph≃0.89 for AM and λel−ph≃0.78 for SPS) and the intermediate to strong coupling regime. The Kadowaki-Woods plot of the compounds is located on the line of the heavy fermion system. The Uemura plot indicates that the compounds are close to the conventional BCS superconductor. The contradictory property of strongly correlated and conventional BCS-type metallic superconductivity may come from the dirty superconducting regime with high Fermi velocity. Enhancing critical current density with significantly suppressed vortex avalanches on the SPS-MEA Nb2/5Hf1/5Zr1/5Ti1/5 compound suggests possible practical applications on the bulk superconductivity.