Extremely strong coupling s-wave superconductivity in the medium-entropy alloy TiHfNbTa

Here we report a TiHfNbTa bulk medium-entropy alloy (MEA) superconductor crystallized in the body-centered cubic structure with the unit cell parameter a=3.35925 Å, which is synthesized by an arc melting method. Superconducting properties of the TiHfNbTa are studied by employing magnetic susceptibility, resistivity, and specific heat measurements. Experimental results show a bulk superconducting transition temperature (Tc) of around 6.75 K. The lower and upper critical fields for TiHfNbTa are 45.8 mT and 10.46 T, respectively. First-principles calculations show that the d electrons of Ti, Hf, Nb, and Ta are the main contribution to the total density of states near the Fermi level. Our results indicate that the superconductivity is a conventional s-wave type with extremely strong coupling (ΔCel/γnTc=2.88, 2Δ0/kBTc=5.02, and λep=2.77). The extremely strong coupling behavior in the s-wave type TiHfNbTa MEA superconductor is unusual because it generally happens in cuprates, pnictides, and other unconventional superconductors.