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, 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 crit-ical fields for TiHfNbTa are 45.8 mT and 10.46 T, respectively. First-principles calculations show that the d electron of Ti, Hf, Nb, and Ta is the main contribution near the Fermi level. Our results indicate that the superconductivity is a conven-tional s-wave type with extremely strong coupling. 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.