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

TL;DR

This study reports a medium-entropy alloy TiHfNbTa that exhibits conventional s-wave superconductivity with an unusually strong coupling strength, demonstrated through experimental and theoretical analyses.

Contribution

It is the first to identify extremely strong coupling s-wave superconductivity in a medium-entropy alloy, expanding understanding of superconductivity in complex alloys.

Findings

Superconducting transition temperature (Tc) is around 6.75 K.

Critical magnetic fields are 45.8 mT (lower) and 10.46 T (upper).

Superconductivity is conventional s-wave with strong coupling.

Abstract

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.