Large upper critical fields and strong coupling superconductivity in the medium-entropy alloy (Ti<sub>1/3</sub>Hf<sub>1/3</sub>Ta<sub>1/3</sub>)<sub>1−x</sub>Nb<sub>x</sub>
Longfu Li, Hongyan Tian, Xunwu Hu, Lingyong Zeng, Kuan Li, Peifeng Yu, Kangwang Wang, Rui Chen, Z. Xiang, Dao‐Xin Yao, Huixia Luo
Abstract
Abstract Since the discovery of high-entropy superconductors in 2014, superconductivity has remained a focal point of interest in medium- and high-entropy alloys (MEAs-HEAs). Here, we report a series of (Ti 1/3 Hf 1/3 Ta 1/3 ) 1− x Nb x (0 ⩽ x ⩽ 0.9) MEA superconductors crystallized in the body-centered cubic structure, whose superconductivity was characterized by resistivity, magnetization, and specific heat measurements. The study found that the (Ti 1/3 Hf 1/3 Ta 1/3 ) 1− x Nb x MEAs exhibit bulk superconductivity. With the doping of Nb, the superconducting transition temperature ( T c ) increases from 5.31 to 9.11 K, and the normalized C el jumps at T c , Δ C el / γT c , and the logarithmically averaged characteristic phonon frequency ω log exhibit dome-shaped curves. Results from specific heat measurements indicate that the superconductivity is of a strongly coupled s -wave type observed at 0 ⩽ x ⩽ 0.75. Furthermore, at low Nb content (0 ⩽ x ⩽ 0.125), the upper critical field of the samples is larger than the Pauli paramagnetic limit. The strongly coupling behavior and large upper critical field in s -wave type (Ti 1/3 Hf 1/3 Ta 1/3 ) 1− x Nb x MEA superconductors are unusual, as they typically occur in other unconventional superconductors. Thus, (Ti 1/3 Hf 1/3 Ta 1/3 ) 1− x Nb x may have significant potential in the research and understanding of physical mechanisms.