Strong correlation behavior and strong coupling superconductivity in (Ti_1/4Hf_1/4Nb_1/4Ta_1/4)_1−<i>x</i>Ni<i> _x </i> with the rich magnetic element Ni

Abstract Searching for new superconductors, especially unconventional superconductors, has been studied extensively for decades but remains one of the major outstanding challenges in condensed matter physics. Medium/high-entropy alloys (MEAs-HEAs) are new fertile soils of unconventional superconductors and generate widespread interest and questions on the existence of superconductivity in highly disordered materials. Here, we report on the effect of Ni-doped on the crystal structure and superconductivity properties of strongly coupled TiHfNbTa MEA. X-ray diffraction results indicate that the maximum solid solution of (Ti 1/4 Hf 1/4 Nb 1/4 Ta 1/4 ) 1− x Ni x is about 7.7%. Resistivity, magnetic susceptibility, and specific heat measurements demonstrated that (Ti 1/4 Hf 1/4 Nb 1/4 Ta 1/4 ) 1− x Ni x HEAs are all bulk type-II superconductors and follow the trend of the increase of T c with the increase of Ni-doped contents. The specific heat jump ( ΔC el / γ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mtext> mid</mml:mtext> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) of all (Ti 1/4 Hf 1/4 Nb 1/4 Ta 1/4 ) 1− x Ni x are much larger than the BCS value of 1.43, suggesting all these HEAs are strongly coupled superconductors. Additionally, large Kadawaki–Woods ratio values suggest that there is a strong electron correlation effect in this system. The (Ti 1/4 Hf 1/4 Nb 1/4 Ta 1/4 ) 1− x Ni x HEA system is a new ideal material platform for the study of strong correlation behavior and strongly coupled superconductivity, which provides an insight into the physics of high-temperature superconductors or other unconventional superconductors.