Superconductivity in Medium-Entropy Alloys: Nb₂TiW and Nb₂TiMo

This paper describes the synthesis and characterization of Nb_2TiW and Nb_2TiMo medium-entropy alloys (MEAs). The Nb_2TiW and Nb_2TiMo MEAs were successfully synthesized using the arc-melting method. Their structures and superconducting properties were investigated through detailed characterization using X-ray diffraction (XRD), resistivity, magnetization, and specific heat measurements. The XRD results confirmed that the obtained Nb_2TiW and Nb_2TiMo compounds have the same body-centered cubic (BCC) structure and crystallize in the Im3m space group (number 229). Experimental results showed that the superconducting transition temperatures (T_\rm cs) of Nb_2TiW and Nb_2TiMo are approximately 4.86 and 3.22 K, respectively. The upper and lower critical fields of Nb_2TiW are 3.52(2) T and 53.36(2) Oe, respectively, and those of Nb_2TiMo are 2.11(2) T and 68.23(3) Oe, respectively. First-principles calculations revealed that the d electrons of Nb, Ti, and Mo or W are the dominant contributors to the density of states near the Fermi level. Specific heat measurement results indicated that Nb_2TiW and Nb_2TiMo exhibit BCS full-gap s-wave superconductivity.