Superconductivity in the medium-entropy alloys: Nb₂TiW and Nb₂TiMo

This study describes the synthesis and characterization of Nb₂TiW and Nb₂TiMo medium-entropy alloys (MEAs). The Nb₂TiW and Nb₂TiMo MEAs can be successfully synthesized by an arc-melting method. Their structures and superconducting properties are investigated by detailed characterization of X-ray diffraction (XRD), resistivity, magnetization, and specific heat measurements. XRD results confirm that the obtained Nb₂TiW and Nb₂TiMo compounds have the same body-centered cubic (BCC) structures and crystalize in the Im\overline3m space group (number 229). Experimental results show that the superconducting transition temperatures Tcs of Nb₂TiW and Nb₂TiMo are around 4.86 K and 3.22 K, respectively. The upper and lower critical fields of Nb₂TiW are 3.52(2) T and 53.36(2) Oe, respectively, and those of Nb₂TiMo are 2.11(2) T and 68.23(3) Oe, respectively. First-principles calculations reveal that the d electrons of Nb, Ti, and Mo or W are the dominant contribution of the density of states near the Fermi level. Specific heat measurement results indicate that Nb₂TiW and Nb₂TiMo display BCS full-gap s-wave superconductivity.