A sharp change of Tc in polycrystalline NbxTa1–xCy and nanocrystals encapsulated in multiwall carbon nanocages

Magnetization measurements for superconducting polycrystalline NbxTa1–xCy (0 ⩽ x ⩽ 1, y < 1) with the NaCl-type structure and nanocrystals encapsulated in multiwall carbon nanocages are reported. Tc for polycrystals shows a sharp change from a value of 9.80 ± 0.02 to 11.1 ± 0.1 K near the lattice parameter of a0 = 0.44630 ± 0.00010 nm. This value of a0 is in good agreement with the lattice parameter calculated for the ideally stoichiometric crystal of Nb0.5Ta0.5C1.0. Hc2(t = T/Tc = 0.4 and 0.7) shows no distinct change around the position at which the sharp change of Tc occurs. The result suggests that a material parameter z/ℓvF compensates for the sharp change, where z is the gap ratio, ℓ is the electron mean free path, and vF is the Fermi velocity of electrons.