Superconductivity in Nb ₄MSi (M=Ni, Co, and Fe) with a quasi-two-dimensional Nb network

The CuAl ₂-type Nb ₄MSi (M = Ni, Co, and Fe) compound is composed of a quasi-two-dimensional Nb network. We found that Nb ₄NiSi with a space group of P4/mcc is a bulk type-II superconductor (lower critical field value of 10.7 mT and upper critical field value of 1.89 T), with the highest critical temperature (T _c of 7.7 K) among these compounds. The electron-phonon coupling constant of ∼0.6 and Debye temperature of 415 K are estimated from the results of heat capacity measurements, and the density functional theory calculation indicates that the electronic density of states around the Fermi level is primarily composed of Nb 4d orbitals. We report the relationship between T _c and Debye temperature in the low-dimensional Nb-based system and attribute the relatively high T _c in Nb ₄NiSi to the higher Debye temperature originating from the shortest Nb-Nb distance in the Nb network.