Superconductivity in Mg-Doped Layered Intermetallic Compound NbB₂

We have performed low temperature resistivity ρ(T) and specific heat C(T) measurements on a superconducting polycrystalline Nb_0.75Mg_0.25B₂ sample.
The results indicate that the superconducting transition temperature is ~4.6K. The zero temperature upper critical field determined from the resistivity and specific heat is 3123Oe. The electronic coefficient of specific heat γ_n=4.51mJmol^-1K² and the Debye temperature θ_D=419K are obtained by fitting the zero-field specific heat data in the normal state. At low temperatures, the electronic specific heat in the superconducting state follows C_esθ_nT_c=2.84exp(1.21T_c/T). This indicates that the superconducting pairing in
Nb_0.75Mg_0.25B₂ has s-wave symmetry.