Low-temperature specific heat of the superconductor Mo 3Sb 7

Abstract

The low-temperature specific heat of a superconductor Mo 3Sb 7 with T c = 2.2 ± 0.05 K has been measured in magnetic fields up to 5 T. In the normal state, the electronic specific heat coefficient γ n, and the Debye temperature Θ D are found to be 34.5(2) mJ mol −1 K −2 and 283(5) K, respectively. The enhanced γ n value is interpreted as due to a narrow Mo-4d band pinned at the Fermi level. The electronic specific heat in the superconducting state can be analyzed in terms a phenomenological two BCS-like gap model with the gap widths 2Δ 1/ k B T c = 4.0 and 2Δ 2/ k B T c = 2.5, and relative weights of the molar electronic heat coefficients γ 1/ γ n = 0.7 and γ 2/ γ n = 0.3. Some characteristic thermodynamic parameters for the studied superconductor, like the specific heat jump at T c, Δ C( T c)/ γ n T c, the electron–phonon coupling constant, λ e−ph, the upper H c2 and thermodynamic critical H c0 fields, the penetration depth λ, coherence length ξ and the Ginzburg–Landau parameter κ are evaluated. The estimated values of parameters such as 2Δ 0/ k B T c, Δ C( T c)/ γ n T c, N( E F) and λ e−ph suggest that Mo 3Sb 7 belongs to an intermediate-coupling regime. The electronic band structure calculations indicate that the density of states near the Fermi level is formed mainly by the Mo-4d orbitals and that there is no overlap between the Mo-4d and Sb-sp orbitals.