Phonon softening and strong coupling superconductivity in metastable Sb_0.83Au_0.17

A central question in superconductivity is how a semimetal such as antimony, normally non superconducting, can host a robust superconducting state upon Au incorporation. We investigate this problem in metastable Sb_0.83Au_0.17, which crystallizes in the simple cubic α−Po–type structure stabilized through high-pressure and high-temperature synthesis. Bulk measurements of magnetic susceptibility, resistivity, and heat capacity establish type-II superconductivity with Tc ∼ 8 K with strong electron–phonon coupling (λ_ep = 0.91). The single-gap s-wave model of heat capacity yields a superconducting energy gap of ∆ = 2.02 k_BT_c, exceeding the BCS weak-coupling limit. Heat-capacity analysis reveal low-energy Einstein modes and a pronounced Boson peak in the C_P/T³ vs. T plot, highlighting phonon softening as a key factor in the enhanced coupling. We argue that the emergence of strong-coupling superconductivity is driven by the cooperative effect of soft phonons and strong electron–phonon interactions. Density-functional theory calculations further indicate a finite density of states at the Fermi level, consistent with a metallic behaviour in contrast to the semimetallic nature of pristine Sb.