Optimum-doped superconductivity associated with phonon softening in vanadium-based compounds with anti-post-perovskite structure

We performed a comprehensive study of the vanadium-based anti-post-perovskite compounds V,inf>3XC (X = Ge, P, As) and V₃XN (X = Ga, Ge, P, As). It was clarified that V₃PN and V₃AsN exhibit superconducting transitions at 4.2 and 2.6 K, respectively, whereas the other compounds not show superconducting transition above 1.8 K. We further investigated substitution effects on superconductivity in V₃PN, and found that the superconducting transition temperature (T_c) shows a dome-shaped curve with maximum T_c values of 5.45K at x = 0.05 and 5.82K at x = 0.10 for V₃P₁-_xAs_xN and V₃P₁-_xSb_xN, respectively. In these optimal compositions, clear implications of the phonon softening and the enhanced electron-phonon coupling were revealed by the analysis of specific heat data. Our findings showed that structural instability is one of the important factors for tuning superconducting properties in vanadium-based superconductors with an anti-post-perovskite structure.