Inverted V-shaped evolution of superconducting temperature in SrBC under pressure

Based on density functional first-principles calculations and anisotropic Eliashberg equations, we have investigated the electronic structure, lattice dynamics, and phonon-mediated superconductivity in newly synthesized layered compound SrBC under pressure. Different from LiBC and MgB₂, our calculations surprisingly reveal that SrBC is isotropic in compressibility, due to the accumulation of substantial electrons in the interstitial region. We find that the Sr phonons strongly couple with B-2p_z orbital and the interstitial states, giving rise to a two-gap superconductivity in SrBC, whose transition temperature shows an inverted V-shaped dependence on pressure. The maximal transition temperature is about 22 K at 50 GPa. On both sides of 50 GPa, the transition temperature exhibits quasi-linear variation with positive and negative slopes, respectively. Such a variation of transition temperature is infrequent among phonon-mediated superconductors. The competition between enhanced electron-phonon matrix element and hardened phonons plays an essential role in governing the behavior of the critical temperature.