Undamped plasmon modes and enhanced superconductivity in metal diborides

The anisotropic geometric and electronic structures of metal diborides (MB₂) leads to many unusual optical and electronic properties. Here, we present a first-principles study on the low-energy collective electronic excitations and the superconducting performance of the MB₂ (M = Be, Mg, Al, and Ca). We demonstrate the undamped cosine-like plasmon modes and the sine-like acoustic plasmons in these metal diborides. Interestingly, the energy of the acoustic plasmons shows a positive correlation with the E _2g phonon modes which are negatively correlated with the superconducting transition temperature (T _c) of the MB₂ materials. Moreover, hole doping can lower the energy of the acoustic plasmons and enhance the T _c of CaB₂ up to 48.2 K. Our work offers a theoretical guidance to regulate the plasmonic properties, as well as a new predictive factor for superconductivity. (paper)