Strong electron-phonon coupling in Be1-xB2C2: ab initio studies

摘要

Several structures for off-stoichiometric beryllium diboride dicarbide Be1-xB2C2 have been designed, and their properties studied from first-principles density functional methods. Among the most stable phases examined, the layered hexagonal structures are shown to exhibit various features in the electronic properties and in the lattice dynamics reminiscent of the superconducting magnesium diboride and alkaline earth-intercalated graphites. For substoichiometric composition x similar to 1/3, the system is found metallic with a moderately strong electron-phonon coupling through a predominant contribution arising from high frequency streching modes modulating the sigma-bonding of the B-C network, and a weaker contribution at medium frequency range of the phonon spectra, arising from the intercalent motion coupled to the pi-bonding states. Further, anharmonicities emerging from the proximity of the Fermi level to the sigma-band edge, contributes to reduce the phonon softening hence stabilizing the structure. All these effects appear to combine favourably to produce a high temperature phonon-superconductivity.