Electric field gradients in s-, p-, and d-metal diborides and the effect of pressure on the band structure and Tc in MgB2 -: art. no. 052501

Results of full-potential linear muffin-tin orbital generalized gradient approximation calculations of the band structure and boron electric field gradients (EFG's) for the new medium-T-c superconductor MgB2 and related diborides MB2, M = Be, Al, Sc, Ti, V, Cr, Mo, and Ta are reported. The boron EFG variations are found to be related to specific features of their band structure and particularly to the M-B hybridization. The strong charge anisotropy at the B site in MgB2 is completely defined by the valence electrons-a property which sets MgB2 apart from other diborides. The boron EFG in MgB2 is weakly dependent on applied pressure: the B p-electron anisotropy increases with pressure, but it is partly compensated by the increase of core charge asymmetry. The concentration of holes in bonding sigma bands is found to decrease slightly from 0.067 to 0.062 holes/B under a pressure of 10 GPa, Despite a small decrease of N(E-F), the Hopfield parameter increases with pressure and we believe that the main reason for the reduction under pressure of the superconducting transition temperature T-c is the strong pressure dependence of phonon frequencies, which is sufficient to compensate for the electronic effects.