Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and T$_c$ in MgB$_2$

Details Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and T$_c$ in MgB$_2$ Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and T$_c$ in MgB$_2$

2001-04-18

arxiv.org/abs/cond-mat/0104346v2

Phys.Rev.B, vol. 65, p.052501 (2002)

Physics

Condensed Matter

Superconductivity

12 pages, 3 figures

Scientific paper

10.1103/PhysRevB.65.052501

Results of FLMTO-GGA (full-potential linear muffin-tin orbital -- generalized gradient approximation) calculations of the band structure and boron electric field gradients (EFG) for the new medium-T$_c$ superconductor (MTSC), MgB$_2$, and related diborides MB$_2$, 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 MgB$_2$ is completely defined by the valence electrons - a property which sets MgB$_2$ apart from other diborides. The boron EFG in MgB$_2$ is weakly dependent of applied pressure: the B p electron anisotropy increases with pressure, but it is partly compensated by the increase of core charge assymetry. 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 the electronic effects.

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