Calculated elastic and electronic properties of MgB 2 at high pressures

The effect of high pressure on structural and electronic properties of the novel super-conductor MgB 2 has been calculated using the full-potential linearized augmented-plane-wave method. Despite the layered crystal structure of MgB 2 nearly isotropic compression (bulk modulus B 0 = 140.1(6) GPa) is found with only a 1.2% decrease of the c/a ratio at 10 GPa. The effect of pressure on the critical temperature has been estimated on the basis of BCS theory and good agreement with experimental data is found. Our results suggest that the primary causes for the reduction of T c are a decrease of the density of electronic states at the Fermi energy and an increase of the zone-center phonon frequencies. A recent report of Akimitsu et al. [1] on superconductivity in MgB 2 with a relatively high critical temperature of T c = 39 K has raised great interest in this compound. At ambient conditions MgB 2 has a layered hexagonal crystal structure of the AlB 2 type (space group P 6/m m m, Z = 1) [2] and shows metallic conductivity [3,4]. Ab initio calculations of the electronic properties [5–7] indicate the coexistence of strong covalent bonding within the honeycomb B layers and metallic bonding between the Mg and B layers. On the basis of BCS theory, the high critical temperature was attributed to a fortunate combination of strong bonding, 'reasonable' electronic density of states at the Fermi level, and high phonon frequencies [6]. This view of phonon-mediated superconductivity is supported by the observation of a sizable boron isotope effect with ∆T c = +1.0 K for substitution of 11 B by 10 B [8]. The effect of high pressure on T c was studied by Lorenz et al. [9]. They found a linear decrease dT c / dP = −1.6 K/GPa up to 1.8 GPa. Hirsch, on the other hand, predicted on the basis of his theory of 'hole superconductivity' an increase of