Detailed electronic structure studies on superconducting MgB 2 and related compounds

By P. Ravindran, P. Vajeeston, R. Vidya, A. Kjekshus, H.Fjellvaag. 2001

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Our recent electronic structure studies on series of transition metal diborides indicated that the electron phonon coupling constant is much smaller in these materials than in superconducting intermetallics. However experimental studies recently show an exceptionally large superconducting transition temperature of 40 K in MgB2. In order to understand the unexpected superconducting behavior of this compound we have made electronic structure calculations for MgB2 and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB2 compared with other superconducting intermetallics and the exceptionally high Tc in this material can be explained through BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identied a doubly-degenerate quasi-two dimensional key-energy band in the vicinity ofEF along -A direction of BZ (having equal amount of B px and py character) which play an important role in deciding the superconducting behavior of this material. Based on this result, we have searched for similar kinds of electronic feature in a series of isoelectronic compounds such as BeB2, CaB2 ,S rB 2 ,L iBC and MgB 2C2 and found that MgB2C2 is one potential material from the superconductivity point of view. We have also investigated closely related compound MgB4 and found that its EF is lying in a pseudogap with a negligibly small density of states at EF which is not favorable for superconductivity. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB2 ranging from isotropic, moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single crystal elastic constants for MgB2 by the accurate full-potential method and derived the directional dependent linear compressibility, Young’s modulus, shear modulus and relevant elastic properties from these results. We have observed large anisotropy in the elastic properties consistent with recent high-pressure measurements. Our calculated polarized optical dielectric tensor shows highly anisotropic behavior even though it possesses isotropic transport property. MgB2 possesses a mixed bonding character and this has been veried from density of states, charge density and

Subfields: Condensed Matter Physics, Materials Chemistry

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