Detailed electronic structure studies on superconducting MgB2 and related compounds

Abstract

In order to understand the unexpected superconducting behavior of MgB2 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 its exceptionally higher Tc can be explained through a BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identified a doubly degenerate quasi-two-dimensional key-energy band in the vicinity of EF along the Γ–A direction of BZ (having equal amount of B px and py character) which plays an important role in deciding the superconducting behavior. Based on this result, we have searched for a similar electronic feature in isoelectronic compounds such as BeB2, CaB2, SrB2, LiBC, and MgB2C2 and found that hole doped LiBC and MgB2C2 are potential superconducting materials. We have found that EF in the closely related compound MgB4 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 to 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 findings. 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 verified from density of states, charge density, and crystal orbital Hamiltonian population analyses.

Source-reported materials — not catalogue approval

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