Detailed electronic structure studies on superconductingMgB2and related compounds

Abstract

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 MgB 2 . In order to understand the unexpected superconducting behavior of this compound we have made electronic structure calculations for MgB 2 and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB 2 compared with other superconducting intermetallics and the exceptionally high T c in this material can be explained through 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 E F along Γ-A direction of BZ (having equal amount of B p x and p y 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 BeB 2 , CaB 2 , SrB 2 , LiBC and MgB 2 C 2 and found that MgB 2 C 2 is one potential material from the superconductivity point of view. We have also investigated closely related compound MgB 4 and found that its E F is lying in a pseudogap with a negligibly small density of states at E F which is not favorable for superconductivity. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB 2 ranging from isotropic, moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single crystal elastic constants for MgB 2 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. MgB 2 possesses a mixed bonding character and this has been verified from density of states, charge density and

Anisotropic optical conductivity and two colors ofMgB2

“…20 (Fig.5) on the base of the full-potential LAPW method, 32 and computed the corresponding dielectric function ǫ IB 1 (ω) by the KK transformation. The plasma frequencies reported by different groups 30,31,32 are also close to each other. We used the values ω p,aσ = 4.14 eV, ω p,aπ = 5.89 eV, ω p,cσ = 0.68 eV and ω p,cπ = 6.85 eV.…”

Structural Arrangements and Bonding Analysis of MgB2C2

“…The topmost valence band is separated from the bottom-most conduction band by an indirect band gap of 2.5 eV. It is apparent from the occurrence of the gap that the material is an insulator which is in agreement with earlier studies of Ravindran et al 11 and Harima. 12 The 2px and 2py orbitals of B and C mainly contribute to the density of states at the top of the valence band, whereas Mg provides very little contribution to the total DOS at this energy.…”

Comparative study of the electronic structure of alkaline-earth borides (MeB2; Me=Mg, Al, Zr, Nb, and Ta) and their normal-state conductivity

“…Similar to graphite, MgB 2 with different B-B in-plane and inter-plane distances should exhibit a strong anisotropy in the thermal, mechanic and electronic properties. Electronic structure calculations have showed that the inter-plane, Mg-B bonding, could be considered small as compared to the main contribution to chemical bond that has σ-character derived from the B:p x +p y electrons (this was also found by P. Ravindran et al 11 ). The in-plane boron bonds are covalent, while the inter-plane Mg-B bond is strongly ionic.…”