Nesting Properties and Anomalous Band Effect in MgB₂

Abstract

First principle FLAPW band calculations of the new superconductor MgB₂ were performed and the polarization function Π₁₂(Q) between the two π-bands mainly formed of boron p_z-orbital was calculated. We found that Π₁₂(Q) is substantially enhanced around Q=(0, 0, π/c), which supports the two-band mechanism of superconductivity for MgB₂. Π₁₂(Q) peaks at Q_z∼ 0.3(2π/c) and Q_z ∼ 0.5(2π/c). These two peaks are related to the nesting of these Fermi surfaces, but significantly deviates from the position expected from the simplest tight-binding bands for the π-bands. From the calculations for different lattice parameters, we have found significant dependences on the isotopic species of B and on the pressure effect of the polarization function in accordance with the respective changes of T_c in the above-mentioned framework.