Nesting Properties and Anomalous Band Effect in MgB2

First principle FLAPW band calculations of the new superconductor MgB 2 were performed and the polarization function Π 12 ( Q ) between the two π-bands mainly formed of boron p z -orbital was calculated. We found that Π 12 ( Q ) is substantially enhanced around Q =(0, 0, π/ c ), which supports the two-band mechanism of superconductivity for MgB 2 . Π 12 ( 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.