Hopping mechanism for superconductivity revealed by Density Functional Theory

Abstract

Electronic band structures of MgB2 show that sigma bands along the G-A reciprocal direction (parallel to the c-axis) in combination with electron-phonon coupling to the E2g mode effect charge transfer. DFT calculation shows that the cosine-shaped sigma band along G-A reveals energy asymmetry matching the superconducting gap. Sigma bands parallel to the G-A direction calculated at intervals along the G-M direction show band structure unlike that at high symmetry nodes. For example, bands in close proximity to EF do not emulate the degeneracy of bands along G-A. Near EF, bands split and align favourably for electron-hole pairing with the nodal inflection point located at EF. With increased isotropic pressure, changes to sigma bands near EF occur with increased intersections of folded Fermi-surfaces and band crossings. Band intersections that define the fraction of coherent nesting at pressure show equivalent estimates for Tc determined by experiment and by the Kohn phonon anomaly. Tight-binding equations, including corrections to describe the observed asymmetry of a cosine-shaped band, show that a hopping mechanism is associated with cosine band asymmetry, the superconducting gap and Fermi surface nesting. Cosine band asymmetry may be inimical to superconductivity mechanisms in multi-element compounds.