Probing two- and three-dimensional electrons inMgB2with soft x-ray angle-resolved photoemission

Abstract

The electronic band structure of ${\mathrm{MgB}}_{2}$ involves a unique combination of two- and three-dimensional (3D) electrons derived from the boron $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ states, respectively. We have mapped out the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands over the complete Brillouin zone, including the full disconnected Fermi surface, using high-resolution soft x-ray angle-resolved photoelectron spectroscopy. The measured band structure, which is closely related to that of graphene, is in overall good agreement with the density functional theory--general gradient approximation (DFT-GGA), though differences in Fermi surface volume are seen. Surprisingly, the measured bands are wider than calculated, by $\ensuremath{\sim}8$% for the $\ensuremath{\sigma}$ and $\ensuremath{\sim}10$--15% for the $\ensuremath{\pi}$ bands. This solves the long-standing challenge of establishing the full 3D electronic structure of the model compound ${\mathrm{MgB}}_{2}$, and it demonstrates the tendency of DFT-GGA to overestimate the band narrowing due to exchange correlations effects.