Surface and bulk electronic structure of aluminium diboride

We report a combined experimental and theoretical study of the surface and bulk electronic structure of aluminium diboride, a nonsuperconducting sister compound of the superconductor ${\mathrm{MgB}}₂$. We perform angle-resolved photoemission measurements with variable photon energy, and compare them to density functional theory calculations to disentangle the surface and bulk contributions to the measured spectra. Aluminium diboride is known to be aluminium deficient, ${\mathrm{Al}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{B}}₂$, which would be expected to lead to a hole doping as compared to the nominally stoichimoetric compound. Nonetheless, we find that the bulk $\ensuremath{\sigma}$ states, which mediate superconductivity in ${\mathrm{MgB}}₂$, remain more than $600\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ below the Fermi level. However, we also observe $\ensuremath{\sigma}$ states originating from the boron terminated surface, with an order of magnitude smaller binding energy of $70\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, and demonstrate how surface hole-doping can bring these across the Fermi level.