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 non-superconducting sister compound of the superconductor MgB₂. We perform angle-resolved photoemission measurements with variable photon energy, and compare them with density functional theory calculations, in order to disentangle the surface and bulk contributions to the measured spectra. Aluminium diboride is known to be aluminium deficient, Al_1−δB₂, which would be expected to lead to a hole doping as compared to the nominally stoichimoetric compound. Nonetheless, we find that the bulk σ states, which mediate superconductivity in MgB₂, remain more than 600meV below the Fermi level. However, we also observe σ states originating from the boron terminated surface, with an order of magnitude smaller binding energy of 70meV, and demonstrate how surface hole-doping can bring these across the Fermi level.