We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the $K$ edge of boron in ${\mathrm{MgB}}_2$. We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron $p$ states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have $p_x{p}_y$ symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty $p_x{p}_y$ states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the $p_x{p}_y$ feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.

• Received 6 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.247002