Unraveling the Symmetry of the Hole States near the Fermi Level in the [Formula presented] Superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the [Formula presented] edge of boron in [Formula presented]. We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron [Formula presented] 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 [Formula presented] symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty [Formula presented] states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the [Formula presented] feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.