Inverse photoemission studies of the empty electronic states and surface stability of La1.85Sr0.15CuO4

Abstract

The energy distribution of the unoccupied electronic states of the high-$T_c$ superconductor ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$ has been identified at room temperature using inverse photoemission. The density of states is very low from $E_F$ to ∼2.5 eV above $E_F$, strong emission features appear 5.8 and 8.7 above the Fermi level $E_F$, and the empty-state spectrum is unstructured thereafter until at least 25 eV. The prominent empty-state features are confined to an energy interval of ≈8 eV and are associated with the La $5d$ and $4f$ levels. The ratio of $4f$ intensity to $5d$ intensity showed a ≈fivefold increase for incident electron energies of ≈1500 eV compared to ≈30 eV. Our empty-state studies showed that the effects of high-energy electron-stimulated desorption were most pronounced ≈1.6 eV above $E_F$, and the exposure of the sample to oxygen showed enhancement of these states.