Inverse photoemission studies of the high-Tcsuperconductors

We present inverse photoemission spectra recorded from the high-$T_c$ superconductors ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and the related compound CuO. We are able to identify the various unoccupied orbitals associated with the La, Y, and Ba sites. For the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ compound we observe the unoccupied O 2p holes above the Fermi level but do not observe the equivalent holes in the La compound. The inverse photoemission spectrum from CuO shows no pronounced structure.