Resonant photoemission study of ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$: Nature of electronic states near the Fermi level

The electronic structures of single-crystal and ceramic samples of the electron-doped superconductor ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ with x=0 and 0.15 have been studied using resonant photoemission spectro- scopy. Comparing these spectra with previous results for ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ shows that for both electron and hole doping the Fermi level lies at nearly the same energy, in states that fill in the x=0 insulator gap. We point out that this would occur if the states at the Fermi level obey a Luttinger-type sum rule, i.e., according to standard theory, if the metallic state is a normal Fermi liquid.