Electronic excitations in electron-doped cuprate superconductors.

Abstract

We calculate the electronic single-particle spectrum of an electron-doped cuprate superconductor such as ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$. The dynamics of holes in the Cu-O planes is described by the extended Hubbard or Emery model. We consider the system at half-filling (one hole per unit cell, ${\mathit{n}}_{\mathit{h}}$=1) and in the case of electron doping where the ground state is paramagnetic. The projection technique of Mori and Zwanzig is applied to derive the equations of motion for the Green's functions of Cu and O holes. These equations are solved self-consistently as in a previous calculation, where we considered the case of hole doping. At half-filling the system exhibits a charge-transfer gap bounded by Zhang-Rice singlet states and the upper Hubbard band. Upon electron doping the upper Hubbard band crosses the Fermi level and the system becomes metallic. With increasing electron doping the singlet band loses intensity and finally vanishes for ${\mathit{n}}_{\mathit{h}}$=0. The corresponding spectral weight is transferred to the upper Hubbard band, which becomes a usual tight-binding band for zero hole concentration. The shape of the flat band crossing the Fermi level fits well to angle-resolved photoemission spectra of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ for x=0.15 and 0.22. Furthermore, our findings are in excellent agreement with exact diagonalization studies of 2\ifmmode\times\else\texttimes\fi{}2 ${\mathrm{CuO}}_{2}$ cluster with periodic boundary conditions.