We propose that the upper Hubbard band (electronlike) and the Zhang-Rice singlet band (holelike) are two essential components in describing low-energy excitations of electron-doped cuprate superconductors. We find that the gap between these two bands is significantly smaller than the charge-transfer gap measured by optics and is further reduced upon doping. This indicates that the charge fluctuation is strong and the system is in the intermediate correlation regime. A two-band model is derived.

In solid-state physics two different paradigms are typically applied. The first is a local picture, in which one visualizes the quantum states of electrons in atomic orbitals or at impurity atoms in real space (r-space). The second is the momentum or reciprocal space (k-space) picture, where electrons are viewed as de Broglie waves completely delocalized throughout the material.