Dispersive single-particle excitations in high-Tc copper oxides

For a Cu4O8 cluster with periodic boundary conditions the single-particle excitation spectrum is calculated rigorously by a numerical method with use of a realistic parameter set. The spectrum shows dispersive character at low energies as well as little dispersion at high excitation energies in accordance with experiments using angle-resolved photoemission. The d8 final state is identified and its energy is found to be close to its experimental one.