Analysis of the valence-band photoemission spectrum ofSr2CuO2Cl2along the high-symmetry directions

Band structure calculations have been used to identify the different bands contributing to the polarization-dependent photoemission spectra of the undoped model cuprate ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$ at the high-symmetry points of the ${\mathrm{CuO}}_2$ plane $\Gamma ,$ $(\pi /a,0)$ and $(\pi /a,\pi /a)$ and along the high-symmetry directions $\Gamma -(\pi /a,\pi /a)$ and $\Gamma -(\pi /a,0)$. Results from calculations within the local density approximation (LDA) have been compared with calculations taking into account the strong electron correlations by LDA+U, with the result that the experimental order of energy levels at the high-symmetry points is better described by the LDA+U calculation than by the simple LDA. All the main peaks in the photoemission spectra at the high symmetry points could be assigned to different Cu $3d$ and O $2p$ orbitals which we have classified according to their point symmetries. The dispersions along the high-symmetry directions were compared with an 11-band tight-binding model which was fitted both to the LDA+U band structure calculation and the angle-resolved photoemission data. The mean field treatment successfully describes the oxygen derived bands but shows discrepancies for the copper ones.