Analysis of the valence band photoemission spectrum of Sr₂CuO₂Cl₂ along the high-symmetry directions

Band structure calculations have been used to identify the different bands contributing to the polarisation-dependent photoemission spectra of the undoped model cuprate Sr$_2$CuO$_2$Cl$_2$ at the high-symmetry points of the 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 3$d$ and O 2$p$ 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.