Analysis of the valence band photoemission spectrum of Sr${}_2$CuO${}_2$Cl${}_2$ along the high-symmetry directions

Abstract

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.

13 pages, 16 figures, 5 tables