E versus k relations and many body effects in the model insulating copper oxide Sr2CuO2Cl2

We report angle resolved photoemission measurements on Sr2CuO2Cl2, an insulating layered copper oxide. We find that the overall width of the highest occupied band and the position of the band maximum at (π2, π2) agree with predictions of the t-J model and other calculations based on the Hubbard model. However, in many regions of the Brillouin zone, the shape of the band differs from these predictions. A comparison of our data with spectra previously reported on metallic samples leads to new suggestions for the phenomenology of doping.