Universality of the electronic structure from a half-filled CuO2 plane

We present angle-resolved photoemission spectroscopy data from insulating A2CuO2X2 (A5Sr, Ca; X 5Cl, Br!, Sr2Cu3O4Cl2, and Bi2Sr2MCu2O8 (M5Er, Dy! single crystals, which illustrate that the low-energy electronic structure of the half-filled cuprates are independent of the apical atom. By performing a photon energy dependent study on Ca2CuO2Cl2, we are able to distinguish between features which are intrinsic and those which are a result of the photoemission matrix elements. We find that the dispersion is independent of photon energy, while an asymmetry in the electron occupation probability across the antiferromagnetic zone boundary is robust to variations in photon energy. Finally, we show that the d-wave-like dispersion which exists in the insulator along the antiferromagnetic zone boundary does not precisely fit the simple d-wave functional form near the nodal direction.