Angle-resolved photoemission spectroscopy of ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$

We have investigated the lowest binding-energy electronic structure of the model cuprate ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$ using angle-resolved photoemission spectroscopy. Our data from about 80 cleavages of ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$ single crystals give a comprehensive, self-consistent picture of the nature of the first electron-removal state in this model undoped ${\mathrm{CuO}}_2$-plane cuprate. First, we show a strong dependence on the polarization of the excitation light which is understandable in the context of the matrix element governing the photoemission process, which gives a state with the symmetry of a Zhang-Rice singlet. Secondly, the strong, oscillatory dependence of the intensity of the Zhang-Rice singlet on the exciting photon energy is shown to be consistent with interference effects connected with the periodicity of the crystal structure in the crystallographic c direction. Thirdly, we measured the dispersion of the first electron-removal states along $\overrightarrow{\Gamma }(\pi ,\pi )$ and $\overrightarrow{\Gamma }(\pi ,0),$ the latter being controversial in the literature, and have shown that the data are best fitted using an extended $\mathrm{tJ}$ model, and extract the relevant model parameters. An analysis of the spectral weight of the first ionization states for different excitation energies within the approach used by Leung et al. [Phys. Rev. B 56, 6320 (1997)] results in a strongly photon-energy dependent ratio between the coherent and incoherent spectral weight. The possible reasons for this observation and its physical implications are discussed.