Role of kz-dispersion in photoemission from quasi-2D cuprates

We discuss how the effects of dispersion of electronic states with kz, which have largely been neglected in discussing the cuprates, play out in the analysis and interpretation of the angle-resolved photoemission (ARPES) spectra from quasi-two-dimensional (2D) materials. Illustrative results on Bi2Sr2CaCu2O8 (Bi2212) and La2−xSrxCuO4 (LSCO) are presented. kz-dispersion induces an irreducible width to ARPES spectral peaks. This width does not have its origin in a scattering mechanism. It is also manifest in ARPES photointensities for emission from the Fermi energy, where the Fermi surface (FS) maps so obtained display k∥-dependent widths, which can be quite large especially in the antinodal region. The first principles photointensities computed in LSCO within the local-density approximation (LDA) based band theory framework are found to reproduce many salient features of the experimental ARPES spectra, even in the underdoped regime, suggesting that some sort of a generalized Luttinger theorem continues to hold in the presence of strong electron correlations. In the lightly doped insulating LSCO, we show via tight-binding model computations that effects of kz-dispersion can give insight into characteristic broadenings observed in the ARPES features associated with the lower Hubbard band and the mid-gap states.