External losses in photoemission from strongly correlated quasi-two-dimensional solids

Expressions are derived for photoemission, which allow experimental electron energy loss data to be used for estimating losses in photoemission. The derivation builds on new results for dielectric response and mean free paths of strongly correlated systems of two-dimensional layers. Numerical evaluations are made for Bi2Sr2CaCu2O8 (Bi2212) by using a parametrized loss function. The mean free path for Bi2212 is calculated and found to be substantially larger than obtained by Norman et al. [Phys. Rev. B 59, 11 191 (1999)] in a recent paper. The photocurrent is expressed as the convolution of the intrinsic approximation for the current from a specific two-dimensional layer with an effective loss function. This effective loss function is the same as the photocurrent from a core level stripped of the dipole matrix elements. The observed current is the sum of such currents from the first few layers. The correlation within one layer is considered as a purely two-dimensional (2D) problem separate from the embedding three-dimensional (3D) environment. When the contribution to the dielectric response from electrons moving in 3D is taken as diagonal in q space, its effect is just to replace bare Coulomb potentials in the (3D) coupling between the 2D layers with dynamically screened ones. The photoelectron from a specific CuO layer is found to excite low-energy acoustic plasmon modes due to the coupling between the CuO layers. These modes give rise to an asymmetric power-law broadening of the photocurrent an isolated two-dimensional layer would have given. We define an asymmetry index where a contribution from a Luttinger line shape is additive to the contribution from our broadening function. Already the loss effect considered here gives broadening comparable to what is observed experimentally. Our theory is not related to the loss mechanism recently discussed by Joynt [R. Joynt, Science 284, 777 (1999); R. Haslinger and R. Joynt, J. Electron Spectrosc. Relat. Phenom. 117-118, 31 (2001)] which adds additional broadening beyond what we calculate. A superconductor with a gapped loss function is predicted to have a peak-dip-hump line shape similar to what has been observed, and with the same qualitative behavior as predicted in the recent work by Campuzano et al. [Phys. Rev. Lett. 83, 3709 (1999)].(35 refs)