Calculation of Photoemission Spectra for the t-J Model and the Extended Hubbard Model

Abstract

There are a wide variety of experimental probes of condensed systems (XPS, UPS, BIS, EELS, etc.) ^1,2 for which a theoretical description may be formulated simply in terms of a single-particle (-hole) propagator. In the particular case of transition metal oxides, these measured excitation spectra cannot be understood fully using conventionally calculated band structures. These discrepancies may however be understood to arise from correlation effects which are not properly treated in single particle band structure calculations. These features may most simply and successfully be handled within an impurity model ³, where a single metal ion coordinated by the appropriate ligands models the crystal. This approach has the advantage of being simple enough to allow for a realistic treatment of many atomic orbitals with crystal-field effects etc. On the other hand, there are interesting questions concerning for example quasiparticle dispersion, i.e. width of bands, which cannot readily be considered within the impurity approach. The exact diagonalization of finite clusters with periodic boundary conditions presented here may be considered as a step from the impurity problem toward the crystal.