On fermionic excitations in doped Mott systems and their observation by ARPES K. Matho * Centre de Recherches sur les Tre Ás Basses Tempe Âratures, Laboratoire Associe Âa Á l' Universite  Joseph Fourier, CNRS, BP 166, 38042 Grenoble Cedex 9, France Abstract The exact Dyson equation for a transition orbital with strong interactions, coupled with an arbitrary number of ligands, is evaluated with a phenomenological interpolation formula that combines microscopic input from the high and low energy sectors. Explicit expressions for momentum-resolved fermionic excitation spectra and photoemission spectra are derived for the case of one ligand. The two types of spectra differ by observable interference effects. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; C. Photoelectron spectroscopy; D. Electronic structure; D. Fermi surface 1. Introduction Microscopic calculations of the selfenergy Sk; v for Mott±Hubbard systems, that have been rendered metallic by doping, focus either on the generics of the lowest excita- tions near a point k F on the Fermi surface FS) and near v  0 chemical potential m as origin), or on the global electro- nic structure on a coarse energy scale, up to and beyond the correlation gap. The experimental possibilities of angle resolved photoemission spectroscopy ARPES) to bridge all these largely different scales and also gather momentum- dependent information are strikingly illustrated by a recent special issue on strongly correlated systems [1]. Accord- ingly, the need for a manybody theory, be it a phenomeno- logical one, to cast a similar bridge on the momentum resolved level, is rising in this community. Combining high and low energy input, we have developed a parameter- less interpolation scheme, which has been applied to the selfenergy of orbitally degenerate Hubbard models with repulsive interaction [2]. In the same paper, it was argued that models with one type of repulsive orbital, hybridised with any number of ligands, could also be interpolated along the same lines. The complete equations, needed to implement spectra for this class of models, are now given in this article. In Section 2, we de®ne the Dyson equation and give the explicit formula which interpolates between a broadened high energy Pade Â; approximant {l 2 1=l} of order l  2 and a strong coupling Fermi liquid as low energy scenario. When the interacting orbital has only twofold degeneracy, the order l  2 is suf®cient to keep track of all spectral features up to the high energy satellites that are due to the repulsive Hubbard U. This claim is not trivial since the system with ligands has more structure at intermediate scales, due to charge transfer and/or hybridisation phenom- ena [3]. To study sets of parameters that are consistent with momentum integrated sum rules, we need to focus ®rst on the simplest, spin degenerate models with one Hubbard like orbital and one ligand per lattice cell. In Section 3, we give the generally valid expressions for the Green functions and partial spectra A dd k; 1 and A pp k; 1 of such models. This allows us to make contact with the most prominent Hamiltonians for lattice fermions that are being studied intensely by many groups and with various approximations. In Section 4, we discuss implications of using the multi- orbital, correlated spectral functions for the analysis of ARPES data [4]. 2. Models with a scalar selfenergy function Consider periodic lattice models with single particle basis spanned by a ®nite set {l} of fermionic orbitals per cell. A single orbital from a partly ®lled d- or f- shell labelled Journal of Physics and Chemistry of Solids 63 2002) 1611±1614 0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0022-369702)00006-9 www.elsevier.com/locate/jpcs * Tel.: 133-4-76-88-78-17; fax: 133-4-76-87-50-60. E-mail address: matho@polycnrs-gre.fr K. Matho).