Electron Energy Loss Spectroscopy of strongly correlated systems in infinite dimensions

We study the electron-energy loss spectra of strongly correlated electronic systems doped away from half-filling using dynamical mean-field theory ($d=\infty$). The formalism can be used to study the loss spectra in the optical (${\bf q=0}$) limit, where it is simply related to the optical response, and hence can be computed in an approximation-free way in $d=\infty$. We apply the general formalism to the one-band Hubbard model off $n=1$, with inclusion of site-diagonal randomness to simulate effects of doping. The interplay between the coherence induced plasmon feature and the incoherence-induced high energy continuum is explained in terms of the evolution in the local spectral density upon hole doping. Inclusion of static disorder is shown to result in qualitative changes in the low-energy features, in particular, to the overdamping of the plasmon feature, resulting in a completely incoherent response. The calculated EELS lineshapes are compared to experimentally observed EELS spectra for the normal state of the high-$T_{c}$ materials near optimal doping and good qualitative agreement is found.Comment: 5 pages, 3 figures, submitted to J. Phys. - Cond. Mat

Quasiparticle bands in cuprates by quantum chemical methods: towards an ab initio description of strong electron correlations

Realistic electronic-structure calculations for correlated Mott insulators are notoriously hard. Here we present an ab initio multiconfiguration scheme that adequately describes strong correlation effects involving Cu 3d and O 2p electrons in layered cuprates. In particular, the O 2p states giving rise to the Zhang-Rice band are explicitly considered. Renormalization effects due to nonlocal spin interactions are also treated consistently. We show that the dispersion of the lowest band observed in photoemission is reproduced with quantitative accuracy. Additionally, the evolution of the Fermi surface with doping follows directly from our ab initio data. Our results thus open a new avenue for the first-principles investigation of the electronic structure of correlated Mott insulators

Theory of Multiband Superconductivity in Iron Pnictides

The precise nature of unconventional superconductivity in Iron Pnictides is presently a hotly debated issue. Here, using insights from normal state electronic structure and symmetry arguments, we show how an unconventional SC emerges from the bad metal "normal" state. Short-ranged, multi-band spin- and charge correlations generates nodeless SC in the active planar $d_{xz,yz}$ bands, and an inter-band proximity effect induces out-of-plane gap nodes in the passive $d_{3z^{2}-r^{2}}$ band. While very good quantitative agreement with various key observations in the SC state and reconciliation with NMR and penetration depth data in the same picture are particularly attractive features of our proposal, clinching evidence would be an experimental confirmation of c-axis nodes in future work.Comment: 4 pages, 2 eps figures, submitted to PRL, text modifie