In this thesis we are going to study the properties of strongly correlated electron systems (SCES) using analytical, phenomenological and computational methods. In the introductory chapter we will discuss generally the problem of strongly correlated electron systems and the methods that are used to analyse their behavior. In chapter 2, we discuss the derivation of low energy effective theories by using perturbation theory. In chapter 3, we point out potential problems with perturbative techniques used to construct low energy theories for SCES. In chapter 4, we introduce an important class of materials, namely the cobaltates NaxCoO2 and we discuss the experiments that establish their strongly correlated electron nature. The cobaltates exhibit a novel insulating state at x = 0.5 which corresponds to a 3/4 filled band. This state is analyzed phenomenologically in chapter 5. In chapter 6, we introduce a non perturbative technique appropriate for strongly correlated electron systems, the Dynamical Mean Field Theory (DMFT) and its cluster extension. The results of the application of cluster DMFT on the cobaltates are presented in chapter 7.In this thesis we are going to study the properties of strongly correlated electron systems (SCES) using analytical, phenomenological and computational methods. In the introductory chapter we will discuss generally the problem of strongly correlated electron systems and the methods that are used to analyse their behavior. In chapter 2, we discuss the derivation of low energy effective theories by using perturbation theory. In chapter 3, we point out potential problems with perturbative techniques used to construct low energy theories for SCES. In chapter 4, we introduce an important class of materials, namely the cobaltates NaxCoO2 and we discuss the experiments that establish their strongly correlated electron nature. The cobaltates exhibit a novel insulating state at x = 0.5 which corresponds to a 3/4 filled band. This state is analyzed phenomenologically in chapter 5. In chapter 6, we introduce a non perturbative technique appropriate for strongly correlated electron systems, the Dynamical Mean Field Theory (DMFT) and its cluster extension. The results of the application of cluster DMFT on the cobaltates are presented in chapter 7.

Jan 1, 20080 citations

New Theoretical Approaches to Strongly Correlated Systems

Springer eBooksJan 1, 200161 citations

Cluster perturbation theory for Hubbard models

David Sénéchal, Danny Pérez, Dany Plouffe

Cluster perturbation theory is a technique for calculating the spectral weight of Hubbard models of strongly correlated electrons, which combines exact diagonalizations on small clusters with strong-coupling perturbation theory at leading order. It is exact in both the strong- and weak-coupling limits and provides a good approximation to the spectral function at any wave vector. Following the paper by S\'en\'echal et al. [Phys. Rev. Lett. $84,$ 522 (2000)], we provide a more complete description and derivation of the method. We illustrate some of its capabilities, in particular regarding the effect of doping, the calculation of ground state energy and double occupancy, the disappearance of the Fermi surface in the $t\ensuremath{-}{t}^{\ensuremath{'}}$ Hubbard model, and so on. The method is applicable to any model with on-site repulsion only.Cluster perturbation theory is a technique for calculating the spectral weight of Hubbard models of strongly correlated electrons, which combines exact diagonalizations on small clusters with strong-coupling perturbation theory at leading order. It is exact in both the strong- and weak-coupling limits and provides a good approximation to the spectral function at any wave vector. Following the paper by S\'en\'echal et al. [Phys. Rev. Lett. $84,$ 522 (2000)], we provide a more complete description and derivation of the method. We illustrate some of its capabilities, in particular regarding the effect of doping, the calculation of ground state energy and double occupancy, the disappearance of the Fermi surface in the $t\ensuremath{-}{t}^{\ensuremath{'}}$ Hubbard model, and so on. The method is applicable to any model with on-site repulsion only.

arXiv:cond-mat/0205044Physical review. B, Condensed matterAug 30, 2002238 citations

Strongly Correlated Systems : Theoretical Methods

Adolfo Avella, Ferdinando Mancini

Jan 1, 201294 citations

Quantum periodic cluster methods for strongly correlated electron systems

Tran Minh-Tien

Quantum periodic cluster methods for strongly correlated electron systems are reformulated and developed. The reformulation and development are based on a canonical transformation which periodizes the fermions in the cluster space. The dynamical cluster approximation and the cellular dynamical mean field theory are related each other through the canonical transformation. A cluster perturbation theory with periodic boundary conditions is developed. It is found that the periodic cluster perturbation theory converges rapidly with corrections $\mathcal{O}(1/L_c^2)$ , where $L_c$ is the linear size of the clusters, whereas the ordinary cluster perturbation theory converges with corrections $\mathcal{O}(1/L_c)$ .Quantum periodic cluster methods for strongly correlated electron systems are reformulated and developed. The reformulation and development are based on a canonical transformation which periodizes the fermions in the cluster space. The dynamical cluster approximation and the cellular dynamical mean field theory are related each other through the canonical transformation. A cluster perturbation theory with periodic boundary conditions is developed. It is found that the periodic cluster perturbation theory converges rapidly with corrections $\mathcal{O}(1/L_c^2)$, where $L_c$ is the linear size of the clusters, whereas the ordinary cluster perturbation theory converges with corrections $\mathcal{O}(1/L_c)$.

arXiv:cond-mat/0602615Feb 27, 20060 citations

84,

522 (2000)], we provide a more complete description and derivation of the method. We illustrate some of its capabilities, in particular regarding the effect of doping, the calculation of ground state energy and double occupancy, the disappearance of the Fermi surface in the $t\ensuremath{-}{t}^{\ensuremath{'}}$ Hubbard model, and so on. The method is applicable to any model with on-site repulsion only.

\mathcal{O}(1/L_c^2)

, where

L_c

is the linear size of the clusters, whereas the ordinary cluster perturbation theory converges with corrections

\mathcal{O}(1/L_c)