59 research outputs found
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
The one dimensional Kondo lattice model at partial band filling
The Kondo lattice model introduced in 1977 describes a lattice of localized
magnetic moments interacting with a sea of conduction electrons. It is one of
the most important canonical models in the study of a class of rare earth
compounds, called heavy fermion systems, and as such has been studied
intensively by a wide variety of techniques for more than a quarter of a
century. This review focuses on the one dimensional case at partial band
filling, in which the number of conduction electrons is less than the number of
localized moments. The theoretical understanding, based on the bosonized
solution, of the conventional Kondo lattice model is presented in great detail.
This review divides naturally into two parts, the first relating to the
description of the formalism, and the second to its application. After an
all-inclusive description of the bosonization technique, the bosonized form of
the Kondo lattice hamiltonian is constructed in detail. Next the
double-exchange ordering, Kondo singlet formation, the RKKY interaction and
spin polaron formation are described comprehensively. An in-depth analysis of
the phase diagram follows, with special emphasis on the destruction of the
ferromagnetic phase by spin-flip disorder scattering, and of recent numerical
results. The results are shown to hold for both antiferromagnetic and
ferromagnetic Kondo lattice. The general exposition is pedagogic in tone.Comment: Review, 258 pages, 19 figure
Atomic-scale electronic structure of the cuprate d-symmetry form factor density wave state
Research on high-temperature superconducting cuprates is at present focused on identifying the relationship between the classic âpseudogapâ phenomenon and the more recently investigated density wave state. This state is generally characterized by a wavevector Q parallel to the planar CuâOâCu bonds along with a predominantly d-symmetry form facto (dFF-DW). To identify the microscopic mechanism giving rise to this state one must identify the momentum-space states contributing to the dFF-DW spectral weight, determine their particleâhole phase relationship about the Fermi energy, establish whether they exhibit a characteristic energy gap, and understand the evolution of all these phenomena throughout the phase diagram. Here we use energy-resolved sublattice visualization14 of electronic structure and reveal that the characteristic energy of the dFF-DW modulations is actually the âpseudogapâ energy Î1. Moreover, we demonstrate that the dFF-DW modulations at Eâ = â âÎ1 (filled states) occur with relative phase Ï compared to those at Eâ = âÎ1 (empty states). Finally, we show that the conventionally defined dFF-DW Q corresponds to scattering between the âhot frontierâ regions of momentum-space beyond which Bogoliubov quasiparticles cease to exist. These data indicate that the cuprate dFF-DW state involves particleâhole interactions focused at the pseudogap energy scale and between the four pairs of âhot frontierâ regions in momentum space where the pseudogap opens.Physic
Experimental Studies of Atomic Behavior at Crystal Surfaces
Coordinated Science Laboratory was formerly known as Control Systems LaboratoryJoint Services Electronics Program / DAAB-07-67-C-019
Valence-Fluctuation Scenario for Cuprate Superconductivity: The Finite-U Pairing Mechanism
The ideal that the normal state of cuprate superconductors is a valence-fluctuation (VF) state is appealing for a number of reasons. We are exploring this scenario quantitatively, focussing especially on the finite-U mechanism for s-wave pairing. The calculations employ a many-body variational technique developed previously for VF and heavy-fermion materials. We are presently exploring the effects of (a) varying the Hamiltonian parameters away from the values obtained from photoemission data analysis, (b) various simple assumptions for the band structure and hybridization k-dependence, and (c) corrections and to the Gutzwiller approximation. These are the topics to be discussed in this report. 17 refs., 1 fig
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