786 research outputs found
Effects of two-site composite excitations in the Hubbard model
The electronic states of the Hubbard model are investigated by use of the
Composite Operator Method. In addition to the Hubbard operators, two other
operators related with two-site composite excitations are included in the
basis. Within the present formulation, higher-order composite excitations are
reduced to the chosen operatorial basis by means of a procedure preserving the
particle-hole symmetry. The positive comparison with numerical simulations for
the double occupancy indicates that such approximation improves over the
two-pole approximation.Comment: 2 pages, 1 figur
Underdoped cuprates phenomenology in the 2D Hubbard model within COM(SCBA)
The two-dimensional Hubbard model is studied within the Composite Operator
Method (COM) with the residual self-energy computed in the Self-Consistent Born
Approximation (SCBA). COM describes interacting electrons in terms of the new
elementary excitations appearing in the system owing to strong correlations;
residual interactions among these excitations are treated within the SCBA. The
anomalous features appearing in the spectral function A(k,\omega), the momentum
distribution function n(k) and the Fermi surface are analyzed for various
values of the filling (from overdoped to underdoped region) in the intermediate
coupling regime at low temperatures. For low doping, in contrast with the
ordinary Fermi-liquid behavior of a weakly-correlated metal found at high
doping, we report the opening of a pseudogap and some non-Fermi-liquid features
as measured for cuprates superconductors. In addition, we show the presence of
kinks in the calculated electronic dispersion in agreement with ARPES data.Comment: 5 pages, 5 figure
The derived category of surface algebras: the case of the torus with one boundary component
In this paper we refine the main result of a previous paper of the author
with Grimeland on derived invariants of surface algebras. We restrict to the
case where the surface is a torus with one boundary component and give an
easily computable derived invariant for such surface algebras. This result
permits to give answers to open questions on gentle algebras: it provides
examples of gentle algebras with the same AG-invariant (in the sense of
Avella-Alaminos and Geiss) that are not derived equivalent and gives a partial
positive answer to a conjecture due to Bobi\'nski and Malicki on gentle
-cycles algebras.Comment: 22 pages, a mistake concerning the computation of the mapping class
group has been fixed, version 3: 25 pages, to appear in Algebras and
Representation Theor
Bosonic sector of the two-dimensional Hubbard model studied within a two-pole approximation
The charge and spin dynamics of the two-dimensional Hubbard model in the
paramagnetic phase is first studied by means of the two-pole approximation
within the framework of the Composite Operator Method. The fully
self-consistent scheme requires: no decoupling, the fulfillment of both Pauli
principle and hydrodynamics constraints, the simultaneous solution of fermionic
and bosonic sectors and a very rich momentum dependence of the response
functions. The temperature and momentum dependencies, as well as the dependency
on the Coulomb repulsion strength and the filling, of the calculated charge and
spin susceptibilities and correlation functions are in very good agreement with
the numerical calculations present in the literature
Eksédra 2008
Il volume affronta temi relativi al disegno, al rilievo ed alla progettazione architettonica
VIRTUAL REALITY FOR HISTORICAL ARCHITECTURE
Abstract. This article shows a first step in the development of an immersive virtual tour of the Cathedral of Palermo, entering the fields of Digital Cultural Heritage and Edutainment. Its purpose is to help people to gain knowledge about the site, highlighting the complex stratifications that have characterized its history.The development of the project has been possible thanks to different phases of work: surveys were initially carried out by laser scanning, then assembled and processed to obtain the 3D model of the current state; at the same time, the model of reconstruction was processed in several phases, based on historical, archival and iconographic sources; both models were, later, subject to post-processing, preparatory to the development of virtual navigation. The tour scenario includes options in order to make it attractive for the player, such as interactive elements, interfaces and animations.</p
High-order correlation effects in the two-dimensional Hubbard model
The electronic states of the two-dimensional Hubbard model are investigated
by means of a 4-pole approximation within the Composite Operator Method. In
addition to the conventional Hubbard operators, we consider other two operators
which come from the hierarchy of the equations of motion and carry information
regarding nearest-neighbor spin and charge configurations. By means of this
operatorial basis, we can study the physics related to the energy scale of
J=4t^2/U in addition to the one of U. Present results show relevant physical
features, well beyond those previously obtained by means of a 2-pole
approximation, such as a four-band structure with shadow bands and a
quasi-particle peak at the Fermi level. The Fermi level stays pinned to the
band flatness located at (pi,0)-point within a wide range of hole-doping (0 <=
delta <= 0.15). A comprehensive analysis of double occupancy, internal energy,
specific heat and entropy features have been also performed. All reported
results are in excellent agreement with the data of numerical simulations.Comment: 13 pages, 8 figure
Positive Operator-Valued Measure reconstruction of a beam-splitter tree based photon-number-resolving detector
Here we present a reconstruction of the Positive Operator-Value Measurement
of a photon-number-resolving detector comprised of three 50:50 beamsplitters in
a tree configuration, terminated with four single-photon avalanche detectors.
The four detectors' outputs are processed by an electronic board that
discriminates detected photon number states from 0 to 4 and implements a "smart
counting" routine to compensate for dead time issues at high count rates
A Study of the Antiferromagnetic Phase in the Hubbard Model by means of the Composite Operator Method
We have investigated the antiferromagnetic phase of the 2D, the 3D and the
extended Hubbard models on a bipartite cubic lattice by means of the Composite
Operator Method within a two-pole approximation. This approach yields a fully
self-consistent treatment of the antiferromagnetic state that respects the
symmetry properties of both the model and the algebra. The complete phase
diagram, as regards the antiferromagnetic and the paramagnetic phases, has been
drawn. We firstly reported, within a pole approximation, three kinds of
transitions at half-filling: Mott-Hubbard, Mott-Heisenberg and Heisenberg. We
have also found a metal-insulator transition, driven by doping, within the
antiferromagnetic phase. This latter is restricted to a very small region near
half filling and has, in contrast to what has been found by similar approaches,
a finite critical Coulomb interaction as lower bound at half filling. Finally,
it is worth noting that our antiferromagnetic gap has two independent
components: one due to the antiferromagnetic correlations and another coming
from the Mott-Hubbard mechanism.Comment: 20 pages, 37 figures, RevTeX, submitted to Phys. Rev.
Nearest-neighbour Attraction Stabilizes Staggered Currents in the 2D Hubbard Model
Using a strong-coupling approach, we show that staggered current vorticity
does not obtain in the repulsive 2D Hubbard model for large on-site Coulomb
interactions, as in the case of the copper oxide superconductors. This trend
also persists even when nearest-neighbour repulsions are present. However,
staggered flux ordering emerges {\bf only} when attractive nearest-neighbour
Coulomb interactions are included. Such ordering opens a gap along the
direction and persists over a reasonable range of doping.Comment: 5 pages with 5 .eps files (Typos in text are corrected
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