18,912 research outputs found
Excited-state quantum phase transitions in a two-fluid Lipkin model
Background: Composed systems have became of great interest in the framework
of the ground state quantum phase transitions (QPTs) and many of their
properties have been studied in detail. However, in these systems the study of
the so called excited-state quantum phase transitions (ESQPTs) have not
received so much attention.
Purpose: A quantum analysis of the ESQPTs in the two-fluid Lipkin model is
presented in this work. The study is performed through the Hamiltonian
diagonalization for selected values of the control parameters in order to cover
the most interesting regions of the system phase diagram. [Method:] A
Hamiltonian that resembles the consistent-Q Hamiltonian of the interacting
boson model (IBM) is diagonalized for selected values of the parameters and
properties such as the density of states, the Peres lattices, the
nearest-neighbor spacing distribution, and the participation ratio are
analyzed.
Results: An overview of the spectrum of the two-fluid Lipkin model for
selected positions in the phase diagram has been obtained. The location of the
excited-state quantum phase transition can be easily singled out with the Peres
lattice, with the nearest-neighbor spacing distribution, with Poincar\'e
sections or with the participation ratio.
Conclusions: This study completes the analysis of QPTs for the two-fluid
Lipkin model, extending the previous study to excited states. The ESQPT
signatures in composed systems behave in the same way as in single ones,
although the evidences of their presence can be sometimes blurred. The Peres
lattice turns out to be a convenient tool to look into the position of the
ESQPT and to define the concept of phase in the excited states realm
Calorons on the lattice - a new perspective
We discuss the manifestation of instanton and monopole solutions on a
periodic lattice at finite temperature and their relation to the infinite
volume analytic caloron solutions with asymptotic non-trivial Polyakov loops.
As a tool we use improved cooling and twisted boundary conditions. Typically we
find 2Q lumps for topological charge Q. These lumps are BPS monopoles.Comment: Latex. 16 pages, 9 figure
Improved superposition schemes for approximate multi-caloron configurations
Two improved superposition schemes for the construction of approximate
multi-caloron-anticaloron configurations, using exact single (anti)caloron
gauge fields as underlying building blocks, are introduced in this paper. The
first improvement deals with possible monopole-Dirac string interactions
between different calorons with non-trivial holonomy. The second one, based on
the ADHM formalism, improves the (anti-)selfduality in the case of small
caloron separations. It conforms with Shuryak's well-known ratio-ansatz when
applied to instantons. Both superposition techniques provide a higher degree of
(anti-)selfduality than the widely used sum-ansatz, which simply adds the
(anti)caloron vector potentials in an appropriate gauge. Furthermore, the
improved configurations (when discretized onto a lattice) are characterized by
a higher stability when they are exposed to lattice cooling techniques.Comment: New version accepted for publication in Nucl. Phys. B. Text partly
shortened, changes in the introduction, new results added on the comparison
with exact solution
Hyperonic crystallization in hadronic matter
Published in Hadrons, Nuclei and Applications, World Scientific, Singapore,
Proc.of the Conference Bologna2000. Structure of the Nucleus at the Dawn of the
Century, G. Bonsignori, M. Bruno, A. Ventura, D. Vretenar Editors, pag. 319.Comment: 4 pages, 2figure
Recent results on self-dual configurations on the torus
We review the recent progress on our understanding of self-dual SU(N)
Yang-Mills configurations on the torus.Comment: Latex 3 pages, 1 figure. Contribution to the Lat99 Proceeding
Infinite boundary conditions for matrix product state calculations
We propose a formalism to study dynamical properties of a quantum many-body
system in the thermodynamic limit by studying a finite system with infinite
boundary conditions (IBC) where both finite size effects and boundary effects
have been eliminated. For one-dimensional systems, infinite boundary conditions
are obtained by attaching two boundary sites to a finite system, where each of
these two sites effectively represents a semi-infinite extension of the system.
One can then use standard finite-size matrix product state techniques to study
a region of the system while avoiding many of the complications normally
associated with finite-size calculations such as boundary Friedel oscillations.
We illustrate the technique with an example of time evolution of a local
perturbation applied to an infinite (translationally invariant) ground state,
and use this to calculate the spectral function of the S=1 Heisenberg spin
chain. This approach is more efficient and more accurate than conventional
simulations based on finite-size matrix product state and density-matrix
renormalization-group approaches.Comment: 10 page
Dynamical windows for real-time evolution with matrix product states
We propose the use of a dynamical window to investigate the real-time
evolution of quantum many-body systems in a one-dimensional lattice. In a
recent paper [H. Phien et al, arxiv:????.????], we introduced infinite boundary
conditions (IBC) in order to investigate real-time evolution of an infinite
system under a local perturbation. This was accomplished by restricting the
update of the tensors in the matrix product state to a finite window, with left
and right boundaries held at fixed positions. Here we consider instead the use
of a dynamical window, namely a window where the positions of left and right
boundaries are allowed to change in time. In this way, all simulation efforts
can be devoted to the space-time region of interest, which leads to a
remarkable reduction in computational costs. For illustrative purposes, we
consider two applications in the context of the spin-1 antiferromagnetic
Heisenberg model in an infinite spin chain: one is an expanding window, with
boundaries that are adjusted to capture the expansion in time of a local
perturbation of the system; the other is a moving window of fixed size, where
the position of the window follows the front of a propagating wave
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