43,257 research outputs found
Production of minimally entangled typical thermal states with the Krylov-space approach
The minimally entangled typical thermal states algorithm is applied to
fermionic systems using the Krylov-space approach to evolve the system in
imaginary time. The convergence of local observables is studied in a
tight-binding system with a site-dependent potential. The temperature
dependence of the superconducting correlations of the attractive Hubbard model
is analyzed on chains, showing an exponential decay with distance and exponents
proportional to the temperature at low temperatures, as expected. In addition,
the non-local parity correlator is calculated at finite temperature. Other
possible applications of the minimally entangled typical thermal states
algorithm to fermionic systems are also discussed.Comment: revtex4, 4 figure
On the Nature of the Phase Transition Triggered by Vortex-Like Deffects in the 2D Ginzburg-Landau Model
The two dimensional lattice Ginzburg-Landau hamiltonian is simulated
numerically for different values of the coherence length in units of the
lattice spacing , a parameter which controls amplitude fluctuations. The
phase diagram on the plane is measured. Amplitude fluctuations change
dramatically the nature of the phase transition: for values of , instead of the smooth Kosterlitz-Thouless transition there is a {\em first
order} transition with a discontinuity in the vortex density and a sharper
drop in the helicity modulus . Both observables and are
analyzed in detail at the crossover region between first and second order which
occurs for intermediate values of .Comment: 9 pages, 7 postscript (eps) figure
Symmetry Conserving Purification of Quantum States within the Density Matrix Renormalization Group
The density matrix renormalization group (DMRG) algorithm was originally
designed to efficiently compute the zero temperature or ground-state properties
of one dimensional strongly correlated quantum systems. The development of the
algorithm at finite temperature has been a topic of much interest, because of
the usefulness of thermodynamics quantities in understanding the physics of
condensed matter systems, and because of the increased complexity associated
with efficiently computing temperature-dependent properties. The ancilla method
is a DMRG technique that enables the computation of these thermodynamic
quantities. In this paper, we review the ancilla method, and improve its
performance by working on reduced Hilbert spaces and using canonical
approaches. We furthermore explore its applicability beyond spins systems to
t-J and Hubbard models.Comment: 10 pages, 7 figure
Spectral Functions with the Density Matrix Renormalization Group: Krylov-space Approach for Correction Vectors
Frequency-dependent correlations, such as the spectral function and the
dynamical structure factor, help understand condensed matter experiments.
Within the density matrix renormalization group (DMRG) framework, an accurate
method for calculating spectral functions directly in frequency is the
correction-vector method. The correction-vector can be computed by solving a
linear equation or by minimizing a functional. This paper proposes an
alternative to calculate the correction vector: to use the Krylov-space
approach. This paper then studies the accuracy and performance of the
Krylov-space approach, when applied to the Heisenberg, the t-J, and the Hubbard
models. The cases studied indicate that Krylov-space approach can be more
accurate and efficient than conjugate gradient, and that the error of the
former integrates best when a Krylov-space decomposition is also used for
ground state DMRG.Comment: 8 pages, 7 figure
Global interactions, information flow, and chaos synchronization
We investigate the relationship between the emergence of chaos
synchronization and the information flow in dynamical systems possessing
homogeneous or heterogeneous global interactions whose origin can be external
(driven systems) or internal (autonomous systems). By employing general models
of coupled chaotic maps for such systems, we show that the presence of a
homogeneous global field, either external or internal, for all times is not
indispensable for achieving complete or generalized synchronization in a system
of chaotic elements. Complete synchronization can also appear with
heterogeneous global fields; it does not requires the simultaneous sharing of
the field by all the elements in a system. We use the normalized mutual
information and the information transfer between global and local variables to
characterize complete and generalized synchronization. We show that these
information measures can characterize both types of synchronized states and
also allow to discern the origin of a global interaction field. A
synchronization state emerges when a sufficient amount of information provided
by a field is shared by all the elements in the system, on the average over
long times. Thus, the maximum value of the top-down information transfer can be
used as a predictor of synchronization in a system, as a parameter is varied.Comment: 9 pages, 5 figure
Cryptanalyzing an improved security modulated chaotic encryption scheme using ciphertext absolute value
This paper describes the security weakness of a recently proposed improved
chaotic encryption method based on the modulation of a signal generated by a
chaotic system with an appropriately chosen scalar signal. The aim of the
improvement is to avoid the breaking of chaotic encryption schemes by means of
the return map attack introduced by Perez and Cerdeira. A method of attack
based on taking the absolute value of the ciphertext is presented, that allows
for the cancellation of the modulation scalar signal and the determination of
some system parameters that play the role of system key. The proposed improved
method is shown to be compromised without any knowledge of the chaotic system
parameter values and even without knowing the transmitter structure.Comment: 12 pages, 8 figures, LaTeX forma
Multi-Orbital Lattice Model for (Ga,Mn)As and Other Lightly Magnetically Doped Zinc-Blende-Type Semiconductors
We present a Hamiltonian in real space which is well suited to study
numerically the behavior of holes introduced in III-V semiconductors by Mn
doping when the III ion is replaced by Mn. We consider the actual
lattice with the diamond structure. Since the focus is on light doping by
acceptors, a bonding combination of III and V p-orbitals is considered since
the top of the valence band, located at the point, has p character in
these materials. As a result, an effective model in which the holes hop between
the sites of an fcc lattice is obtained. We show that around the point
in momentum space the Hamiltonian for the undoped case is identical to the one
for the Luttinger-Kohn model. The spin-orbit interaction is included as well as
the on-site interaction between the spin of the magnetic impurity and the spin
of the doped holes. The effect of Coulomb interactions between Mn and
holes, as well as Mn doping are discussed. Through large-scale Monte
Carlo simulations on a Cray XT3 supercomputer, we show that this model
reproduces many experimental results for and
, and that the Curie temperature does not
increase monotonically with . The cases of Mn doped GaP and GaN, in which
Mn is believed to play a role, are also studied.Comment: 17 pages, 11 figure
Methods for analyzing surface texture effects of volcanoes with Plinian and subplinian eruptions types: Cases of study Lascar (23 S) and Chaiten (42 S), Chile
This paper presents a new methodology that provides the analysis of surface
texture changes in areas adjacent to the volcano and its impact product of
volcanic activity. To do this, algorithms from digital image processing such as
the co-occurrence matrix and the wavelet transform are used. These methods are
working on images taken by the Landsat satellite platform sensor 5 TM and
Landsat 7 ETM + sensor, and implemented with the purpose of evaluating
superficial changes that can warn of surface movements of the volcano. The
results were evaluated by similarity metrics for grayscale images, and
validated in two different scenarios that have the same type of eruption, but
differ, essentially, in climate and vegetation. Finally, the proposed algorithm
is presented, setting the parameters and constraints for implementation and
use.Comment: 18 pages, 9 figure
Chaotic singular maps
We consider a family of singular maps as an example of a simple model of
dynamical systems exhibiting the property of robust chaos on a well defined
range of parameters. Critical boundaries separating the region of robust chaos
from the region where stable fixed points exist are calculated on the parameter
space of the system. It is shown that the transitions to robust chaos in these
systems occur either through the routes of type-I or type-III intermittency and
the critical boundaries for each type of transition have been determined on the
phase diagram of the system. The simplicity of these singular maps and the
robustness of their chaotic dynamics make them useful ingredients in the
construction of models and in applications that require reliable operation
under chaos.Comment: 4 pages, 4 figures, accepted in Cienci
Scaling of the Lyapunov exponent in type-III intermittent chaos
The scaling behaviour of the Lyapunov exponent near the transition to chaos
via type-III intermittency is determined for a generic map. A critical exponent
expressing the scaling of the Lyapunov exponent as a function of both,
the reinjection probability and the nonlinearity of the map is calculated. It
is found that the critical exponent varies on the interval .
This contrasts with earlier predictions for the scaling behaviour of the
Lyapunov exponent in type-III intermittency.Comment: 4 pages, 5 Figs, Submitted to IJB
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