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G. Alvarez

Publications and source records attributed to G. Alvarez.

66 records · Page 4Linked to original sources
arXiv2004

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.

nlin.CD↗
arXiv2004

Insulator to Metal Transition Induced by Disorder in a Model for Manganites

The physics of manganites appears to be dominated by phase competition among ferromagnetic metallic and charge-ordered antiferromagnetic insulating states. Previous investigations (Burgy {\it et al.}, Phys. Rev. Lett. {\bf 87}, 277202 (2001)) have shown that quenched disorder is important to smear the first-order transition between those competing states, and induce nanoscale inhomogeneities that produce the colossal magnetoresistance effect. Recent studies (Motome {\it et al.} Phys. Rev. Lett. {\bf 91}, 167204 (2003)) have provided further evidence that disorder is important in the manganite context, unveiling an unexpected insulator-to-metal transition triggered by disorder in a one-orbital model with cooperative phonons. In this paper, a qualitative explanation for this effect is presented. It is argued that the transition occurs for disorder in the form of local random energies. Acting over an insulating states made out of a checkerboard arrangement of charge, with ``effective'' site energies positive and negative, this form of disorder can produce lattice sites with an effective energy near zero, favorable for the transport of charge. This explanation is based on Monte Carlo simulations and the study of simplified toy models, measuring the density-of-states, cluster conductances using the Landauer formalism, and other observables. The applicability of these ideas to real manganites is discussed.

cond-mat.str-el↗
arXiv2003

Breaking parameter modulated chaotic secure communication system

This paper describes the security weakness of a recently proposed secure communication method based on parameter modulation of a chaotic system and adaptive observer-based synchronization scheme. We show that the security is compromised even without precise knowledge of the chaotic system used.

nlin.CD↗
arXiv2003

Cryptanalysis of dynamic look-up table based chaotic cryptosystems

In recent years many chaotic cryptosystems based on Baptista's seminal work have been proposed. We analyze the security of two of the newest and most interesting ones, which use a dynamically updated look-up table and also work as stream ciphers. We provide different attack techniques to recover the keystream used by the algorithms. The knowledge of this keystream provides the attacker with the same information as the key and thus the security is broken. We also show that the dependence on the plaintext, and not on the key, of the look-up table updating mechanism facilitates cryptanalysis.

nlin.CD↗
arXiv2003

Single-Band Model for Diluted Magnetic Semiconductors: Dynamical and Transport Properties and Relevance of Clustered States

Dynamical and transport properties of a simple single-band spin-fermion lattice model for (III,Mn)V diluted magnetic semiconductors (DMS) is here discussed using Monte Carlo simulations. This effort is a continuation of previous work (G. Alvarez, Phys. Rev. Lett. 89, 277202 (2002)) where the static properties of the model were studied. The present results support the view that the relevant regime of J/t (standard notation) is that of intermediate coupling, where carriers are only partially trapped near Mn spins, and locally ordered regions (clusters) are present above the Curie temperature T_C. This conclusion is based on the calculation of the resistivity vs. temperature, that shows a soft metal to insulator transition near T_C, as well on the analysis of the density-of-states and optical conductivity. In addition, in the clustered regime a large magnetoresistance is observed in simulations. Formal analogies between DMS and manganites are also discussed.

cond-mat.str-el↗
arXiv2003

Clustered States as a New Paradigm of Condensed Matter Physics

We argue that several materials of much current interest in condensed matter physics share common phenomenological aspects that only very recent investigations are unveiling. The list includes colossal magnetoresistance manganites, high temperature superconducting cuprates, diluted magnetic semiconductors, and others. The common aspect is the relevance of intrinsic inhomogeneities in the form of "clustered states", as explained in the text.

cond-mat.str-el↗
arXiv2000

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 $ξ$ in units of the lattice spacing $a$, a parameter which controls amplitude fluctuations. The phase diagram on the plane $T-ξ$ is measured. Amplitude fluctuations change dramatically the nature of the phase transition: for values of $ξ/a \simeq 1$, instead of the smooth Kosterlitz-Thouless transition there is a {\em first order} transition with a discontinuity in the vortex density $v$ and a sharper drop in the helicity modulus $Γ$. Both observables $v$ and $Γ$ are analyzed in detail at the crossover region between first and second order which occurs for intermediate values of $ξ/a$.

cond-mat.supr-con↗
arXiv2000

The Effect of Large Amplitude Fluctuations in the Ginzburg-Landau Phase Transition

The lattice Ginzburg-Landau model in d=3 and d=2 is simulated, for different values of the coherence length $ξ$ in units of the lattice spacing $a$, using a Monte Carlo method. The energy, specific heat, vortex density $v$, helicity modulus $Γ_μ$ and mean square amplitude are measured to map the phase diagram on the plane $T-ξ$. When amplitude fluctuations, controlled by the parameter $ξ$, become large ($ξ\sim 1$) a proliferation of vortex excitations occurs changing the phase transition from continuous to first order.

cond-mat.supr-con↗
arXiv2000

Phase Transitions Driven by Vortices in 2D Superfluids and Superconductors: From Kosterlitz-Thouless to 1st Order

The Landau-Ginzburg-Wilson hamiltonian is studied for different values of the parameter $λ$ which multiplies the quartic term (it turns out that this is equivalent to consider different values of the coherence length $ξ$ in units of the lattice spacing $a$). It is observed that amplitude fluctuations can change dramatically the nature of the phase transition: for small values of $λ$ ($ξ/a > 0.7$), instead of the smooth Kosterlitz-Thouless transition there is a {\em first order} transition with a discontinuous jump in the vortex density $v$ and a larger non-universal drop in the helicity modulus. In particular, for $λ$ sufficiently small ($ξ/a \cong 1$), the density of bound pairs of vortex-antivortex below $T_c$ is so low that, $v$ drops to zero almost for all temperature $T<Tc$.

cond-mat.supr-con↗
arXiv1999

Summability of the perturbative expansion for a zero-dimensional disordered spin model

We show analytically that the perturbative expansion for the free energy of the zero dimensional (quenched) disordered Ising model is Borel-summable in a certain range of parameters, provided that the summation is carried out in two steps: first, in the strength of the original coupling of the Ising model and subsequently in the variance of the quenched disorder. This result is illustrated by some high-precision calculations of the free energy obtained by a straightforward numerical implementation of our sequential summation method.

cond-mat.stat-mech↗