297 research outputs found
Dynamic heterogeneity in the glass-like monoclinic phases of some halogen methane compounds
In this work we study the heterogeneity of the dynamics on the low-temperature monoclinic phases of the simple molecular glassy systems CBrnCl4−nCBrnCl4−n, n = 0, 1, 2. In these systems the disorder comes exclusively from reorientational jumps mainly around the C3 molecular axes. The different time scales are determined by means of the analysis of the spin-lattice relaxation time obtained through Nuclear Quadrupole Resonance (NQR) technique. Results are compared with those obtained from dielectric spectroscopy, from which two α- and β-relaxation times appear. NQR results enable us to ascribe with no doubt that the existence of two relaxations is due to dynamical heterogeneities which are the consequence of the different molecular surroundings of the molecules in the asymmetric unit cell of systems here studied.Fil: Zuriaga, Mariano Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Perez, S. C.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Pardo, L. C.. Universidad Politecnica de Catalunya; EspañaFil: Tamarit, J. L.. Universidad Politecnica de Catalunya; Españ
Comparison of short-range-order in liquid- and rotator-phase states of a simple molecular liquid: A reverse Monte Carlo and molecular dynamics analysis of neutron diffraction data
6 págs.; 6 figs.; 1 tab. ; PACS number s : 61.25.Em, 61.20. p, 61.43. j, 61.12. qThe short-range order (SRO) correlations in liquid- and rotator-phase states of carbon tetrachloride are revisited here. The correlation of some angular magnitudes is used to evaluate the positional and orientational correlations in the liquid as well as in the rotator phase. The results show significant similitudes in the relative position of the molecules surrounding a central one but striking differences in their relative orientations, which could explain the changes in SRO between the two phases and the puzzling behavior of the local density in the liquid and rotator phases. © 2007 The American Physical Society.We would also like to acknowledge “Generalitat de Catalunya” Project No. 2005SGR-
00535 and MEC Project No. FIS2005-00975 for partial support of this work.Peer Reviewe
Implanted muon spin spectroscopy on 2-O-adamantane: a model system that mimics the liquid
The transition taking place between two metastable phases in 2-O-adamantane, namely the [Formula: see text] cubic, rotator phase and the lower temperature P21/c, Z = 4 substitutionally disordered crystal is studied by means of muon spin rotation and relaxation techniques. Measurements carried out under zero, weak transverse and longitudinal fields reveal a temperature dependence of the relaxation parameters strikingly similar to those exhibited by structural glass[Formula: see text]liquid transitions (Bermejo et al 2004 Phys. Rev. B 70 214202; Cabrillo et al 2003 Phys. Rev. B 67 184201). The observed behaviour manifests itself as a square root singularity in the relaxation rates pointing towards some critical temperature which for amorphous systems is located some tens of degrees above that shown as the characteristic transition temperature if studied by thermodynamic means. The implications of such findings in the context of current theoretical approaches concerning the canonical liquid-glass transition are discussed.Postprint (author's final draft
Dynamical properties of the hypercell spin glass model
The spreading of damage technique is used to study the sensibility to initial
conditions in a heath bath Monte Carlo simulation of the spin glass hypercubic
cell model. Since the hypercubic cell in dimension 2D and the hypercubic
lattice in dimension D resemble each other closely at finite dimensions and
both converge to mean field when dimension goes to infinity, it allows us to
study the effect of dimensionality on the dynamical behavior of spin glasses.Comment: 13 pages, RevTex, 8 ps figure
Fitting in a complex chi^2 landscape using an optimized hypersurface sampling
Fitting a data set with a parametrized model can be seen geometrically as
finding the global minimum of the chi^2 hypersurface, depending on a set of
parameters {P_i}. This is usually done using the Levenberg-Marquardt algorithm.
The main drawback of this algorithm is that despite of its fast convergence, it
can get stuck if the parameters are not initialized close to the final
solution. We propose a modification of the Metropolis algorithm introducing a
parameter step tuning that optimizes the sampling of parameter space. The
ability of the parameter tuning algorithm together with simulated annealing to
find the global chi^2 hypersurface minimum, jumping across chi^2{P_i} barriers
when necessary, is demonstrated with synthetic functions and with real data
Aging in a Two-Dimensional Ising Model with Dipolar Interactions
Aging in a two-dimensional Ising spin model with both ferromagnetic exchange
and antiferromagnetic dipolar interactions is established and investigated via
Monte Carlo simulations. The behaviour of the autocorrelation function
is analyzed for different values of the temperature, the waiting
time and the quotient , and being the
strength of exchange and dipolar interactions respectively. Different
behaviours are encountered for at low temperatures as is
varied. Our results show that, depending on the value of , the dynamics
of this non-disordered model is consistent either with a slow domain dynamics
characteristic of ferromagnets or with an activated scenario, like that
proposed for spin glasses.Comment: 4 pages, RevTex, 5 postscript figures; acknowledgment added and some
grammatical corrections in caption
Experimental And Topological Determination of the Pressure-Temperature Phase Diagram of Racemic Etifoxine, a Pharmaceutical Ingredient with Anxyolitic Properties
International audienc
Non-equilibrium structures and slow dynamics in a two dimensional spin system with competitive long range and short range interactions
We introduce a lattice spin model that mimics a system of interacting
particle through a short range repulsive potential and a long range attractive
power law decaying potential. We performed a detailed analysis of the general
equilibrium phase diagram of the model at finite temperature, showing that the
only possible equilibrium pases are the ferromagnetic and the antiferromagnetic
ones. We then studied the non equilibrium behavior of the model after a quench
to subcritical temperatures, in the antiferromagnetic region of the phase
diagram region, where the pair interaction potential behaves in the same
qualitative way as in a Lennard-Jones gas. We found that, even in the absence
of quenched disorder or geometric frustration, the competition between
interactions gives rise to non--equilibrium disordered structures at low enough
temperatures that strongly slow down the relaxation of the system.Comment: 14 pages, 20 figure
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