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G Ruocco

Publications and source records attributed to G Ruocco.

At least 55 records · Page 3Linked to original sources
PubMed2000

Experimental evidence of the acousticlike character of the high frequency excitations in glasses

The dynamic structure factor S(Q,E) of glassy glycerol has been measured by inelastic x-ray scattering as a function of momentum transfer Q and at constant energy transfer E. This allows one to establish independently from specific models of S(Q,E) the following: (i) Propagating collective excitations exist in glasses at high Q. (ii) Their dispersion up to E higher than E(BP) (the boson peak energy) confirms that E(BP) is not the onset of modes localization. (iii) The observation of an almost Q independent plateau on the high Q side of the Brillouin peak supports numerical simulations on glasses, describing the vibrational eigenvectors in terms of acousticlike and "random" components.

Journal Article↗
PubMed2000

Off-equilibrium effective temperature in monatomic Lennard-Jones glass.

The off-equilibrium dynamics of a monatomic Lennard-Jones glass is numerically investigated after sudden isothermal density jumps (crunch) from well equilibrated liquid configurations towards the glassy state. The generalized fluctuation-dissipation relation has been studied and the temperature dependence of the violation factor m is found in agreement with the one step replica symmetry breaking scenario, i.e., at low temperature m(T) is found proportional to T up to an off-equilibrium effective temperature T(eff), where m(T(eff)) = 1. We report T(eff) as a function of the density and compare it with the glass transition temperature T(g) as determined by equilibrium calculations.

Journal Article↗
PubMed2000

Relaxation processes in harmonic glasses?

A relaxation process, with the associated phenomenology of sound attenuation and sound velocity dispersion, is found in a simulated harmonic Lennard-Jones glass. We propose to identify this process with the so-called microscopic (or, instantaneous) relaxation process observed in real glasses and supercooled liquids. A model based on the memory function approach accounts for the observation and allows one to relate to each other (1) the characteristic time and strength of this process, (2) the low frequency limit of the dynamic structure factor of the glass, and (3) the high frequency sound attenuation coefficient, with its observed quadratic dependence on the momentum transfer.

Journal Article↗
PubMed2000

Frustration and sound attenuation in structural glasses

Three classes of harmonic disorder systems (Lennard-Jones-like glasses, percolators above threshold, and spring disordered lattices) have been numerically investigated in order to clarify the effect of different types of disorder on the mechanism of high frequency sound attenuation. We introduce the concept of frustration in structural glasses as a measure of the internal stress, and find a strong correlation between the degree of frustration and the exponent alpha that characterizes the momentum dependence of the sound attenuation gamma(Q) approximately Qalpha. In particular, alpha decreases from approximately d+1 in low-frustration systems (where d is the spectral dimension) to approximately 2 for high-frustration systems such as the realistic glasses examined.

Journal Article↗
PubMed2000

Determination of the short-wavelength propagation threshold in the collective excitations of liquid ammonia

The dynamics structure factor S(Q,E) of liquid ammonia l-NH3 at T = 200 K and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the 1-15 nm(-1) momentum transfer ( Q) range. Contrary to previous IXS studies on other associated liquids and glasses, in l-NH3 a large inelastic signal is observed up to Q = 15 nm(-1). This, enabling S(Q,E) measurements as a function of Q at constant E transfer, allows us to demonstrate experimentally the transition from a propagating dynamics regime, where the acoustic excitation energy linearly disperses with Q, to a high-Q regime, where it is no longer possible to observe a dominant excitation in the S(Q,E).

Journal Article↗
PubMed2000

Potential energy landscape and long-time dynamics in a simple model glass

We analyze the properties of a Lennard-Jones system at the level of the potential energy landscape. After an exhaustive investigation of the topological features of the landscape of the systems, obtained by studying small size samples, we describe the dynamics of the systems in multidimensional configurational space by means of a simple model. This considers the configurational space as a connected network of minima where the dynamics proceeds by jumps described by an appropriate master equation. Using this model we are able to reproduce the long-time dynamics and the low temperature regime. We investigate both the equilibrium regime and the off-equilibrium one, finding those typical glassy behaviors usually observed in the experiments such as (i) a stretched exponential relaxation, (ii) a temperature-dependent stretching parameter, (iii) a breakdown of the Stokes-Einstein relation, and (iv) the appearance of a critical temperature below which one observes a deviation from the fluctuation-dissipation relation as a consequence of the lack of equilibrium in the system.

Journal Article↗
PubMed2000

Molecular dynamics simulation of the fragile glass-former orthoterphenyl: A flexible molecule model

We present a realistic model of the fragile glass-former orthoterphenyl and the results of extensive molecular dynamics simulations in which we investigated its basic static and dynamic properties. In this model the internal molecular interactions between the three rigid phenyl rings are described by a set of force constants, including harmonic and anharmonic terms; the interactions among different molecules are described by Lennard-Jones site-site potentials. Self-diffusion properties are discussed in detail together with the temperature and momentum dependencies of the self-intermediate scattering function. The simulation data are compared with existing experimental results and with the main predictions of the mode-coupling theory.

Journal Article↗
PubMed2000

Intramolecular origin of the fast relaxations observed in the brillouin light scattering spectra of molecular glass formers

The Brillouin light scattering spectra of the o-terphenyl single crystal are compared with those of the liquid and the glass phases. This shows (i) the direct evidence of a fast relaxation at frequencies nu approximately 5 GHz in both the single crystal and the glass; (ii) a similar temperature dependence for the attenuation of the longitudinal sound waves in the single crystal and the glass; and (iii) the absence of coupling between the fast relaxation and the transverse acoustic waves. These results allow us to assign such a relaxation to the coupling between the longitudinal acoustic waves and intramolecular vibrations, and therefore to exclude any relationship between it and the glass transition.

Journal Article↗
PubMed1999

Viscoelastic behavior of water in the terahertz-frequency range: an inelastic x-ray scattering study.

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S(q,omega) of liquid water in the THz frequency range have been performed as a function of wave vector q (1-7 nm(-1)) and temperature T (273-473 K), using pressure (0-1.5 kbar) to keep the density at rho approximately 1 g/cm(3). We show that, for q 2 nm(-1), we find that the viscoelastic contribution to the memory function becomes q dependent. Thus this work provides a view on the evolution of the collective dynamics of water across the q region where the continuum approximation inherent in the hydrodynamic formulation begins to fail. The physical consequences of such a result are discussed in some detail.

Journal Article↗
PubMed1999

Brillouin and Umklapp scattering in polybutadiene: comparison of neutron and x-ray scattering.

We report a comparison of high resolution inelastic x-ray Brillouin scattering to coherent inelastic neutron scattering for amorphous deuterated polybutadiene, done for one temperature in the glass phase and another one in the melt. The x-ray scattering proves to be by far the better technique for such a polymer within its present resolution bounds. The neutron scattering allows one to extend these measurements to a much better resolution, showing an additional quasielastic signal in the melt. The results suggest x-ray measurements at higher momentum transfer, to see whether they are complementary to neutrons.

Journal Article↗
PubMed1999

[Vaccination against Haemophilus influenzae type b in Uruguay: experience and impact].

Between 1979 and 1994, epidemiological surveillance of meningitides in Uruguay showed a progressive increase in suppurative meningitides due mainly to Neisseria meningitidis and Haemophilus influenzae type b (Hib). The cases were concentrated in children under 5; however, among the cases caused by Hib, 70% affected children from 1 to 11 months old. Facing this situation, the Ministry of Public Health resolved, as of August 1994, to include the Hib vaccine in the country's Expanded Program on Immunization, which has been in place since 1982. The Hib vaccination is done without charge and is obligatory for all children under 5 years of age. It is done using the following series of vaccinations: a) three doses, given at 2, 4, and 6 months, with a booster dose at age 1; b) children from 7 to 11 months old receive two doses two months apart and a booster dose a year later; and c) a single dose for children 12 months to 4 years old. Between August and December 1994 a coverage rate of 76.6% was reached among children between 2 months and 4 years old, and the coverage has remained above 80% in the new cohorts. In Uruguay, this vaccination strategy had a spectacular impact on morbidity and mortality due to meningitides caused by Hib. One of the results was that the incidence of 15.6 per 100,000 registered in children under 5 in the prevaccination years declined to 0.03 per 100,000 in 1996.

Child, Preschool↗
PubMed1999

[Introduction of a conjugate vaccine against Hib in Chile and Uruguay].

In some countries, the invasive disease caused by Haemophilus influenzae type b (Hib) has been practically eliminated thanks to vaccination. However, in much of the developing world, meningitides and pneumonias caused by these bacteria continue to be a major cause of childhood morbidity and mortality, as well as high hospitalization costs. Because safe and effective conjugate vaccines are now available, the Special Program for Vaccines and Immunization of the Pan American Health Organization has recommended introducing them into the regular vaccination regimen of as many countries as possible. This has been done in Chile and Uruguay, where the Hib vaccine now forms part of the regular vaccination routine. When the vaccine was being introduced, both countries had difficulties they could have avoided if they had known of the experiences of other nations. Therefore, these two countries now offer the lessons they learned to other nations considering introducing the vaccine into their immunization programs. The most important lessons were to: strengthen the epidemiological surveillance system sufficiently in advance of introducing the vaccine; with the support of scientific societies, present the technical information that justifies introducing the vaccine; seek community backing and acceptance; precisely establish in advance the presentation and dosage of the vaccine that is most appropriate for the country; and be certain to have the political and legal decisions needed to ensure the continuity of Hib vaccination in the future.

Child↗
PubMed1997

Angiotensin converting enzyme inhibitors and reflux nephropathy: 2-year follow-up.

We evaluated the effect of 2 years' therapy with an angiotensin converting enzyme inhibitor (captopril) in 16 patients who had severe reflux nephropathy and microalbuminuria. During the period of therapy, microalbuminuria decreased, glomerular filtration rate measured by diethylenetriamine pentaacetate scan, serum creatinine, and blood pressure remained stable. We suggest the captopril was useful in reducing microalbuminuria and may have slowed the progression of renal damage in our patients.

Adolescent↗