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At least 919 records · Page 51Linked to original sources
PubMed1999

Effect of angular momentum conservation in the phase transitions of collapsing systems.

The effect of angular momentum conservation within microcanonical thermodynamics is considered. This is relevant in self-gravitating systems, where angular momentum is conserved and the collapsing nature of the forces makes the microcanonical ensemble the proper statistical description of the physical processes. The microcanonical distribution function with nonvanishing angular momentum is obtained as a function of the coordinates of the particles. As an example, a simple model, introduced by Thirring long ago [Z. Phys. 235, 339 (1970)], is worked out. The phase diagram contains three phases: For low values of the angular momentum L the system behaves as the original model, showing a complete collapse at low energies and a convex intruder in the entropy. For intermediate values of L the collapse at low energies is not complete, and the entropy still has a convex intruder. For large L there is neither collapse nor anomalies in the thermodynamical quantities. A short discussion of the extension of these results to more realistic situations is given.

Journal Article↗
PubMed2003

Two-vibron bound states in alpha-helix proteins: the interplay between the intramolecular anharmonicity and the strong vibron-phonon coupling.

The influence of the intramolecular anharmonicity and the strong vibron-phonon coupling on the two-vibron dynamics in an alpha-helix protein is studied within a modified Davydov model. The intramolecular anharmonicity of each amide-I vibration is considered, and the vibron dynamics is described according to the small polaron approach. A unitary transformation is performed to remove the intramolecular anharmonicity, and a modified Lang-Firsov transformation is applied to renormalize the vibron-phonon interaction. Then a mean field procedure is realized to obtain the dressed anharmonic vibron Hamiltonian. It is shown that the anharmonicity modifies the vibron-phonon interaction, which results in an enhancement of the dressing effect. In addition, both the anharmonicity and the dressing favor the occurrence of two different bound states whose properties strongly depend on the interplay between the anharmonicity and the dressing. This dependence was summarized in a phase diagram which characterizes the number and the nature of the bound states as a function of the relevant parameters of the problem. For a significant anharmonicity, the low-frequency bound states describe two vibrons trapped onto the same amide-I vibration, whereas the high-frequency bound states refer to the trapping of the two vibrons onto nearest neighbor amide-I vibrations.

Biophysical Phenomena↗
PubMed2004

Variational procedure for time-dependent processes.

A simple variational Lagrangian is proposed for the time development of an arbitrary density matrix, employing the "factorization" of the density. Only the "kinetic energy" appears in the Lagrangian. The formalism applies to pure and mixed state cases, the Navier-Stokes equations of hydrodynamics, transport theory, etc. It recaptures the least dissipation function condition of Rayleigh-Onsager and in practical applications is flexible. The variational proposal is tested on a two-level system interacting that is subject, in one instance, to an interaction with a single oscillator and, in another, that evolves in a dissipative mode.

Kinetics↗
PubMed2006

Universal finite-sample effect on the perturbation growth in chaotic dynamical systems.

The finite-sample effect on the growth of moments of the perturbation observed in numerical simulations of chaotic dynamical systems is studied. To numerically estimate the moments, only a limited number of sample trajectories can be utilized, and therefore the moments exhibit pure exponential growth only initially, and give way to relaxed growth thereafter. Such transition is a consequence of the unobservability of rare events in finite sample sets. Using the large-deviation formalism for chaotic time series, we estimate the relaxation time and derive the post-relaxation growth law. We demonstrate that even after the relaxation, each moment still obeys a universal growth law of different type, which reflects physical information on the statistics of chaotic expansion rates.

Journal Article↗
PubMed2004

Channel formation and intermediate range order in sodium silicate melts and glasses.

We use inelastic neutron scattering and molecular dynamics simulation to investigate the interplay between the structure and the fast sodium ion diffusion in various sodium silicates. With increasing temperature and decreasing density the structure factors exhibit an emerging prepeak around 0.9 A(-1). We show that this prepeak has its origin in the formation of sodium rich channels in the static structure. The channels serve as preferential ion conducting pathways in the relative immobile Si-O matrix. On cooling below the glass transition this intermediate range order is frozen in.

Diffusion↗
PubMed2006

Dynamical slowdown of polymers in laminar and random flows.

The influence of an external flow on the relaxation dynamics of a single polymer is investigated theoretically and numerically. We show that a pronounced dynamical slowdown occurs in the vicinity of the coil-stretch transition, especially when the dependence on polymer conformation of the drag is accounted for. For the elongational flow, relaxation times are exceedingly larger than the Zimm relaxation time, resulting in the observation of conformation hysteresis. For random smooth flows, hysteresis is not present. Yet, relaxation dynamics is significantly slowed down because of the large variety of accessible polymer configurations. The implications of these results for the modeling of dilute polymer solutions in turbulent flows are addressed.

Acrylic Resins↗
PubMed2004

On the robustness of stride frequency estimation.

The robustness of stride frequency estimation (location and spread) from stride period data is investigated using influence functions. Theoretical analysis reveals that stride frequency estimates by Stokes et al. and by direct calculation have unbounded influence functions and zero breakdown points, implying a lack of both local and global robustness. Comparison of estimates obtained from an ensemble of pathological gait stride time series shows that on average, differences among estimators are not statistically significant (p > or = 0.59) for long time series (hundreds of strides). Specific circumstances under which nonrobust estimates depart from robust estimates are investigated in terms of outlier influence. We recommend some heuristic rules-of-thumb for prudent selection of nonrobust stride frequency estimators for a given stride time series. The theoretical and empirical estimator comparisons suggest that in general, more research on estimator robustness in quantitative gait analysis is warranted.

Adult↗
PubMed2004

Intersection of microwire electrodes with proximal CA1 stratum-pyramidale neurons at insertion for multiunit recordings predicted by a 3-D computer model.

It is of broad interest in the context of neuronal multiunit extracellular recordings to understand electrode-tissue interactions in order to maximize the number of recordable units and to minimize experimental artifacts due to mechanical tissue alteration. Toward this goal, a computer model of microwire electrode insertion in hippocampus CA1 area was developed, firstly to provide estimates of the number of electrode-neuron intersections affecting recordable (local) neurons and, secondly, to determine optimal insertion/electrode parameters that minimize the number of intersections. The model predicts that in hippocampus CA1 area, using an electrode 50 microm in diameter, only 10% of the recordable neurons (those within 50 microm of the electrode), would remain collision free. Moreover, the model also predicts that inhibitory neurons are less prone to be intersected by the electrode, resulting in a 2 to threefold higher percentage of collision-free interneurons than expected from the relative densities of pyramidal cells and interneurons. Furthermore, the model confirms, in agreement with experimental observations, that electrode tilting with respect to the main neuronal axis increases the number of intact neurons (fourfold for a 50-microm electrode at 45 degrees when compared to 0 degrees , i.e., an insertion normal to the cell body layer).

Animals↗
PubMed2005

Investigating the stationarity of paediatric aspiration signals.

An aspiration signal is the time-varying anterior-posterior acceleration measured infero-anterior to the thyroid notch when foreign material enters the airway during inspiration. The hypothesis of weak stationarity is tested on aspiration signals by the reverse arrangements test. Results indicate that aspiration signals cannot be uniformly regarded as weakly stationary. Forty-five percent of the examined signals violated the stationarity hypothesis. For these signals, time-varying variance and spectral density structure are identified as major sources of nonstationarity. Stationarity test results generally corroborate qualitative clinical descriptions of aspiration. However, stationarity analysis indicates that aspiration signals are highly heterogenous, a finding which poses significant challenges to the automatic detection of aspirations by accelerometry.

Acceleration↗
PubMed2006

A composite score for predicting errors in protein structure models.

Reliable prediction of model accuracy is an important unsolved problem in protein structure modeling. To address this problem, we studied 24 individual assessment scores, including physics-based energy functions, statistical potentials, and machine learning-based scoring functions. Individual scores were also used to construct approximately 85,000 composite scoring functions using support vector machine (SVM) regression. The scores were tested for their abilities to identify the most native-like models from a set of 6000 comparative models of 20 representative protein structures. Each of the 20 targets was modeled using a template of <30% sequence identity, corresponding to challenging comparative modeling cases. The best SVM score outperformed all individual scores by decreasing the average RMSD difference between the model identified as the best of the set and the model with the lowest RMSD (DeltaRMSD) from 0.63 A to 0.45 A, while having a higher Pearson correlation coefficient to RMSD (r=0.87) than any other tested score. The most accurate score is based on a combination of the DOPE non-hydrogen atom statistical potential; surface, contact, and combined statistical potentials from MODPIPE; and two PSIPRED/DSSP scores. It was implemented in the SVMod program, which can now be applied to select the final model in various modeling problems, including fold assignment, target-template alignment, and loop modeling.

Models, Molecular↗
PubMed2006

Are transient increases in blood pressure during the treadmill stress test associated with headache?

We aimed to investigate whether transient increases in blood pressure (BP) are associated with headache, by provoking hypertension in 50 non-hypertensive cephalalgia sufferers (Ce), 50 hypertensive cephalalgia sufferers (HyCe) and 50 healthy subjects (C) with a treadmill stress test. Occurrence of headache among groups did not differ (P = 0.81); however, women developed headache more frequently than men (P < 0.0001). The 35 individuals who developed headache during the test compared with those who did not (regardless of their original group) had higher absolute diastolic BP and higher absolute and relative diastolic BP increase during the test (P < 0.0001). HyCe patients that developed headache had higher diastolic BP than those that did not (P < 0.002). For each group, the absolute/relative mean increases of systolic/diastolic BP did not differ between patients that developed headache and those that did not excepting relative diastolic BP increase in Ce (28% vs. 15%, P = 0.006). The appearance of headache during dynamic exercise is not associated with a history of headache or hypertension. In non-hypertensive cephalalgia sufferers, relative diastolic BP increases of >27% are associated with headache attacks whereas in HyCe sufferers absolute diastolic BP values may be more important.

Adult↗
PubMed1979

Alcohol problems and the availability of alcohol.

This study assesses the statistical effects of the physical availability of alcohol as measured by the number of liquor store employees per 100,000 persons. Controlling for the effects of per capita income and urbanism, it is found that there are effects of physical availability on current tangible consequences but not on alcoholism rates or frequent heavy drinking.

Alcohol Drinking↗