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Results for “crossover”

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At least 307 records · Page 17Linked to original sources

PubMed2005

Case-crossover analyses of air pollution exposure data: referent selection strategies and their implications for bias.

The case-crossover design has been widely used to study the association between short-term air pollution exposure and the risk of an acute adverse health event. The design uses cases only; for each individual case, exposure just before the event is compared with exposure at other control (or "referent") times. Time-invariant confounders are controlled by making within-subject comparisons. Even more important in the air pollution setting is that time-varying confounders can also be controlled by design by matching referents to the index time. The referent selection strategy is important for reasons in addition to control of confounding. The case-crossover design makes the implicit assumption that there is no trend in exposure across the referent times. In addition, the statistical method that is used-conditional logistic regression-is unbiased only with certain referent strategies. We review here the case-crossover literature in the air pollution context, focusing on key issues regarding referent selection. We conclude with a set of recommendations for choosing a referent strategy with air pollution exposure data. Specifically, we advocate the time-stratified approach to referent selection because it ensures unbiased conditional logistic regression estimates, avoids bias resulting from time trend in the exposure series, and can be tailored to match on specific time-varying confounders.

Acute Disease↗

PubMed2005

Strong correlation between meiotic crossovers and haplotype structure in a 2.5-Mb region on the long arm of chromosome 21.

Although the haplotype structure of the human genome has been studied in great detail, very little is known about the mechanisms underlying its formation. To investigate the role of meiotic recombination on haplotype block formation, single nucleotide polymorphisms were selected at a high density from a 2.5-Mb region of human chromosome 21. Direct analysis of meiotic recombination by high-throughput multiplex genotyping of 662 single sperm identifies 41 recombinants. The crossovers were nonrandomly distributed within 16 small areas. All, except one, of these crossovers fall in areas where the haplotype structure exhibits breakdown, displaying a strong statistically positive association between crossovers and haplotype block breaks. The data also indicate a particular clustered distribution of recombination hotspots within the region. This finding supports the hypothesis that meiotic recombination makes a primary contribution to haplotype block formation in the human genome.

Chromosome Mapping↗

PubMed1999

Thermally activated escape rates of uniaxial spin systems with transverse field: uniaxial crossovers.

Classical escape rates of uniaxial spin systems are characterized by a prefactor differing from and much smaller than that of the particle problem, since the maximum of the spin energy is attained everywhere on the line of constant latitude: theta =const, 0 < or= phi < or = 2 pi. If a transverse field is applied, a saddle point of the energy is formed, and high, moderate, and low damping regimes (similar to those for particles) appear. Here we present the first analytical and numerical study of crossovers between the uniaxial and other regimes for spin systems. It is shown that there is one HD-Uniaxial crossover, whereas at low damping the uniaxial and LD regimes are separated by two crossovers.

Journal Article↗

PubMed2000

Crossover behavior in 3He and Xe near their liquid-vapor critical point

We present a detailed discussion of the crossover from mean-field to Ising critical behavior upon approach of the critical point, both for 3He and Xe. By combining different sets of experimental data, we are able to cover an unusually large temperature range on either side of the critical temperature T(c). Below T(c), we thus can make an accurate comparison with a recent calculation for the crossover of the coexistence curve. For the regime above T(c), an analysis of the compressibility demonstrates that the crossover regime in 3He is unexpectedly widened by a subtle interplay between quantum and critical fluctuations.

Journal Article↗

PubMed2000

Crossover from the hydrodynamic regime to the thermal fluctuation regime in a two-dimensional phase-separating binary fluid containing surfactants

Extensive simulations were carried out to investigate the crossover between the hydrodynamic regime at intermediate stage and the thermal fluctuation regime at late stage in a phase-separating binary fluid/surfactant system in two dimensions. The existence of the crossover and its dependence on the surfactant concentration were analyzed using Kawasaki and Ohta's interface kinetic equation [Physica A 118, 175 (1983)]. The analysis showed that there should exist a critical surfactant concentration, above which thermal fluctuations dominate phase separation at all times. Simulations suggested that the crossover exists and the hydrodynamic regime shrinks when surfactant concentration increases. Simulations also demonstrated that the trapped surfactants seen in a previous study [Phys. Rev. E 59, 2109 (1999)] can remain trapped for a time much longer than the time needed to form well segregated domains, in spite of the presence of significant thermal fluctuations.

Journal Article↗

PubMed2001

Crossover behavior in the isothermal susceptibility near the 3He critical point.

We present high-resolution measurements of the isothermal susceptibility of pure 3He near the liquid-gas critical point. PVT measurements were performed in the single-phase region over the reduced temperature range 3 x 10(-5)<T/Tc-1<1.5 x 10(-1). The crossover behavior of the susceptibility along the critical isochore was analyzed using a field-theoretical renormalization-group calculation based on the phi4 model. A similar crossover analysis was performed on previously obtained Xe susceptibility measurements. A comparison of the rescaled susceptibility for 3He and Xe shows theoretically predicted universal crossover behavior.

Journal Article↗

PubMed2001

Crossover parametric equation of state for Ising-like systems.

We present a parametric equation for the thermodynamic properties in the critical region of three-dimensional Ising-like systems which include fluids and fluid mixtures. The equation of state incorporates a crossover from singular Ising behavior asymptotically close to the critical point to classical (mean-field) behavior further away from the critical point, characterized by two physical crossover parameters: a coupling constant related to the strength and range of molecular interactions and a "cutoff" wave number for the critical fluctuations. In the asymptotic Ising limit, the crossover equation reproduces the most recent theoretical estimates for the universal ratios of the leading and correction-to-scaling critical amplitudes. The equation has been tested by comparing it with recent experimental thermodynamic-property data for 3He near its vapor-liquid critical point.

Journal Article↗

PubMed2002

Crossover effects in a discrete deposition model with Kardar-Parisi-Zhang scaling.

We simulated a growth model in (1+1) dimensions in which particles are aggregated according to the rules of ballistic deposition with probability p or according to the rules of random deposition with surface relaxation (Family model) with probability 1-p. For any p>0, this system is in the Kardar-Parisi-Zhang (KPZ) universality class, but it presents a slow crossover from the Edwards-Wilkinson class (EW) for small p. From the scaling of the growth velocity, the parameter p is connected to the coefficient lambda of the nonlinear term of the KPZ equation, giving lambda approximately p(gamma), with gamma=2.1+/-0.2. Our numerical results confirm the interface width scaling in the growth regime as W approximately lambda(beta)t(beta) and the scaling of the saturation time as tau approximately lambda(-1)L(z), with the expected exponents beta=1/3 and z=3/2, and strong corrections to scaling for small lambda. This picture is consistent with a crossover time from EW to KPZ growth in the form t(c) approximately lambda(-4) approximately p(-8), in agreement with scaling theories and renormalization group analysis. Some consequences of the slow crossover in this problem are discussed and may help investigations of more complex models.

Journal Article↗

PubMed2003

Dynamics of crossover from a chaotic to a power-law state in jerky flow.

We study the dynamics of an intriguing crossover from a chaotic to a power-law state as a function of strain rate within the context of a recently introduced model that reproduces the crossover. While the chaotic regime has a small set of positive Lyapunov exponents, interestingly, the scaling regime has a power-law distribution of null exponents which also exhibits a power law. The slow-manifold analysis of the model shows that while a large proportion of dislocations are pinned in the chaotic regime, most of them are pushed to the threshold of unpinning in the scaling regime, thus providing an insight into the mechanism of crossover.

Journal Article↗

PubMed2004

Self-affine crossover length in a layered silicate deposit.

Self-affine dehydrated colloidal deposits on fresh mica surfaces of the synthetic layered silicate 2:1 smectite clay laponite have been studied by means of atomic force microscopy (AFM). AFM images of these prepared assemblies of sol and gel aggregates have been analyzed both by means of standard AFM Fourier software and a wavelet method. The deposited surfaces show a persistence to antipersistent crossover with a clay concentration dependent crossover length. It is concluded that the crossover length is associated with aggregate size, and further that the persistent roughness at small length scales signals near compact clusters of fractal dimension three, whereas the antipersistent roughness at large length scales signals a sedimentation process.

Journal Article↗

PubMed2004

Crossover behavior in three-dimensional dilute spin systems.

We study the crossover behaviors that can be observed in the high-temperature phase of three-dimensional dilute spin systems, using a field-theoretical approach. In particular, for randomly dilute Ising systems we consider the Gaussian-to-random and the pure-Ising-to-random crossover, determining the corresponding crossover functions for the magnetic susceptibility and the correlation length. Moreover, for the physically interesting cases of dilute Ising, XY, and Heisenberg systems, we estimate several universal ratios of scaling-correction amplitudes entering the high-temperature Wegner expansion of the magnetic susceptibility, of the correlation length, and of the zero-momentum quartic couplings.

Journal Article↗

PubMed2004

Violation of the Widom scaling law for effective crossover exponents.

In this work we consider the universal crossover behavior of two nonequilibrium systems exhibiting a continuous phase transition. Focusing on the field driven crossover from mean-field to non-mean-field scaling behavior we show that the well-known Widom scaling law is violated for the effective exponents in the so-called crossover regime.

Journal Article↗

PubMed2004

Comparison of theoretical models of crossover behavior near the 3He liquid-vapor critical point.

Parametric expressions, based on three different theoretical models, are used to calculate the isothermal susceptibility, specific heat, and order parameter along the critical isochore and coexistence curve from the asymptotic region to the crossover region. These models are (i) the minimal-subtraction renormalization scheme, (ii) the massive renormalization scheme within the phi(4) model, and (iii) the crossover parametric model based on the crossover Landau model. We fit these theories globally to experimental measurements of the isothermal susceptibility and specific heat along the critical isochore and coexistence curve, and to the coexistence curve of 3He near its liquid-vapor critical point. All of the theories provide good agreement with the experimental measurements within the reduced temperature range |t| < or =2 x 10(-2) . The differences in the fits between the theories and the correlations between the adjustable model parameters are discussed.

Journal Article↗

PubMed2005

Possible crossover of a nonuniversal quantity at the upper critical dimension.

We report on a possible crossover of a nonuniversal quantity at the upper critical dimensionality in the field of percolation. Plotting recent estimates for site percolation thresholds of hypercubes in dimension 6 < or =d < or =13 against corresponding predictions from the Galam-Mauger (GM) formula p(c) = p(0) [(d-1) (q-1)](-a) d(b) for percolation thresholds, a significant departure of p(c) is observed for d > or =6 . This result is reminiscent of the crossover undergone by universal quantities in critical phenomena. For bond percolation, evidence of such a crossover of dimensionality would require an improvement of the GM formula to reach a relative error of typically 0.2%, while it is currently at 0.9% for hypercubes.

Journal Article↗

PubMed2005

Crossover behavior in a mixed-mode fiber bundle model.

We introduce a mixed-mode load sharing scheme in a fiber bundle model. This model reduces exactly to equal-load-sharing (ELS) and local-load-sharing (LLS) models at the two extreme limits of a single-load-sharing parameter. We identify two distinct regimes: (a) the mean-field regime where the ELS mode dominates and (b) the short-range regime dominated by the LLS mode. The crossover behavior is explored through a numerical study of the strength variation, the avalanche statistics, susceptibility and relaxation time variations, the correlations among the broken fibers, and their cluster analysis. Analyzing the moments of the cluster size distributions we locate the crossover point of these regimes. We thus conclude that even in one dimension, the fiber bundle model shows crossover behavior from mean-field to short-range interactions.

Journal Article↗

PubMed2000

Measurement of wave chaotic eigenfunctions in the time-reversal symmetry-breaking crossover regime

We present experimental results on eigenfunctions of a wave chaotic system in the continuous crossover regime between time-reversal symmetric and time-reversal symmetry-broken states. The statistical properties of the eigenfunctions of a two-dimensional microwave resonator are analyzed as a function of an experimentally determined time-reversal symmetry-breaking parameter. We test four theories of one-point eigenfunction statistics and introduce a new theory relating the one-point and two-point statistical properties in the crossover regime. We also find a universal correlation between the one-point and two-point statistical parameters for the crossover eigenfunctions.

Journal Article↗

PubMed2000

Supercooled confined water and the mode coupling crossover temperature

We present a molecular dynamics study of the single-particle dynamics of supercooled water confined in a silica pore. Two dynamical regimes are found. Close to the hydrophilic substrate molecules are below the mode coupling crossover temperature, T(C), already at ambient temperature. The water closer to the center of the pore (free water) approaches upon supercooling T(C) as predicted by mode coupling theories. For free water the crossover temperature and crossover exponent gamma are extracted from power-law fits to both the diffusion coefficient and the relaxation time of the late alpha region.

Journal Article↗

PubMed2002

Two crossovers in the Pseudogap regime of YBa 2Cu 3O (7-delta) superconductors observed by ultrafast spectroscopy.

We have investigated the temperature dependence of the optical reflectivity on a femtosecond scale in a near optimally doped YBa 2Cu 3O (7-delta) superconductor. The combined study of the lattice and carrier dynamics at temperatures above T(c) allows us to identify two crossover temperatures in the normal state, giving evidence for an inhomogeneity of the pseudogap regime. These crossovers exhibit a clear hysteresis behavior depending on the direction of temperature change. The carrier and lattice dynamics within the crossover regimes show distinct differences from and similarities to the superconducting state, which may help in choosing between the competing theories for the pseudogap state.

Journal Article↗