321 research outputs found
Low density expansion for Lyapunov exponents
In some quasi-one-dimensional weakly disordered media, impurities are large
and rare rather than small and dense. For an Anderson model with a low density
of strong impurities, a perturbation theory in the impurity density is
developed for the Lyapunov exponent and the density of states. The Lyapunov
exponent grows linearly with the density. Anomalies of the Kappus-Wegner type
appear for all rational quasi-momenta even in lowest order perturbation theory
Random Fibonacci Sequences
Solutions to the random Fibonacci recurrence x_{n+1}=x_{n} + or - Bx_{n-1}
decrease (increase) exponentially, x_{n} = exp(lambda n), for sufficiently
small (large) B. In the limits B --> 0 and B --> infinity, we expand the
Lyapunov exponent lambda(B) in powers of B and B^{-1}, respectively. For the
classical case of we obtain exact non-perturbative results. In
particular, an invariant measure associated with Ricatti variable
r_n=x_{n+1}/x_{n} is shown to exhibit plateaux around all rational.Comment: 11 Pages (Multi-Column); 3 EPS Figures ; Submitted to J. Phys.
Persistent Currents in Multichannel Interacting Systems
Persistent currents of disordered multichannel mesoscopic rings of spinless
interacting fermions threaded by a magnetic flux are calculated using exact
diagonalizations and self-consistent Hartree-Fock methods. The validity of the
Hartree-Fock approximation is controled by a direct comparison with the exact
results on small clusters. For sufficiently large disorder
(diffusive regime), the effect of repulsive interactions on the current
distribution is to slightly decrease its width (mean square current) but to
{\it increase} its mean value (mean current). This effect is stronger in the
case of a long range repulsion. Our results suggest that the coupling between
the chains is essential to understand the large currents observed
experimentally.Comment: Revised version, uuencoded compressed file including fig
Shaping Biological Knowledge: Applications in Proteomics
The central dogma of molecular biology has provided a meaningful principle
for data integration in the field of genomics. In this context, integration reflects
the known transitions from a chromosome to a protein sequence: transcription,
intron splicing, exon assembly and translation. There is no such clear principle for
integrating proteomics data, since the laws governing protein folding and interactivity
are not quite understood. In our effort to bring together independent pieces of
information relative to proteins in a biologically meaningful way, we assess the bias of
bioinformatics resources and consequent approximations in the framework of small-scale
studies. We analyse proteomics data while following both a data-driven (focus
on proteins smaller than 10 kDa) and a hypothesis-driven (focus on whole bacterial
proteomes) approach. These applications are potentially the source of specialized
complements to classical biological ontologies
The Effects of a Multiflavonoid Supplement on Vascular and Hemodynamic Parameters following Acute Exercise
Antioxidants can decrease oxidative stress and combined with acute exercise they may lead to further decreases in blood pressure. The purpose of this study was to investigate the effects of 2 weeks of antioxidant supplementation on vascular distensibility and cardiovascular hemodynamics during postexercise hypotension. Methods. Twenty young subjects were randomized to placebo (n = 10) or antioxidant supplementation (n = 10) for two weeks. Antioxidant status, vascular distensibility, and hemodynamics were obtained before, immediately, and 30 minutes after an acute bout of aerobic exercise both before and after supplementation. Results. Two weeks of antioxidant supplementation resulted in a greater systolic blood pressure (SBP) decrease during postexercise hypotension (PEH) and significant decreases in augmentation index versus placebo (12.5% versus 3.5%, resp.). Also ferric-reducing ability of plasma (FRAP) increased significantly (interaction P = 0.024) after supplementation. Conclusion. Supplementation showed an additive effect on PEH associated with increased FRAP values and decreases in systolic blood pressure and augmentation index
Suppression of Persistent Currents in 1-D Disordered Rings by Coulomb Interaction
Effects of Coulomb interaction on persistent currents in disordered
one-dimensional rings are numerically investigated. First of all effectiveness
of the Hartree-Fock approximation is established on small systems. Then the
calculations are done for systems with 40 electrons in 100 sites. It is found
that the amplitude of the average persistent current in the diffusive regime is
suppressed as the strength of the Coulomb interaction increases. The
suppression of the current is stronger in larger rings than in smaller ones.
The enhancement of the current by the electron-electron interaction was not
observed in the diffusive regime.Comment: 9 pages (RevTeX), 4 figures available upon request
([email protected]), KCMG-preprint-HK
Anderson-localization versus delocalization of interacting fermions in one dimension
Using the density matrix renormalization group algorithm, we investigate the
lattice model for spinless fermions in one dimension in the presence of a
strong interaction and disorder. The phase sensitivity of the ground state
energy is determined with high accuracy for systems up to a size of 60 lattice
constants. This quantity is found to be log-normally distributed. The
fluctuations grow algebraically with system size with a universal exponent of
~2/3 in the localized region of the phase diagram. Surprizingly, we find, for
an attractive interaction, a delocalized phase of finite extension. The
boundary of this delocalized phase is determined.Comment: 5 pages, 6 figures, revte
Differential Post-Exercise Blood Pressure Responses between Blacks and Caucasians
Post-exercise hypotension (PEH) is widely observed in Caucasians (CA) and is associated with histamine receptors 1- and 2- (H1R and H2R) mediated post-exercise vasodilation. However, it appears that blacks (BL) may not exhibit PEH following aerobic exercise. Hence, this study sought to determine the extent to which BL develop PEH, and the contri- bution of histamine receptors to PEH (or lack thereof) in this population. Forty-nine (22 BL, 27 CA) young and healthy subjects completed the study. Subjects were randomly assigned to take either a combined H1R and H2R antagonist (fexofenadine and ranitidine) or a con- trol placebo. Supine blood pressure (BP), cardiac output and peripheral vascular resistance measurements were obtained at baseline, as well as at 30 min, 60 min and 90 min after 45 min of treadmill exercise at 70% heart rate reserve. Exercise increased diastolic BP in young BL but not in CA. Post-exercise diastolic BP was also elevated in BL after exercise with histamine receptor blockade. Moreover, H1R and H2R blockade elicited differential responses in stroke volume between BL and CA at rest, and the difference remained follow- ing exercise. Our findings show differential BP responses following exercise in BL and CA, and a potential role of histamine receptors in mediating basal and post-exercise stroke vol- ume in BL. The heightened BP and vascular responses to exercise stimulus is consistent with the greater CVD risk in BL
Failure of single-parameter scaling of wave functions in Anderson localization
We show how to use properties of the vectors which are iterated in the
transfer-matrix approach to Anderson localization, in order to generate the
statistical distribution of electronic wavefunction amplitudes at arbitary
distances from the origin of disordered systems. For
our approach is shown to reproduce exact diagonalization results
available in the literature. In , where strips of width sites
were used, attempted fits of gaussian (log-normal) forms to the wavefunction
amplitude distributions result in effective localization lengths growing with
distance, contrary to the prediction from single-parameter scaling theory. We
also show that the distributions possess a negative skewness , which is
invariant under the usual histogram-collapse rescaling, and whose absolute
value increases with distance. We find for the
range of parameters used in our study, .Comment: RevTeX 4, 6 pages, 4 eps figures. Phys. Rev. B (final version, to be
published
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