36,295 research outputs found
Quantum Levy flights and multifractality of dipolar excitations in a random system
We consider dipolar excitations propagating via dipole-induced exchange among
immobile molecules randomly spaced in a lattice. The character of the
propagation is determined by long-range hops (Levy flights). We analyze the
eigen-energy spectra and the multifractal structure of the wavefunctions. In 1D
and 2D all states are localized, although in 2D the localization length can be
extremely large leading to an effective localization-delocalization crossover
in realistic systems. In 3D all eigenstates are extended but not always
ergodic, and we identify the energy intervals of ergodic and non-ergodic
states. The reduction of the lattice filling induces an ergodic to non-ergodic
transition, and the excitations are mostly non-ergodic at low filling.Comment: 5 pages, 6 figure
Gravitational waves in the generalized Chaplygin gas model
The consequences of taking the generalized Chaplygin gas as the dark energy
constituent of the Universe on the gravitational waves are studied and the
spectrum obtained from this model, for the flat case, is analyzed. Besides its
importance for the study of the primordial Universe, the gravitational waves
represent an additional perspective (besides the CMB temperature and
polarization anisotropies) to evaluate the consistence of the different dark
energy models and establish better constraints to their parameters. The
analysis presented here takes this fact into consideration to open one more
perspective of verification of the generalized Chapligin gas model
applicability. Nine particular cases are compared: one where no dark energy is
present; two that simulate the -CDM model; two where the gas acts like
the traditional Chaplygin gas; and four where the dark energy is the
generalized Chaplygin gas. The different spectra permit to distinguish the
-CDM and the Chaplygin gas scenarios.Comment: Latex file, 9 pages, 11 figures eps forma
Birth size and breast cancer risk: Re-analysis of individual participant data from 32 studied
Background Birth size, perhaps a proxy for prenatal environment, might be a correlate of subsequent breast cancer risk, but findings from epidemiological studies have been inconsistent. We re-analysed individual participant data from published and unpublished studies to obtain more precise estimates of the magnitude and shape of the birth size–breast cancer association. Methods and Findings Studies were identified through computer-assisted and manual searches, and personal communication with investigators. Individual participant data from 32 studies, comprising 22,058 breast cancer cases, were obtained. Random effect models were used, if appropriate, to combine study-specific estimates of effect. Birth weight was positively associated with breast cancer risk in studies based on birth records (pooled relative risk [RR] per one standard deviation [SD] [= 0.5 kg] increment in birth weight: 1.06; 95% confidence interval [CI] 1.02–1.09) and parental recall when the participants were children (1.02; 95% CI 0.99–1.05), but not in those based on adult self-reports, or maternal recall during the woman's adulthood (0.98; 95% CI 0.95–1.01) (p for heterogeneity between data sources = 0.003). Relative to women who weighed 3.000–3.499 kg, the risk was 0.96 (CI 0.80–1.16) in those who weighed < 2.500 kg, and 1.12 (95% CI 1.00–1.25) in those who weighed ≥ 4.000 kg (p for linear trend = 0.001) in birth record data. Birth length and head circumference from birth records were also positively associated with breast cancer risk (pooled RR per one SD increment: 1.06 [95% CI 1.03–1.10] and 1.09 [95% CI 1.03–1.15], respectively). Simultaneous adjustment for these three birth size variables showed that length was the strongest independent predictor of risk. The birth size effects did not appear to be confounded or mediated by established breast cancer risk factors and were not modified by age or menopausal status. The cumulative incidence of breast cancer per 100 women by age 80 y in the study populations was estimated to be 10.0, 10.0, 10.4, and 11.5 in those who were, respectively, in the bottom, second, third, and top fourths of the birth length distribution. Conclusions This pooled analysis of individual participant data is consistent with birth size, and in particular birth length, being an independent correlate of breast cancer risk in adulthood
Plane waves in noncommutative fluids
We study the dynamics of the noncommutative fuid in the Snyder space
perturbatively at the first order in powers of the noncommutative parameter.
The linearized noncommutative fluid dynamics is described by a system of
coupled linear partial differential equations in which the variables are the
fluid density and the fluid potentials. We show that these equations admit a
set of solutions that are monocromatic plane waves for the fluid density and
two of the potentials and a linear function for the third potential. The
energy-momentum tensor of the plane waves is calculated.Comment: 11 pages. Version published as a Lette
Dynamical control of electron spin coherence in a quantum dot
We investigate the performance of dynamical decoupling methods at suppressing
electron spin decoherence from a low-temperature nuclear spin reservoir in a
quantum dot. The controlled dynamics is studied through exact numerical
simulation, with emphasis on realistic pulse delays and long-time limit. Our
results show that optimal performance for this system is attained by a periodic
protocol exploiting concatenated design, with control rates substantially
slower than expected from the upper spectral cutoff of the bath. For a known
initial electron spin state, coherence can saturate at long times, signaling
the creation of a stable ``spin-locked'' decoherence-free subspace. Analytical
insight on saturation is obtained for a simple echo protocol, in good agreement
with numerical results.Comment: 4 pages, 4 figures with 3 of them in colo
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