1,328 research outputs found
Stimulated wave of polarization in spin chains
Stimulated wave of polarization, triggered by a flip of a single spin,
presents a simple model of quantum amplification. Previously, it has been found
that such wave can be excited in a 1D Ising chain with nearest-neighbor
interactions, irradiated by a weak resonant transverse field. Here we explore
models with more realistic Hamiltonians, in particular, with natural
dipole-dipole interactions. Results of simulations for 1D spin chains and rings
with up to nine spins are presented.Comment: 15 pages, 5 figure
PND67 Impact of Caregiving for Patients with Alzheimer's Disease and Dementia on Psychiatric and Clinical Comorbidities in Brazil
Direct Measurement of the System-Environment Coupling as a Tool For Understanding Decoherence and Dynamical Decoupling
Decoherence is a major obstacle to any practical implementation of quantum
information processing. One of the leading strategies to reduce decoherence is
dynamical decoupling --- the use of an external field to average out the effect
of the environment. The decoherence rate under any control field can be
calculated if the spectrum of the coupling to the environment is known. We
present a direct measurement of the bath coupling spectrum in an ensemble of
optically trapped ultracold atoms, by applying a spectrally narrow-band control
field. The measured spectrum follows a Lorentzian shape at low frequencies, but
exhibits non-monotonic features at higher frequencies due to the oscillatory
motion of the atoms in the trap. These features agree with our analytical
models and numerical Monte-Carlo simulations of the collisional bath. From the
inferred bath-coupling spectrum, we predict the performance of well-known
dynamical decoupling sequences: CPMG, UDD and CDD. We then apply these
sequences in experiment and compare the results to predictions, finding good
agreement in the weak-coupling limit. Thus, our work establishes experimentally
the validity of the overlap integral formalism, and is an important step
towards the implementation of an optimal dynamical decoupling sequence for a
given measured bath spectrum.Comment: 9 pages, 6 figure
Dynamics and Selection of Giant Spirals in Rayleigh-Benard Convection
For Rayleigh-Benard convection of a fluid with Prandtl number \sigma \approx
1, we report experimental and theoretical results on a pattern selection
mechanism for cell-filling, giant, rotating spirals. We show that the pattern
selection in a certain limit can be explained quantitatively by a
phase-diffusion mechanism. This mechanism for pattern selection is very
different from that for spirals in excitable media
The Effect of Stimulus Size on the Reliable Stimulus Range of Perimetry
Purpose: Automated perimetry uses a 3.5 log unit (35dB) range of stimulus contrasts to assess function within the visual field. Using 'Size III' stimuli (0.438), presenting stimuli within the highest 15dB of available contrast may not increase the response probability at locations damaged by glaucoma, due to retinal ganglion cell response saturation. This experiment examines the effect of instead using 'Size V' (1.728) stimuli. Methods: Luminance increment thresholds for circular spot stimuli of each stimulus size were measured in 35 participants (mean deviation À20.9 to À3.4 dB, ages 52-87) using the method of constant stimuli, at four locations per participant. Frequency-ofseeing curves were fit at each size and location, with three free parameters: mean, standard deviation, and asymptotic maximum response probability. These were used to estimate the contrasts to which each participant would respond on 25% of presentations (c25). Results: Using segmented orthogonal regression, the maximum observed response probabilities for size III stimuli began to decline at c25 ¼ 25.2 dB (95% confidence interval 23.3-29.0 dB from bootstrap resampling). This decline started at similar contrast for the size V stimulus: c25 ¼ 25.0dB (22.0-26.8 dB). Among locations at which the sensitivity was above these split-points for both stimulus sizes, c25 averaged 5.6 dB higher for size V than size III stimuli. Conclusions: The lower limit of the reliable stimulus range did not differ significantly between stimulus sizes. However, more locations remained within the reliable stimulus range when using the size V stimulus. Translational Relevance: Size V stimuli enable reliable clinical testing later into the glaucomatous disease process
The Shapes of Flux Domains in the Intermediate State of Type-I Superconductors
In the intermediate state of a thin type-I superconductor magnetic flux
penetrates in a disordered set of highly branched and fingered macroscopic
domains. To understand these shapes, we study in detail a recently proposed
"current-loop" (CL) model that models the intermediate state as a collection of
tense current ribbons flowing along the superconducting-normal interfaces and
subject to the constraint of global flux conservation. The validity of this
model is tested through a detailed reanalysis of Landau's original conformal
mapping treatment of the laminar state, in which the superconductor-normal
interfaces are flared within the slab, and of a closely-related straight-lamina
model. A simplified dynamical model is described that elucidates the nature of
possible shape instabilities of flux stripes and stripe arrays, and numerical
studies of the highly nonlinear regime of those instabilities demonstrate
patterns like those seen experimentally. Of particular interest is the buckling
instability commonly seen in the intermediate state. The free-boundary approach
further allows for a calculation of the elastic properties of the laminar
state, which closely resembles that of smectic liquid crystals. We suggest
several new experiments to explore of flux domain shape instabilities,
including an Eckhaus instability induced by changing the out-of-plane magnetic
field, and an analog of the Helfrich-Hurault instability of smectics induced by
an in-plane field.Comment: 23 pages, 22 bitmapped postscript figures, RevTex 3.0, submitted to
Phys. Rev. B. Higher resolution figures may be obtained by contacting the
author
Postnatal DNA demethylation and its role in tissue maturation.
Development in mammals is accompanied by specific de novo and demethylation events that are thought to stabilize differentiated cell phenotypes. We demonstrate that a large percentage of the tissue-specific methylation pattern is generated postnatally. Demethylation in the liver is observed in thousands of enhancer-like sequences associated with genes that undergo activation during the first few weeks of life. Using. conditional gene ablation strategy we show that the removal of these methyl groups is stable and necessary for assuring proper hepatocyte gene expression and function through its effect on chromatin accessibility. These postnatal changes in methylation come about through exposure to hormone signaling. These results define the molecular rules of 5-methyl-cytosine regulation as an epigenetic mechanism underlying cellular responses to. changing environment
Uneven integration for perception and action cues in children’s working memory
We examined the development of visual cue integration in a desktop working-memory task using boxes with different visual action cues (opening actions) and perceptual surface cues (colours, monochromatic textures, or images of faces). Children had to recall which box held a hidden toy, based on (a) the action cue, (b) the surface cue, or (c) a conjunction of the two. Results from three experiments show a set of asymmetries in children's integration of action and surface cues. The 18–24-month-olds disregarded colour in conjunction judgements with action; 30–36-month-olds used colour but disregarded texture. Images of faces were not disregarded at either age. We suggest that 18–24-month-olds' disregard of colour, seen previously in reorientation tasks (Hermer & Spelke, 1994), may represent a general phenomenon, likened to uneven integration between the dorsal and ventral streams in early development
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