2,256 research outputs found
Feedback cooling of a cantilever's fundamental mode below 5 mK
We cool the fundamental mechanical mode of an ultrasoft silicon cantilever
from a base temperature of 2.2 K to 2.9 +/- 0.3 mK using active optomechanical
feedback. The lowest observed mode temperature is consistent with limits
determined by the properties of the cantilever and by the measurement noise.
For high feedback gain, the driven cantilever motion is found to suppress or
"squash" the optical interferometer intensity noise below the shot noise level.Comment: 4 pages, 6 figure
Nuclear spin relaxation induced by a mechanical resonator
We report on measurements of the spin lifetime of nuclear spins strongly
coupled to a micromechanical cantilever as used in magnetic resonance force
microscopy. We find that the rotating-frame correlation time of the statistical
nuclear polarization is set by the magneto-mechanical noise originating from
the thermal motion of the cantilever. Evidence is based on the effect of three
parameters: (1) the magnetic field gradient (the coupling strength), (2) the
Rabi frequency of the spins (the transition energy), and (3) the temperature of
the low-frequency mechanical modes. Experimental results are compared to
relaxation rates calculated from the spectral density of the magneto-mechanical
noise.Comment: 4 pages, 4 figure
Towards Efficient Sequential Pattern Mining in Temporal Uncertain Databases
Uncertain sequence databases are widely used to model data with inaccurate or imprecise timestamps in many real world applications. In this paper, we use uniform distributions to model uncertain timestamps and adopt possible world semantics to interpret temporal uncertain database. We design an incremental approach to manage temporal uncertainty efficiently, which is integrated into the classic pattern-growth SPM algorithm to mine uncertain sequential patterns. Extensive experiments prove that our algorithm performs well in both efficiency and scalability
Dynamical Properties of a Growing Surface on a Random Substrate
The dynamics of the discrete Gaussian model for the surface of a crystal
deposited on a disordered substrate is investigated by Monte Carlo simulations.
The mobility of the growing surface was studied as a function of a small
driving force and temperature . A continuous transition is found from
high-temperature phase characterized by linear response to a low-temperature
phase with nonlinear, temperature dependent response. In the simulated regime
of driving force the numerical results are in general agreement with recent
dynamic renormalization group predictions.Comment: 10 pages, latex, 3 figures, to appear in Phys. Rev. E (RC
MRI Shows More Severe Hippocampal Atrophy and Shape Deformation in Hippocampal Sclerosis Than in Alzheimer's Disease
While hippocampal atrophy is a key feature of both hippocampal sclerosis (HS) and Alzheimer's disease (AD), the pathology underlying this finding differs in these two conditions. In AD, atrophy is due primarily to loss of neurons and neuronal volume as a result of neurofibrillary tangle formation. While the etiology of HS is unknown, neuron loss in the hippocampus is severe to complete. We compared hippocampal volume and deformations from premortem MRI in 43 neuropathologically diagnosed cases of HS, AD, and normal controls (NC) selected from a longitudinal study of subcortical ischemic vascular disease (IVD Program Project). HS cases (n = 11) showed loss of neurons throughout the rostral-caudal extent of the hippocampus in one or both hemispheres. AD cases (n = 24) met NIA-Reagan criteria for high likelihood of AD. Normal control cases (n = 8) were cognitively intact and showed no significant AD or hippocampal pathology. The mean hippocampal volumes were significantly lower in HS versus AD groups (P < .001). Mean shape deformations in the CA1 and subiculum differed significantly between HS versus AD, HS versus NC, and AD versus NC (P < .0001). Additional study is needed to determine whether these differences will be meaningful for clinical diagnosis of individual cases
Melting of the classical bilayer Wigner crystal: influence of the lattice symmetry
The melting transition of the five different lattices of a bilayer crystal is
studied using the Monte-Carlo technique. We found the surprising result that
the square lattice has a substantial larger melting temperature as compared to
the other lattice structures, which is a consequence of the specific topology
of the temperature induced defects. A new melting criterion is formulated which
we show to be universal for bilayers as well as for single layer crystals.Comment: 4 pages, 5 figures (postscript files). Accepted in Physical Review
Letter
A Bias-Aware EnKF Estimator for Aerodynamic Flows
Ensemble methods can integrate measurement data and CFD-based models to estimate the state of fluid systems in a robust and cost-efficient way. However, discretization errors can render numerical solutions a biased representation of reality. Left unaccounted for, biased forecast and observation models can lead to poor estimator performance. In this work, we propose a low-rank representation for the bias whose dynamics is represented by a colorednoise process. System state and bias parameters are simultaneously corrected on-line with the Ensemble Kalman Filter (EnKF) algorithm. The proposed methodology is demonstrated to achieve a 70% error reduction for the problem of estimating the state of the two-dimensional low-Re flow past a flat plate at high angle of attack using an ensemble of coarse-mesh simulations and pressure measurements at the surface of the body, compared to a bias-blind estimator. Strategies to determine the bias statistics and to deal with nonlinear observation functions in the context of ensemble methods are discussed
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