1,876 research outputs found
Surface-induced near-field scaling in the Knudsen layer of a rarefied gas
We report on experiments performed within the Knudsen boundary layer of a
low-pressure gas. The non-invasive probe we use is a suspended
nano-electro-mechanical string (NEMS), which interacts with He gas at
cryogenic temperatures. When the pressure is decreased, a reduction of the
damping force below molecular friction had been first reported in
Phys. Rev. Lett. Vol 113, 136101 (2014) and never reproduced since. We
demonstrate that this effect is independent of geometry, but dependent on
temperature. Within the framework of kinetic theory, this reduction is
interpreted as a rarefaction phenomenon, carried through the boundary layer by
a deviation from the usual Maxwell-Boltzmann equilibrium distribution induced
by surface scattering. Adsorbed atoms are shown to play a key role in the
process, which explains why room temperature data fail to reproduce it.Comment: Article plus supplementary materia
Geometrical Nonlinearity of Circular Plates and Membranes: an Alternative Method
We apply the well-established theoretical method developed for geometrical
nonlinearities of micro/nano-mechanical clamped beams to circular drums. The
calculation is performed under the same hypotheses, the extra difficulty being
to analytically describe the (coordinate-dependent) additional stress generated
in the structure by the motion. Specifically, the model applies to
non-axisymmetric mode shapes. An analytic expression is produced for the
Duffing (hardening) nonlinear coefficient, which requires only the knowledge of
the mode shape functions to be evaluated. This formulation is simple to handle,
and does not rely on complex numerical methods. Moreover, no hypotheses are
made on the drive scheme and the nature of the in-plane stress: it is not
required to be of electrostatic origin. We confront our predictions with both
typical experimental devices and relevant theoretical results from the
literature. Generalization of the presented method to Duffing-type
mode-coupling should be a straightforward extension of this work. We believe
that the presented modeling will contribute to the development of nonlinear
physics implemented in 2D micro/nano-mechanical structures
Force dependent fragility in RNA hairpins
We apply Kramers theory to investigate the dissociation of multiple bonds
under mechanical force and interpret experimental results for the
unfolding/refolding force distributions of an RNA hairpin pulled at different
loading rates using laser tweezers. We identify two different kinetic regimes
depending on the range of forces explored during the unfolding and refolding
process. The present approach extends the range of validity of the two-states
approximation by providing a theoretical framework to reconstruct free-energy
landscapes and identify force-induced structural changes in molecular
transition states using single molecule pulling experiments. The method should
be applicable to RNA hairpins with multiple kinetic barriers.Comment: Latex file, 4 pages+3 figure
Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2
75As nuclear magnetic resonance (NMR) experiments were performed on
Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer
magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift
and is found similar to that of BaFe2As2, implying that Mn does not induce
charge doping. A satellite line associated with the Mn nearest neighbours
(n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries
a local magnetic moment. This is confirmed by the main line broadening typical
of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to
the localization of the additional Mn hole. These findings explain why Mn does
not induce superconductivity in the pnictides contrary to other dopants such as
Co, Ni, Ru or K.Comment: 6 pages, 7 figure
Systematic Study of High p_T Hadron Spectra in pp, pA and AA Collisions from SPS to RHIC Energies
High- particle spectra in (), and
collisions are calculated within a QCD parton model in which intrinsic
transverse momentum, its broadening due to initial multiple parton scattering,
and jet quenching due to parton energy loss inside a dense medium are included
phenomenologically. The intrinsic and its broadening in and
collisions due to initial multiple parton scattering are found to be very
important at low energies ( GeV). Comparisons with ,
and data with different centrality cuts show that the differential
cross sections of large transverse momentum pion production ( GeV/)
in collisions scale very well with the number of binary nucleon-nucleon
collisions (modulo effects of multiple initial scattering). This indicates that
semi-hard parton scattering is the dominant particle production mechanism
underlying the hadron spectra at moderate GeV/.
However, there is no evidence of jet quenching or parton energy loss. Within
the parton model, one can exclude an effective parton energy loss
GeV/fm and a mean free path fm from the
experimental data of collisions at the SPS energies. Predictions for high
particle spectra in and collisions with and without jet
quenching at the RHIC energy are also given. Uncertainties due to initial
multiple scattering and nuclear shadowing of parton distributions are also
discussed.Comment: 13 pages in RevTex with 14 figures, the final published version (with
some typos corrected
Motif Discovery in Physiological Datasets: A Methodology for Inferring Predictive Elements
In this article, we propose a methodology for identifying predictive physiological patterns in the absence of prior knowledge. We use the principle of conservation to identify activity that consistently precedes an outcome in patients, and describe a two-stage process that allows us to efficiently search for such patterns in large datasets. This involves first transforming continuous physiological signals from patients into symbolic sequences, and then searching for patterns in these reduced representations that are strongly associated with an outcome.
Our strategy of identifying conserved activity that is unlikely to have occurred purely by chance in symbolic data is analogous to the discovery of regulatory motifs in genomic datasets. We build upon existing work in this area, generalizing the notion of a regulatory motif and enhancing current techniques to operate robustly on non-genomic data. We also address two significant considerations associated with motif discovery in general: computational efficiency and robustness in the presence of degeneracy and noise. To deal with these issues, we introduce the concept of active regions and new subset-based techniques such as a two-layer Gibbs sampling algorithm. These extensions allow for a framework for information inference, where precursors are identified as approximately conserved activity of arbitrary complexity preceding multiple occurrences of an event.
We evaluated our solution on a population of patients who experienced sudden cardiac death and attempted to discover electrocardiographic activity that may be associated with the endpoint of death. To assess the predictive patterns discovered, we compared likelihood scores for motifs in the sudden death population against control populations of normal individuals and those with non-fatal supraventricular arrhythmias. Our results suggest that predictive motif discovery may be able to identify clinically relevant information even in the absence of significant prior knowledge.CIMIT: Center for Integration of Medicine and Innovative TechnologyHarvard University--MIT Division of Health Sciences and Technolog
On-chip thermometry for microwave optomechanics implemented in a demagnetization cryostat
We report on microwave optomechanics measurements performed on a nuclear
adiabatic demagnetization cryostat, whose temperature is determined by accurate
thermometry from below 500K to about 1Kelvin. We describe a method for
accessing the on-chip temperature, building on the blue-detuned parametric
instability and a standard microwave setup. The capabilities and sensitivity of
both the experimental arrangement and the developed technique are demonstrated
with a very weakly coupled silicon-nitride doubly-clamped beam mode of about
4MHz and a niobium on-chip cavity resonating around 6GHz. We report on an
unstable intrinsic driving force in the coupled microwave-mechanical system
acting on the mechanics that appears below typically 100mK. The origin of
this phenomenon remains unknown, and deserves theoretical input. It prevents us
from performing reliable experiments below typically 10-30mK; however no
evidence of thermal decoupling is observed, and we propose that the same
features should be present in all devices sharing the microwave technology, at
different levels of strengths. We further demonstrate empirically how most of
the unstable feature can be annihilated, and speculate how the mechanism could
be linked to atomic-scale two level systems. The described
microwave/microkelvin facility is part of the EMP platform, and shall be used
for further experiments within and below the millikelvin range.Comment: 14 pages with appendi
Beyond linear coupling in microwave optomechanics
We explore the nonlinear dynamics of a cavity optomechanical system. Our realization consisting of a drumhead nano-electro-mechanical resonator (NEMS) coupled to a microwave cavity, allows for a nearly ideal platform to study the nonlinearities arising purely due to radiation-pressure physics. Experiments are performed under a strong microwave Stokes pumping which triggers mechanical self-sustained oscillations. We analyze the results in the framework of an extended nonlinear optome-chanical theory, and demonstrate that quadratic and cubic coupling terms in the opto-mechanical Hamiltonian have to be considered. Quantitative agreement with the measurements is obtained considering only genuine geometrical nonlinearities: no thermo-optical instabilities are observed, in contrast with laser-driven systems. Based on these results, we describe a method to quantify nonlin-ear properties of microwave optomechanical devices. Such a technique, available now in the quantum electro-mechanics toolbox, but completely generic, is mandatory for the development of new schemes where higher-order coupling terms are proposed as a new resource, like Quantum Non-Demolition measurements, or in the search for new fundamental quantum signatures, like Quantum Gravity. We also find that the motion imprints a wide comb of extremely narrow peaks in the microwave output field, which could also be exploited in specific microwave-based measurements, potentially limited only by the quantum noise of the optical and the mechanical fields for a ground-state cooled NEMS device
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