642 research outputs found
Compressed correlation functions and fast aging dynamics in metallic glasses
We present x-ray photon correlation spectroscopy measurements of the atomic
dynamics in a Zr67Ni33 metallic glass, well below its glass transition
temperature. We find that the decay of the density fluctuations can be well
described by compressed, thus faster than exponential, correlation functions
which can be modeled by the well-known Kohlrausch-Williams-Watts function with
a shape exponent {\beta} larger than one. This parameter is furthermore found
to be independent of both waiting time and wave-vector, leading to the
possibility to rescale all the correlation functions to a single master curve.
The dynamics in the glassy state is additionally characterized by different
aging regimes which persist in the deep glassy state. These features seem to be
universal in metallic glasses and suggest a non diffusive nature of the
dynamics. This universality is supported by the possibility of describing the
fast increase of the structural relaxation time with waiting time using a
unique model function, independently of the microscopic details of the system.Comment: 7 pages, 4 figures. To be published in J. Chem. Phy
Simulations of aging and plastic deformation in polymer glasses
We study the effect of physical aging on the mechanical properties of a model
polymer glass using molecular dynamics simulations. The creep compliance is
determined simultaneously with the structural relaxation under a constant
uniaxial load below yield at constant temperature. The model successfully
captures universal features found experimentally in polymer glasses, including
signatures of mechanical rejuvenation. We analyze microscopic relaxation
timescales and show that they exhibit the same aging characteristics as the
macroscopic creep compliance. In addition, our model indicates that the entire
distribution of relaxation times scales identically with age. Despite large
changes in mobility, we observe comparatively little structural change except
for a weak logarithmic increase in the degree of short-range order that may be
correlated to an observed decrease in aging with increasing load.Comment: 9 pages, 12 figure
Intermittent quakes and record dynamics in the thermoremanent magnetization of a spin-glass
A novel method for analyzing the intermittent behavior of linear response
data in aging systems is presented and applied to spin-glass thermoremanent
magnetization (TRM) (Rodriguez et al. Phys. Rev. Lett. 91, 037203, 2003).
The probability density function (PDF) of magnetic fluctuations is shown to
have an asymmetric exponential tail, demonstrating that the demagnetization
process is carried by intermittent, significant, spin rearrangements or
\emph{quakes}. These quakes are most pronounced shortly after the field
removal, and in the non-equilibrium aging regime .
For a broad temperature range, we study the dependence of the TRM decay rate on
, the time since the initial quench and on , the time at which the
magnetic field is cut. The and dependence of the rate is extracted
numerically from the data and described analytically using the assumption that
the linear response is subordinated to the intermittent process which
spasmodically release the initial imbalances created by the quench.Comment: 8 pages, 9 figures. The paper has been expanded and restructured, the
figures have been enlarged and improved. Final version, to appear in Phy.
Rev.
Wrapping an adhesive sphere with a sheet
We study the adhesion of an elastic sheet on a rigid spherical substrate.
Gauss'Theorema Egregium shows that this operation necessarily generates metric
distortions (i.e. stretching) as well as bending. As a result, a large variety
of contact patterns ranging from simple disks to complex branched shapes are
observed as a function of both geometrical and material properties. We describe
these different morphologies as a function of two non-dimensional parameters
comparing respectively bending and stretching energies to adhesion. A complete
configuration diagram is finally proposed
Field-scale validation of an automated soil nitrate extraction and measurement system
One of the many gaps that needs to be solved by precision agriculture technologies is the availability of an economic, automated, on-the-go mapping system that can be used to obtain intensive and accurate ‘real-time’ data on the levels of nitrate nitrogen (NO3–N) in the soil. A soil nitrate mapping system (SNMS) has been developed to provide a way to collect such data. This study was done to provide extensive field-scale validation testing of the system’s nitrate extraction and measurement sub-unit (NEMS) in two crop (wheat and carrot) production systems. Field conditions included conventional tillage (CT) versus no tillage (NT), inorganic versus organic fertilizer application, four soil groups and three points in time throughout the season. Detailed data analysis showed that: (i) the level of agreement, as measured by root mean squared error (RMSE), mean absolute error (MAE) and coefficient of efficiency (CE), between NEMS soil NO3–N and standard laboratory soil NO3–N measurements was excellent; (ii) at the field-scale, there was little practical difference when using either integer or real number data processing; (iii) regression equations can be used to enable field measurements of soil NO3–N using the NEMS to be obtained with laboratory accuracy; (iv) future designs of the SNMS’s control system can continue to use cheaper integer chip technology for processing the nitrate ion-selective electrode (NO3 -–ISE) readings; and (v) future designs of the SNMS would not need a soil moisture sensor, ultimately saving on manufacturing costs of a more simple syste
Full Aging in Spin Glasses
The discovery of memory effects in the magnetization decays of spin glasses
in 1983 began a large effort to determine the exact nature of the decay. While
qualitative arguments have suggested that the decay functions should scale as
, the only time scale in the system, this type of scaling has not yet
been observed. In this letter we report strong evidence for the scaling of the
TRM magnetization decays as a function of . By varying the rate and the
profile that the sample is cooled through its transition temperature to the
measuring temperature, we find that the cooling plays a major role in
determining scaling. As the effective cooling time decreases, scaling improves and for we find almost perfect
scaling. We also find that subtraction of a stationary term
from the magnetization decay has a small effect on the scaling but changes the
form of the magnetization decay and improves overlap between curves produced
with different .Comment: 4 pages, 3 figure
Boost invariant marginally trapped surfaces in Minkowski 4-space
The extremal and partly marginally trapped surfaces in Minkowski 4-space,
which are invariant under the group of boost isometries, are classified.
Moreover, it is shown that there do not exist extremal surfaces of this kind
with constant Gaussian curvature. A procedure is given in order to construct a
partly marginally trapped surface by gluing two marginally trapped surfaces
which are invariant under the group of boost isometries. As an application, a
proper star-surface is constructed.Comment: 13 pages, comment added in section
Effective field theory of 3He
3He and the triton are studied as three-body bound states in the effective
field theory without pions. We study 3He using the set of integral equations
developed by Kok et al. which includes the full off-shell T-matrix for the
Coulomb interaction between the protons. To leading order, the theory contains:
two-body contact interactions whose renormalized strengths are set by the NN
scattering lengths, the Coulomb potential, and a three-body contact
interaction. We solve the three coupled integral equations with a sharp
momentum cutoff, Lambda, and find that a three-body interaction is required in
3He at leading order, as in the triton. It also exhibits the same limit-cycle
behavior as a function of Lambda, showing that the Efimov effect remains in the
presence of the Coulomb interaction. We also obtain the difference between the
strengths of the three-body forces in 3He and the triton.Comment: 18 pages, 6 figures; further discussion and references adde
Effects of non-invasive positive pressure ventilation (NIPPV) in stable chronic obstructive pulmonary disease (COPD)
This review provides an overview of the randomised controlled trials covering the topic of chronic non-invasive positive pressure ventilation (NIPPV) in severe stable COPD patients. Studies investigating patients receiving bilevel NIPPV via nasal, oronasal or total face mask interfaces for at least 1 week or more, were described. Eight RCTs were included, from which six trials used NIPPV for up to 3 months (short-term) and two trials also obtained long-term effects (3 months to ≥2 years). Outcome parameters were: arterial blood gases, pulmonary function, respiratory mechanics, respiratory muscle strength, dyspnoea, exercise tolerance, health-related quality of life, neuropsychologic function, sleep quality, hospital admissions and survival. We found that NIPPV in addition to standard care can have beneficial effects on certain outcome measures, however results are conflicting. Therefore, evidence is insufficient to recommend NIPPV routinely in stable but severe COPD patients. Nevertheless, it seems that hypercapnic patients, who receive enough time to adjust to the ventilator and so obtain improved ventilation, could benefit from NIPPV
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