3,074 research outputs found
Out of equilibrium dynamics of a Quantum Heisenberg Spin Glass
We study the out of equilibrium dynamics of the infinite range quantum
Heisenberg spin glass model coupled to a thermal relaxation bath. The SU(2)
spin algebra is generalized to SU(N) and we analyse the large-N limit. The
model displays a dynamical phase transition between a paramagnetic and a glassy
phase. In the latter, the system remains out of equilibrium and displays an
aging phenomenon, which we characterize using both analytical and numerical
methods. In the aging regime, the quantum fluctuation-dissipation relation is
violated and replaced at very long time by its classical generalization, as in
models involving simple spin algebras studied previously. We also discuss the
effect of a finite coupling to the relaxation baths and their possible forms.
This work completes and justifies previous studies on this model using a static
approach.Comment: Minor change
Simulation of pore-scale flow using finite element-methods
I present a new finite element (FE) simulation method to simulate pore-scale
flow. Within the pore-space, I solve a simplified form of the incompressible
Navier-Stoke’s equation, yielding the velocity field in a two-step solution
approach. First, Poisson’s equation is solved with homogeneous boundary
conditions, and then the pore pressure is computed and the velocity field
obtained for no slip conditions at the grain boundaries. From the computed
velocity field I estimate the effective permeability of porous media samples
characterized by thin section micrographs, micro-CT scans and synthetically
generated grain packings. This two-step process is much simpler than solving
the full Navier Stokes equation and therefore provides the opportunity to
study pore geometries with hundreds of thousands of pores in a computationally
more cost effective manner than solving the full Navier-Stoke’s equation.
My numerical model is verified with an analytical solution and validated on
samples whose permeabilities and porosities had been measured in laboratory
experiments (Akanji and Matthai, 2010). Comparisons were also made with
Stokes solver, published experimental, approximate and exact permeability
data. Starting with a numerically constructed synthetic grain packings, I also
investigated the extent to which the details of pore micro-structure affect the
hydraulic permeability (Garcia et al., 2009). I then estimate the hydraulic
anisotropy of unconsolidated granular packings.
With the future aim to simulate multiphase flow within the pore-space, I also compute the radii and derive capillary pressure from the Young-Laplace
equation (Akanji and Matthai,2010
4D electron imaging: principles and perspectives
In this perspective we highlight developments and concepts in the field of 4D electron imaging. With spatial and temporal resolutions reaching the picometer and femtosecond, respectively, the field is now embracing ultrafast electron diffraction, crystallography and microscopy. Here, we overview the principles involved in the direct visualization of structural dynamics with applications in chemistry, materials science and biology. The examples include the studies of complex isolated chemical reactions, phase transitions and cellular structures. We conclude with an outlook on the potential of the approach and with some questions that may define new frontiers of research
From Quantum Systems to L-Functions: Pair Correlation Statistics and Beyond
The discovery of connections between the distribution of energy levels of
heavy nuclei and spacings between prime numbers has been one of the most
surprising and fruitful observations in the twentieth century. The connection
between the two areas was first observed through Montgomery's work on the pair
correlation of zeros of the Riemann zeta function. As its generalizations and
consequences have motivated much of the following work, and to this day remains
one of the most important outstanding conjectures in the field, it occupies a
central role in our discussion below. We describe some of the many techniques
and results from the past sixty years, especially the important roles played by
numerical and experimental investigations, that led to the discovery of the
connections and progress towards understanding the behaviors. In our survey of
these two areas, we describe the common mathematics that explains the
remarkable universality. We conclude with some thoughts on what might lie ahead
in the pair correlation of zeros of the zeta function, and other similar
quantities.Comment: Version 1.1, 50 pages, 6 figures. To appear in "Open Problems in
Mathematics", Editors John Nash and Michael Th. Rassias. arXiv admin note:
text overlap with arXiv:0909.491
A quantum mechanical NMR simulation algorithm for protein-scale spin systems
Nuclear magnetic resonance spectroscopy is one of the few remaining areas of
physical chemistry for which polynomially scaling simulation methods have not
so far been available. Here, we report such a method and illustrate its
performance by simulating common 2D and 3D liquid state NMR experiments
(including accurate description of spin relaxation processes) on isotopically
enriched human ubiquitin - a protein containing over a thousand nuclear spins
forming an irregular polycyclic three-dimensional coupling lattice. The
algorithm uses careful tailoring of the density operator space to only include
nuclear spin states that are populated to a significant extent. The reduced
state space is generated by analyzing spin connectivity and decoherence
properties: rapidly relaxing states as well as correlations between
topologically remote spins are dropped from the basis set. In the examples
provided, the resulting reduction in the quantum mechanical simulation time is
by many orders of magnitude.Comment: Submitted for publicatio
An automated archival VLA transients survey
In this paper we present the results of a survey for radio transients using
data obtained from the Very Large Array archive. We have reduced, using a
pipeline procedure, 5037 observations of the most common pointings - i.e. the
calibrator fields. These fields typically contain a relatively bright point
source and are used to calibrate `target' observations: they are therefore
rarely imaged themselves. The observations used span a time range ~ 1984 - 2008
and consist of eight different pointings, three different frequencies (8.4, 4.8
and 1.4 GHz) and have a total observing time of 435 hours. We have searched for
transient and variable radio sources within these observations using components
from the prototype LOFAR transient detection system. In this paper we present
the methodology for reducing large volumes of Very Large Array data; and we
also present a brief overview of the prototype LOFAR transient detection
algorithms. No radio transients were detected in this survey, therefore we
place an upper limit on the snapshot rate of GHz frequency transients > 8.0 mJy
to rho less than or equal to 0.032 deg^-2 that have typical timescales 4.3 to
45.3 days. We compare and contrast our upper limit with the snapshot rates -
derived from either detections or non-detections of transient and variable
radio sources - reported in the literature. When compared with the current Log
N - Log S distribution formed from previous surveys, we show that our upper
limit is consistent with the observed population. Current and future radio
transient surveys will hopefully further constrain these statistics, and
potentially discover dominant transient source populations. In this paper we
also briefly explore the current transient commissioning observations with
LOFAR, and the impact they will make on the field.Comment: Accepted for publication in MNRA
Weakly Supervised Object Localization with Multi-fold Multiple Instance Learning
Object category localization is a challenging problem in computer vision.
Standard supervised training requires bounding box annotations of object
instances. This time-consuming annotation process is sidestepped in weakly
supervised learning. In this case, the supervised information is restricted to
binary labels that indicate the absence/presence of object instances in the
image, without their locations. We follow a multiple-instance learning approach
that iteratively trains the detector and infers the object locations in the
positive training images. Our main contribution is a multi-fold multiple
instance learning procedure, which prevents training from prematurely locking
onto erroneous object locations. This procedure is particularly important when
using high-dimensional representations, such as Fisher vectors and
convolutional neural network features. We also propose a window refinement
method, which improves the localization accuracy by incorporating an objectness
prior. We present a detailed experimental evaluation using the PASCAL VOC 2007
dataset, which verifies the effectiveness of our approach.Comment: To appear in IEEE Transactions on Pattern Analysis and Machine
Intelligence (TPAMI
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