840 research outputs found
3-manifolds which are spacelike slices of flat spacetimes
We continue work initiated in a 1990 preprint of Mess giving a geometric
parameterization of the moduli space of classical solutions to Einstein's
equations in 2+1 dimensions with cosmological constant 0 or -1 (the case +1 has
been worked out in the interim by the present author). In this paper we make a
first step toward the 3+1-dimensional case by determining exactly which closed
3-manifolds M^3 arise as spacelike slices of flat spacetimes, and by finding
all possible holonomy homomorphisms pi_1(M^3) to ISO(3,1).Comment: 10 page
Efficient Behavior of Small-World Networks
We introduce the concept of efficiency of a network, measuring how
efficiently it exchanges information. By using this simple measure small-world
networks are seen as systems that are both globally and locally efficient. This
allows to give a clear physical meaning to the concept of small-world, and also
to perform a precise quantitative a nalysis of both weighted and unweighted
networks. We study neural networks and man-made communication and
transportation systems and we show that the underlying general principle of
their construction is in fact a small-world principle of high efficiency.Comment: 1 figure, 2 tables. Revised version. Accepted for publication in
Phys. Rev. Let
Probing the Sensitivity of Electron Wave Interference to Disorder-Induced Scattering in Solid-State Devices
The study of electron motion in semiconductor billiards has elucidated our
understanding of quantum interference and quantum chaos. The central assumption
is that ionized donors generate only minor perturbations to the electron
trajectories, which are determined by scattering from billiard walls. We use
magnetoconductance fluctuations as a probe of the quantum interference and show
that these fluctuations change radically when the scattering landscape is
modified by thermally-induced charge displacement between donor sites. Our
results challenge the accepted understanding of quantum interference effects in
nanostructures.Comment: 8 pages, 5 figures, Submitted to Physical Review
Effect of resonant magnetic perturbations on low collisionality discharges in MAST and a comparison with ASDEX Upgrade
Sustained ELM mitigation has been achieved on MAST and AUG using RMPs with a
range of toroidal mode numbers over a wide region of low to medium
collisionality discharges. The ELM energy loss and peak heat loads at the
divertor targets have been reduced. The ELM mitigation phase is typically
associated with a drop in plasma density and overall stored energy. In one
particular scenario on MAST, by carefully adjusting the fuelling it has been
possible to counteract the drop in density and to produce plasmas with
mitigated ELMs, reduced peak divertor heat flux and with minimal degradation in
pedestal height and confined energy. While the applied resonant magnetic
perturbation field can be a good indicator for the onset of ELM mitigation on
MAST and AUG there are some cases where this is not the case and which clearly
emphasise the need to take into account the plasma response to the applied
perturbations. The plasma response calculations show that the increase in ELM
frequency is correlated with the size of the edge peeling-tearing like response
of the plasma and the distortions of the plasma boundary in the X-point region.Comment: 31 pages, 28 figures. This is an author-created, un-copyedited
version of an article submitted for publication in Nuclear Fusion. IoP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from i
Domain-Adversarial Learning for Multi-Centre, Multi-Vendor, and Multi-Disease Cardiac MR Image Segmentation
Cine cardiac magnetic resonance (CMR) has become the gold standard for the
non-invasive evaluation of cardiac function. In particular, it allows the
accurate quantification of functional parameters including the chamber volumes
and ejection fraction. Deep learning has shown the potential to automate the
requisite cardiac structure segmentation. However, the lack of robustness of
deep learning models has hindered their widespread clinical adoption. Due to
differences in the data characteristics, neural networks trained on data from a
specific scanner are not guaranteed to generalise well to data acquired at a
different centre or with a different scanner. In this work, we propose a
principled solution to the problem of this domain shift. Domain-adversarial
learning is used to train a domain-invariant 2D U-Net using labelled and
unlabelled data. This approach is evaluated on both seen and unseen domains
from the M\&Ms challenge dataset and the domain-adversarial approach shows
improved performance as compared to standard training. Additionally, we show
that the domain information cannot be recovered from the learned features.Comment: Accepted at the STACOM workshop at MICCAI 202
From modular to centralized organization of synchronization in functional areas of the cat cerebral cortex
Recent studies have pointed out the importance of transient synchronization
between widely distributed neural assemblies to understand conscious
perception. These neural assemblies form intricate networks of neurons and
synapses whose detailed map for mammals is still unknown and far from our
experimental capabilities. Only in a few cases, for example the C. elegans, we
know the complete mapping of the neuronal tissue or its mesoscopic level of
description provided by cortical areas. Here we study the process of transient
and global synchronization using a simple model of phase-coupled oscillators
assigned to cortical areas in the cerebral cat cortex. Our results highlight
the impact of the topological connectivity in the developing of
synchronization, revealing a transition in the synchronization organization
that goes from a modular decentralized coherence to a centralized synchronized
regime controlled by a few cortical areas forming a Rich-Club connectivity
pattern.Comment: 24 pages, 8 figures. Final version published in PLoS On
First data and preliminary experimental results from a new Doppler Backscattering system on the MAST-U spherical tokamak
A new Doppler backscattering (DBS) system, consisting of Q-band and V-band,
has been installed and achieved its first data on the MAST-U spherical tokamak.
The Q-band and V-band have separate microwave source systems, but share the
same optical front-end components. The Q-band and V-band sources simultaneously
generate eight (34, 36, 38, 40, 42, 44, 46 and 48 GHz) and seven (52.5, 55,
57.5, 60, 62.5, 65 and 67.5 GHz) fixed frequency probe beams, respectively.
These frequencies provide a large range of radial positions from the
low-field-side edge plasma to the core, and possibly to the high-field-side
edge, depending on the plasma conditions. The quasi-optical system consists of
a remotely-tunable polarizer, a focusing lens and a remotely-steerable mirror.
By steering the mirror, the system provides remote control of the probed
density fluctuation wavenumber, and allow the launch angle to match the
magnetic field. The range of accessible turbulence wavenumbers (k_\theta) is
reasonably large with normalized wavenumber k_\theta\rho_s ranging from <0.5 to
9. The first data acquired by this DBS system is validated by comparing with
the data from the other DBS system on MAST-U (introduced in Ref. [21]). An
example of measuring the velocity profile spanning from the edge to the center
in a high-density plasma is presented, indicating the robust capabilities of
the integrated Q-band and V-band DBS systems
Magnetic Reconnection Triggering Magnetohydrodynamic Instabilities during a Sawtooth Crash in a Tokamak Plasma
Thomson scattering measurements with subcentimeter spatial resolution have been made during a sawtooth crash in a Mega Ampere Spherical Tokamak fusion plasma. The unparalleled resolution of the temperature profile has shed new light on the mechanisms that underlie the sawtooth. As magnetic reconnection occurs, the temperature gradient at the island boundary increases. The increased local temperature gradient is sufficient to make the helical core unstable to ideal magnetohydrodynamic instabilities, thought to be responsible for the rapidity of the collapse
Law of Genome Evolution Direction : Coding Information Quantity Grows
The problem of the directionality of genome evolution is studied. Based on
the analysis of C-value paradox and the evolution of genome size we propose
that the function-coding information quantity of a genome always grows in the
course of evolution through sequence duplication, expansion of code, and gene
transfer from outside. The function-coding information quantity of a genome
consists of two parts, p-coding information quantity which encodes functional
protein and n-coding information quantity which encodes other functional
elements except amino acid sequence. The evidences on the evolutionary law
about the function-coding information quantity are listed. The needs of
function is the motive force for the expansion of coding information quantity
and the information quantity expansion is the way to make functional innovation
and extension for a species. So, the increase of coding information quantity of
a genome is a measure of the acquired new function and it determines the
directionality of genome evolution.Comment: 16 page
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