367,784 research outputs found
Quantum renormalization of high energy excitations in the 2D Heisenberg antiferromagnet
We find using Monte Carlo simulations of the spin-1/2 2D square lattice
nearest neighbour quantum Heisenberg antiferromagnet that the high energy peak
locations at (pi,0) and (pi/2,pi/2) differ by about 6%, (pi/2,pi/2) being the
highest. This is a deviation from linear spin wave theory which predicts equal
magnon energies at these points.Comment: Final version, Latex using iopart & epsfi
A summary of the behavior of materials at cryogenic temperatures
Summary of material behavior at cryogenic temperature
The Peierls--Nabarro FE model in two-phase microstructures -- a comparison with atomistics
This paper evaluates qualitatively as well as quantitatively the accuracy of
a recently proposed Peierls--Nabarro Finite Element (PN-FE) model for
dislocations by a direct comparison with an equivalent molecular statics
simulation. To this end, a two-dimensional microstructural specimen subjected
to simple shear is considered, consisting of a central soft phase flanked by
two hard-phase regions. A hexagonal atomic structure with equal lattice spacing
is adopted, the interactions of which are described by the Lennard--Jones
potential with phase specific depths of its energy well. During loading, edge
dislocation dipoles centred in the soft phase are introduced, which progress
towards the phase boundaries, where they pile up. Under a sufficiently high
external shear load, the leading dislocation is eventually transmitted into the
harder phase. The homogenized PN-FE model is calibrated to an atomistic model
in terms of effective elasticity constants and glide plane properties as
obtained from simple uniform deformations. To study the influence of different
formulations of the glide plane potential, multiple approaches are employed,
ranging from a simple sinusoidal function of the tangential disregistry to a
complex model that couples the influence of the tangential and the normal
disregistries. The obtained results show that, qualitatively, the dislocation
structure, displacement, strain fields, and the dislocation evolution are
captured adequately. The simplifications of the PN-FE model lead, however, to
some discrepancies within the dislocation core. Such discrepancies play a
dominant role in the dislocation transmission process, which thus cannot
quantitatively be captured properly. Despite its simplicity, the PN-FE model
proves to be an elegant tool for a qualitative study of edge dislocation
behaviour in two-phase microstructures, although it may not be quantitatively
predictive.Comment: 29 pages, 11 figures, 5 tables, abstract shortened to fulfill 1920
character limit, small changes after revie
Development of Muon Drift-Tube Detectors for High-Luminosity Upgrades of the Large Hadron Collider
The muon detectors of the experiments at the Large Hadron Collider (LHC) have
to cope with unprecedentedly high neutron and gamma ray background rates. In
the forward regions of the muon spectrometer of the ATLAS detector, for
instance, counting rates of 1.7 kHz/square cm are reached at the LHC design
luminosity. For high-luminosity upgrades of the LHC, up to 10 times higher
background rates are expected which require replacement of the muon chambers in
the critical detector regions. Tests at the CERN Gamma Irradiation Facility
showed that drift-tube detectors with 15 mm diameter aluminum tubes operated
with Ar:CO2 (93:7) gas at 3 bar and a maximum drift time of about 200 ns
provide efficient and high-resolution muon tracking up to the highest expected
rates. For 15 mm tube diameter, space charge effects deteriorating the spatial
resolution at high rates are strongly suppressed. The sense wires have to be
positioned in the chamber with an accuracy of better than 50 ?micons in order
to achieve the desired spatial resolution of a chamber of 50 ?microns up to the
highest rates. We report about the design, construction and test of prototype
detectors which fulfill these requirements
Relationship between clustering and algorithmic phase transitions in the random k-XORSAT model and its NP-complete extensions
We study the performances of stochastic heuristic search algorithms on
Uniquely Extendible Constraint Satisfaction Problems with random inputs. We
show that, for any heuristic preserving the Poissonian nature of the underlying
instance, the (heuristic-dependent) largest ratio of constraints per
variables for which a search algorithm is likely to find solutions is smaller
than the critical ratio above which solutions are clustered and
highly correlated. In addition we show that the clustering ratio can be reached
when the number k of variables per constraints goes to infinity by the
so-called Generalized Unit Clause heuristic.Comment: 15 pages, 4 figures, Proceedings of the International Workshop on
Statistical-Mechanical Informatics, September 16-19, 2007, Kyoto, Japan; some
imprecisions in the previous version have been correcte
Advanced multilateration theory, software development, and data processing: The MICRODOT system
The process of geometric parameter estimation to accuracies of one centimeter, i.e., multilateration, is defined and applications are listed. A brief functional explanation of the theory is presented. Next, various multilateration systems are described in order of increasing system complexity. Expected systems accuracy is discussed from a general point of view and a summary of the errors is listed. An outline of the design of a software processing system for multilateration, called MICRODOT, is presented next. The links of this software, which can be used for multilateration data simulations or operational data reduction, are examined on an individual basis. Functional flow diagrams are presented to aid in understanding the software capability. MICRODOT capability is described with respect to vehicle configurations, interstation coordinate reduction, geophysical parameter estimation, and orbit determination. Numerical results obtained from MICRODOT via data simulations are displayed both for hypothetical and real world vehicle/station configurations such as used in the GEOS-3 Project. These simulations show the inherent power of the multilateration procedure
Frustrated spin- Heisenberg magnet on a square-lattice bilayer: High-order study of the quantum critical behavior of the ---- model
The zero-temperature phase diagram of the spin-
---- model on an -stacked square-lattice
bilayer is studied using the coupled cluster method implemented to very high
orders. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor
Heisenberg exchange interactions, of strengths and , respectively, are included in each layer. The two layers are
coupled via a NN interlayer Heisenberg exchange interaction with a strength
. The magnetic order parameter (viz.,
the sublattice magnetization) is calculated directly in the thermodynamic
(infinite-lattice) limit for the two cases when both layers have
antiferromagnetic ordering of either the N\'{e}el or the striped kind, and with
the layers coupled so that NN spins between them are either parallel (when
) to one another. Calculations
are performed at th order in a well-defined sequence of approximations,
which exactly preserve both the Goldstone linked cluster theorem and the
Hellmann-Feynman theorem, with . The sole approximation made is to
extrapolate such sequences of th-order results for to the exact limit,
. By thus locating the points where vanishes, we calculate
the full phase boundaries of the two collinear AFM phases in the
-- half-plane with . In particular, we provide the
accurate estimate, (), for the
position of the quantum triple point (QTP) in the region . We also
show that there is no counterpart of such a QTP in the region ,
where the two quasiclassical phase boundaries show instead an ``avoided
crossing'' behavior, such that the entire region that contains the nonclassical
paramagnetic phases is singly connected
Geoneutrinos in Borexino
This paper describes the Borexino detector and the high-radiopurity studies
and tests that are integral part of the Borexino technology and development.
The application of Borexino to the detection and studies of geoneutrinos is
discussed.Comment: Conference: Neutrino Geophysics Honolulu, Hawaii December 14-16, 200
Universal decay law in charged-particle emission and exotic cluster radioactivity
A linear universal decay formula is presented starting from the microscopic
mechanism of the charged-particle emission. It relates the half-lives of
monopole radioactive decays with the -values of the outgoing particles as
well as the masses and charges of the nuclei involved in the decay. This
relation is found to be a generalization of the Geiger-Nuttall law in
radioactivity and explains well all known cluster decays. Predictions on the
most likely emissions of various clusters are presented.Comment: 2 figure
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