15,907 research outputs found
Resonances, Unstable Systems and Irreversibility: Matter Meets Mind
The fundamental time-reversal invariance of dynamical systems can be broken
in various ways. One way is based on the presence of resonances and their
interactions giving rise to unstable dynamical systems, leading to well-defined
time arrows. Associated with these time arrows are semigroups bearing time
orientations. Usually, when time symmetry is broken, two time-oriented
semigroups result, one directed toward the future and one directed toward the
past. If time-reversed states and evolutions are excluded due to resonances,
then the status of these states and their associated backwards-in-time oriented
semigroups is open to question. One possible role for these latter states and
semigroups is as an abstract representation of mental systems as opposed to
material systems. The beginnings of this interpretation will be sketched.Comment: 9 pages. Presented at the CFIF Workshop on TimeAsymmetric Quantum
Theory: The Theory of Resonances, 23-26 July 2003, Instituto Superior
Tecnico, Lisbon, Portugal; and at the Quantum Structures Association Meeting,
7-22 July 2004, University of Denver. Accepted for publication in the
Internation Journal of Theoretical Physic
Highly frustrated spin-lattice models of magnetism and their quantum phase transitions: A microscopic treatment via the coupled cluster method
We outline how the coupled cluster method of microscopic quantum many-body
theory can be utilized in practice to give highly accurate results for the
ground-state properties of a wide variety of highly frustrated and strongly
correlated spin-lattice models of interest in quantum magnetism, including
their quantum phase transitions. The method itself is described, and it is
shown how it may be implemented in practice to high orders in a systematically
improvable hierarchy of (so-called LSUB) approximations, by the use of
computer-algebraic techniques. The method works from the outset in the
thermodynamic limit of an infinite lattice at all levels of approximation, and
it is shown both how the "raw" LSUB results are themselves generally
excellent in the sense that they converge rapidly, and how they may accurately
be extrapolated to the exact limit, , of the truncation
index , which denotes the {\it only} approximation made. All of this is
illustrated via a specific application to a two-dimensional, frustrated,
spin-half -- model on a honeycomb lattice with
nearest-neighbor and next-nearest-neighbor interactions with exchange couplings
and , respectively, where both
interactions are of the same anisotropic type. We show how the method can
be used to determine the entire zero-temperature ground-state phase diagram of
the model in the range of the frustration parameter and
of the spin-space anisotropy parameter. In particular,
we identify a candidate quantum spin-liquid region in the phase space
A frustrated spin-1/2 Heisenberg antiferromagnet on a chevron-square lattice
The coupled cluster method (CCM) is used to study the zero-temperature
properties of a frustrated spin-half () -- Heisenberg
antiferromagnet (HAF) on a 2D chevron-square lattice. Each site on an
underlying square lattice has 4 nearest-neighbor exchange bonds of strength
and 2 next-nearest-neighbor (diagonal) bonds of strength , with each square plaquette having only one diagonal bond.
The diagonal bonds form a chevron pattern, and the model thus interpolates
smoothly between 2D HAFs on the square () and triangular () lattices,
and also extrapolates to disconnected 1D HAF chains (). The
classical () version of the model has N\'{e}el order for and a form of spiral order for , where
. For the model we use both these classical
states, as well as other collinear states not realized as classical
ground-state (GS) phases, as CCM reference states, on top of which the
multispin-flip configurations resulting from quantum fluctuations are
incorporated in a systematic truncation scheme, which we carry out to high
orders and extrapolate to the physical limit. We calculate the GS energy, GS
magnetic order parameter, and the susceptibilities of the states to various
forms of valence-bond crystalline (VBC) order, including plaquette and two
different dimer forms. We find that the model has two quantum
critical points, at and ,
with N\'{e}el order for , a form of spiral order for
that includes the correct three-sublattice
spin ordering for the triangular-lattice HAF at , and
parallel-dimer VBC order for
Spin-1/2 - Heisenberg model on a cross-striped square lattice
Using the coupled cluster method (CCM) we study the full (zero-temperature)
ground-state (GS) phase diagram of a spin-half () -
Heisenberg model on a cross-striped square lattice. Each site of the square
lattice has 4 nearest-neighbour exchange bonds of strength and 2
next-nearest-neighbour (diagonal) bonds of strength . The bonds
are arranged so that the basic square plaquettes in alternating columns have
either both or no bonds included. The classical () version of the model has 4 collinear phases when and
can take either sign. Three phases are antiferromagnetic (AFM), showing
so-called N\'{e}el, double N\'{e}el and double columnar striped order
respectively, while the fourth is ferromagnetic. For the quantum model
we use the 3 classical AFM phases as CCM reference states, on top of which the
multispin-flip configurations arising from quantum fluctuations are
incorporated in a systematic truncation hierarchy. Calculations of the
corresponding GS energy, magnetic order parameter and the susceptibilities of
the states to various forms of valence-bond crystalline (VBC) order are thus
carried out numerically to high orders of approximation and then extrapolated
to the (exact) physical limit. We find that the model has 5 phases,
which correspond to the four classical phases plus a new quantum phase with
plaquette VBC order. The positions of the 5 quantum critical points are
determined with high accuracy. While all 4 phase transitions in the classical
model are first order, we find strong evidence that 3 of the 5 quantum phase
transitions in the model are of continuous deconfined type
BOOTSTRAPPING YOUR FISH OR FISHING FOR BOOTSTRAPS?: PRECISION OF WELFARE LOSS ESTIMATES FROM A GLOBALLY CONCAVE INVERSE DEMAND MODEL OF COMMERCIAL FISH LANDINGS IN THE U.S. GREAT LAKES
Replaced with revised version of paper 06/30/04.Demand and Price Analysis, Resource /Energy Economics and Policy,
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Integrity static analysis of COTS/SOUP
This paper describes the integrity static analysis approach developed to support the justification of commercial off-the-shelf software (COTS) used in a safety-related system. The static analysis was part of an overall software qualification programme, which also included the work reported in our paper presented at Safecomp 2002. Integrity static analysis focuses on unsafe language constructs and “covert” flows, where one thread can affect the data or control flow of another thread. The analysis addressed two main aspects: the internal integrity of the code (especially for the more critical functions), and the intra-component integrity, checking for covert channels. The analysis process was supported by an aggregation of tools, combined and engineered to support the checks done and to scale as necessary. Integrity static analysis is feasible for industrial scale software, did not require unreasonable resources and we provide data that illustrates its contribution to the software qualification programme
Associations between differing magnitudes of inter-limb asymmetry and linear and change of direction speed performance in male youth soccer players
Abstract Study aim: This study examines the relationship between different magnitudes of asymmetry and their effects on speed performance. Material and methods: Forty-two sub-elite male youth soccer players performed a 30-m sprint, change of direction, single leg countermovement jump and single leg hop. Subjects were divided into groups with vertical and horizontal asymmetry, and both groups were then divided into three groups according to magnitudes of inter-limb asymmetry (<5%, 5–10%, and >10%). Results: The results showed no significant correlation between different jump asymmetry magnitudes and the mentioned outcomes of speed performance (p > 0.05). In addition, larger asymmetries resulted in faster linear speed, even if small differences (g range = 0.00 to 0.57; p > 0.05). But this was not similar for change of direction speed (g range = –0.42 to 0.34; p > 0.05). Conclusions: There are inconsistent findings for the effects of inter-limb asymmetries on speed performance. The results of the present study indicate that the magnitude of asymmetry had no meaningful association with independent measures of performance in soccer players. Therefore, it seems more likely to explain the effects of individual asymmetries on performance rather than the idea that asymmetry negatively affects performance
Vibration effects on heat transfer in cryogenic systems Quarterly progress report no. 1, Jun. 1 - Aug. 31, 1966
Vibration effects on natural convection and fluid transport properties in cryogenic system
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