31,995 research outputs found
Spin Wave Theory of Spin 1/2 XY Model with Ring Exchange on a Triangular Lattice
We present the linear spin wave theory calculation of the superfluid phase of
a hard-core boson - model with nearest neighbour exchange and
four-particle ring-exchange at half filling on the triangular lattice, as
well as the phase diagrams of the system at zero and finite temperatures. We
find that the pure model (XY model) which has a well known uniform
superfluid phase with an ordered parameter at zero
temperature is quickly destroyed by the inclusion of a negative-
ring-exchange interactions, favouring a state with a
ordering wavevector. We further study the behaviour of the finite-temperature
Kosterlitz-Thouless phase transition () in the uniform superfluid
phase, by forcing the universal quantum jump condition on the
finite-temperature spin wave superfluid density. We find that for K \textless
0, the phase boundary monotonically decreases to T=0 at , where a
phase transition is expected and decreases rapidly while for positive
, reaches a maximum at some . It has been shown on a
square lattice using quantum Monte Carlo(QMC) simulations that for small
K\textgreater 0 away from the XY point, the zero-temperature spin stiffness
value of the XY model is decreased\cite{F}. Our result seems to agree with this
trend found in QMC simulations
Coherent pairing states for the Hubbard model
We consider the Hubbard model and its extensions on bipartite lattices. We
define a dynamical group based on the -pairing operators introduced by
C.N.Yang, and define coherent pairing states, which are combinations of
eigenfunctions of -operators. These states permit exact calculations of
numerous physical properties of the system, including energy, various
fluctuations and correlation functions, including pairing ODLRO to all orders.
This approach is complementary to BCS, in that these are superconducting
coherent states associated with the exact model, although they are not
eigenstates of the Hamiltonian.Comment: 5 pages, RevTe
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Models for discriminating image blur from loss of contrast
Observers can discriminate between blurry and low-contrast images (Morgan, 2017). Wang and Simoncelli (2004) demonstrated that a code for blur is inherent to the phase relationships between localized pattern detectors of different scale. To test whether human observers actually use local phase coherence when discriminating between image blur and loss of contrast, we compared phase-scrambled chessboards with unscrambled chessboards. Although both stimuli had identical amplitude spectra, local phase coherence was disrupted by phase-scrambling. Human observers were required to concurrently detect and identify (as contrast or blur) image manipulations in the 2x2 forced-choice paradigm (Nachmias & Weber, 1975; Watson & Robson, 1981) traditionally considered to be a litmus test for "labelled lines" (i.e. detection mechanisms that can be distinguished on the basis of their preferred stimuli). Phase scrambling reduced some observers’ ability to discriminate between blur and a reduction in contrast. However, none of our observers produced data consistent with Watson & Robson’s most stringent test for labelled lines, regardless whether phases were scrambled or not. Models of performance fit significantly better when either a) the blur detector also responded to contrast modulations, b) the contrast detector also responded to blur modulations, or c) noise in the two detectors was anticorrelate
Coherent States from Combinatorial Sequences
We construct coherent states using sequences of combinatorial numbers such as
various binomial and trinomial numbers, and Bell and Catalan numbers. We show
that these states satisfy the condition of the resolution of unity in a natural
way. In each case the positive weight functions are given as solutions of
associated Stieltjes or Hausdorff moment problems, where the moments are the
combinatorial numbers.Comment: 4 pages, Latex; Conference 'Quantum Theory and Symmetries 2', Krakow,
Poland, July 200
Ultrastructural alteration of mouse lung by prolonged exposure to mixtures of helium and oxygen
Observed changes consist mainly of blebbing of capillary endothelium and alveolar epithelium, which is quite possibly indicative of cellular edema; also, there can be observed highly-convoluted basement membrane, alveolar debris, and increased numbers of platelets
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A visual search asymmetry for novelty in the visual field based on sensory adaptation
The ability to detect sudden changes in the environment is important for survival. However, studies of “change blindness” have shown that image differences are hard to detect when a time delay or a mask is imposed between the different images. However, when sensory adaptation is permitted by accuratefixation, we find that change detection is not only possible but asymmetrical: a single changed target amongst 15 unchanging distractors is much easier to detect than a target defined by its lack of change. Although adaptation may selectively reduce the apparent contrast of unchanged objects, the asymmetry in “change salience” cannot be attributed to any such reduction because genuine reductions in target contrast increase, rather than decrease, target detectability. Analogous results preclude attribution to apparent differences between a) target onset and distractor onset and b) their temporal frequencies (both flickered at 7.5 Hz, minimizing afterimages). Our results demonstrate a hitherto underappreciated (or unappreciated) advantage conferred by low-level sensory adaptation: it automatically elevates the salience of previously absent objects
Condition for tripartite entanglement
We propose a scheme for classifying the entanglement of a tripartite pure
qubit state. This classification scheme consists of an ordered list of seven
elements. These elements are the Cayley hyper-determinant, and its six
associated subdeterminants. In particular we show that this
classification provides a necessary and sufficient condition for separability.Comment: 8 pages, to appear in the Proceedings of "Quantum Theory and
Symmetries 7", Prague, Aug 7-13, 201
Transform fault earthquakes in the North Atlantic: Source mechanisms and depth of faulting
The centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern Mid-Atlantic Ridge were determined from an inversion of long-period body waveforms. The earthquakes occurred on the Gibbs, Oceanographer, Hayes, Kane, 15 deg 20 min, and Vema transforms. The depth extent of faulting during each earthquake was estimated from the centroid depth and the fault width. The source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes; slip vector azimuths agree to 2 to 3 deg of the local strike of the zone of active faulting. The only anomalies in mechanism were for two earthquakes near the western end of the Vema transform which occurred on significantly nonvertical fault planes. Secondary faulting, occurring either precursory to or near the end of the main episode of strike-slip rupture, was observed for 5 of the 12 earthquakes. For three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform. In all three cases, the site of secondary reverse faulting is near a compression jog in the current trace of the active transform fault zone. No evidence was found to support the conclusions of Engeln, Wiens, and Stein that oceanic transform faults in general are either hotter than expected from current thermal models or weaker than normal oceanic lithosphere
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