5,116 research outputs found
Gas of self-avoiding loops on the brickwork lattice
An exact calculation of the phase diagram for a loop gas model on the
brickwork lattice is presented. The model includes a bending energy. In the
dense limit, where all the lattice sites are occupied, a phase transition
occuring at an asymmetric Lifshitz tricritical point is observed as the
temperature associated with the bending energy is varied. Various critical
exponents are calculated. At lower densities, two lines of transitions (in the
Ising universality class) are observed, terminated by a tricritical point,
where there is a change in the modulation of the correlation function. To each
tricritical point an associated disorder line is found.Comment: 19 pages, 6 figures. to appear in J. Phys. A : Math. & Ge
Brood patch and sex-ratio observations indicate breeding provenance and timing in New Zealand storm petrel (Fregetta maoriana)
We used measurements of brood patch and moult status to estimate the breeding phenology of New Zealand Storm-Petrel, using birds caught at sea within the Hauraki Gulf Marine Park near Auckland, New Zealand. Birds caught October–January had completely downy brood patches, whereas birds caught February–April had bare brood patches with an observed male bias in the February sex-ratio, consistent with a female pre-laying exodus typical of petrels and with the existence of an unknown colony in the region. No birds captured exhibited primary moult, which is known to occur in storm-petrels during their non-breeding season. Our data support the conclusion that the New Zealand storm-petrel breeds during January–June in northern New Zealand and that field surveys for the species on offshore islands in this region during this period are warrante
Epoch Dependent Dark Energy
We present a model in which the equation of state parameter w approaches -1
near a particular value of z, and has significant negative values in a
restricted range of z. For example, one can have w ~ -1 near z = 1, and w >
-0.2 from z = 0 to z = 0.3, and for z > 9. The ingredients of the model are
neutral fermions (which may be neutrinos, neutralinos, etc) which are very
weakly coupled to a light scalar field. This model emphasises the importance of
the proposed studies of the properties of dark energy into the region z > 1.Comment: 7pp., 2 figs. Invited talk at the 5th Int'l. Wkshp. on the Dark Side
of the Universe, 1-5 June 2009 Melbourne, DSU09; to appear in the proceeding
MSW-like Enhancements without Matter
We study the effects of a scalar field, coupled only to neutrinos, on
oscillations among weak interaction current eigenstates. The effect of a real
scalar field appears as effective masses for the neutrino mass eigenstates, the
same for \nbar as for \n. Under some conditions, this can lead to a
vanishing of , giving rise to MSW-like effects. We discuss some
examples and show that it is possible to resolve the apparent discrepancy in
spectra required by r-process nucleosynthesis in the mantles of supernovae and
by Solar neutrino solutions.Comment: 9 pages, latex, 1 figur
A dynamics-driven approach to precision machines design for micro-manufacturing and its implementation perspectives
Precision machines are essential elements in fabricating high quality micro products or micro features and directly affect the machining accuracy, repeatability and efficiency. There are a number of literatures on the design of industrial machine elements and a couple of precision machines commercially available. However, few researchers have systematically addressed the design of precision machines from the dynamics point of view. In this paper, the design issues of precision machines are presented with particular emphasis on the dynamics aspects as the major factors affecting the performance of the precision machines and machining processes. This paper begins with a brief review of the design principles of precision machines with emphasis on machining dynamics. Then design processes of precision machines are discussed, and followed by a practical modelling and simulation approaches. Two case studies are provided including the design and analysis of a fast tool servo system and a 5-axis bench-top micro-milling machine respectively. The design and analysis used in the two case studies are formulated based on the design methodology and guidelines
High-rate, high-fidelity entanglement of qubits across an elementary quantum network
We demonstrate remote entanglement of trapped-ion qubits via a
quantum-optical fiber link with fidelity and rate approaching those of local
operations. Two Sr qubits are entangled via the polarization
degree of freedom of two photons which are coupled by high-numerical-aperture
lenses into single-mode optical fibers and interfere on a beamsplitter. A novel
geometry allows high-efficiency photon collection while maintaining unit
fidelity for ion-photon entanglement. We generate remote Bell pairs with
fidelity at an average rate (success
probability ).Comment: v2 updated to include responses to reviewers, as published in PR
Irrelevant operators in the two-dimensional Ising model
By using conformal-field theory, we classify the possible irrelevant
operators for the Ising model on the square and triangular lattices. We analyze
the existing results for the free energy and its derivatives and for the
correlation length, showing that they are in agreement with the conformal-field
theory predictions. Moreover, these results imply that the nonlinear scaling
field of the energy-momentum tensor vanishes at the critical point. Several
other peculiar cancellations are explained in terms of a number of general
conjectures. We show that all existing results on the square and triangular
lattice are consistent with the assumption that only nonzero spin operators are
present.Comment: 32 pages. Added comments and reference
Frustrated quantum Heisenberg ferrimagnetic chains
We study the ground-state properties of weakly frustrated Heisenberg
ferrimagnetic chains with nearest and next-nearest neighbor antiferromagnetic
exchange interactions and two types of alternating sublattice spins S_1 > S_2,
using 1/S spin-wave expansions, density-matrix renormalization group, and
exact- diagonalization techniques. It is argued that the zero-point spin
fluctuations completely destroy the classical commensurate- incommensurate
continuous transition. Instead, the long-range ferrimagnetic state disappears
through a discontinuous transition to a singlet state at a larger value of the
frustration parameter. In the ferrimagnetic phase we find a disorder point
marking the onset of incommensurate real-space short-range spin-spin
correlations.Comment: 16 pages (LaTex 2.09), 6 eps figure
Potential for Forecasting UK Summer Grass Growth from the North Atlantic Oscillation
The North Atlantic Oscillation (NAO) is a large-scale atmospheric circulation pattern which is well-known to influence the UK winter climate (Wilby et al., 1997). Recently, it has been shown that the winter NAO also affects summer rainfall in the UK (Kettlewell et al., 2003). Since water supply is an important limitation to summer grass growth in many parts of the UK, the winter NAO may influence summer growth. The objective of this study was to test the hypothesis that there is a relationship between the winter NAO and summer grass growth using data from reference plots at North Wyke in Devon
A Possible Connection Between Massive Fermions and Dark Energy
In a dense cloud of massive fermions interacting by exchange of a light
scalar field, the effective mass of the fermion can become negligibly small. As
the cloud expands, the effective mass and the total energy density eventually
increase with decreasing density. In this regime, the pressure-density relation
can approximate that required for dark energy. We apply this phenomenon to the
expansion of the Universe with a very light scalar field and infer relations
between the parameters available and cosmological observations. Majorana
neutrinos at a mass that may have been recently determined, and fermions such
as the Lightest Supersymmetric Particle (LSP) may both be consistent with
current observations of dark energy.Comment: 15 pages, 8 figures, to appear in Proceedings of the Seventh
International Heidelberg Conference on Dark Matter in Astro and Particle
Physics, DARK'0
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