4,302 research outputs found
Phase transition of the three-dimensional chiral Ginzburg-Landau model -- search for the chiral phase
Nature of the phase transition of regularly frustrated vector spin systems in
three dimensions is investigated based on a Ginzburg-Landau-type effective
Hamiltonian. On the basis of the variational analysis of this model, Onoda et
al recently suggested the possible occurrence of a chiral phase, where the
vector chirality exhibits a long-range order without the long-range order of
the spin [Phys. Rev. Lett. 99, 027206 (2007)]. In the present paper, we
elaborate their analysis by considering the possibility of a first-order
transition which was not taken into account in their analysis. We find that the
first-order transition indeed occurs within the variational approximation,
which significantly reduces the stability range of the chiral phase, while the
chiral phase still persists in a restricted parameter range. Then, we perform
an extensive Monte Carlo simulation focusing on such a parameter range.
Contrary to the variational result, however, we do not find any evidence of the
chiral phase. The range of the chiral phase, if any, is estimated to be less
than 0.1% in the temperature width.Comment: 19 pages, 17 figure
Comment on "Valence QCD: Connecting QCD to the Quark Model"
I criticize certain conclusions about the physics of hadrons drawn from a
"valence QCD" approximation to QCD.Comment: 12 pages, 8 figures; some minor improvements made to the tex
Synthesis and textural properties of unsupported and supported rutile (TiO2) membranes
Two approaches were postulated for improving the stability of porous texture of titania membranes: (1) retarding the phase transformation and grain growth; (2) avoiding the phase transformation. Based on the second approach, rutile membranes were made directly from a rutile sol, prepared by the precipitation of titania on SnO2 nuclei. The rutile membranes were stable up to 800 °C, with a porosity of ca. 40%, whereas normal titania membranes (starting with anatase) show very little porosity above 600 °C. Alumina substitution retards grain growth and pore growth at 850 °C for unsupported as well as supported membranes. \u
High-temperature catalyst supports and ceramic membranes: Metastability and particle packing
Parameters and/or processes responsible for the stability of catalyst supports and ceramic membranes are discussed. Two major parameters/processes were identified which are responsible for the stability of sol-gel derived nanostructured oxides at elevated temperatures. They are metastable-to-stable phase transformation and structure and packing of primary particles within the aggregate. Based on these observations, strategies to develop thermostable nanostructured oxides for high-temperature membrane and catalyst applications are discussed by taking titania and titania-alumina nanocomposites as examples
Textural evolution and phase transformation in titania membranes: Part 2. - Supported membranes
Nanostructural evolution and phase transformation in supported and unsupported titania membranes have been studied using Raman spectroscopy, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). Densification of unsupported membranes started at ca. 450 °C and reached more than 97% density at 600 °C, whereas the supported membranes had a density of only ca. 70–75% even at 700 °C when calcined for 8 h. At 700 °C the average crystallite size of supported and unsupported membranes was ca. 20 and 70 nm, respectively. This behaviour is primarily attributed to the decrease in the driving force for sintering due to the stress developed during the constrained sintering of a film attached to a rigid support and to the inhibition of the reorganization process within the film, resulting in lower coordination numbers in supported membranes. Supported membranes showed a higher transformation temperature (slower rate of transformation) than did the unsupported. Supported and unsupported membranes, calcined for 8 h, transformed to ca. 90% rutile (calculated from Raman spectrum) after calcination at 850 and 650 °C, respectively. This difference in phase transformation behaviour is attributed primarily to the large stress which is developed in a constrained environment owing to the negative volume change during the anatase–rutile transformation
Low-momentum Hyperon-Nucleon Interactions
We present a first exploratory study for hyperon-nucleon interactions using
renormalization group techniques. The effective two-body low-momentum potential
V_low-k is obtained by integrating out the high-momentum components from
realistic Nijmegen YN potentials. A T-matrix equivalence approach is employed,
so that the low-energy phase shifts are reproduced by V_low-k up to a momentum
scale Lambda ~ 500 MeV. Although the various bare Nijmegen models differ
somewhat from each other, the corresponding V_low-k interactions show
convergence in some channels, suggesting a possible unique YN interaction at
low momenta.Comment: 4 pages, 6 figure
Field Strength Correlators For 2D Yang-Mills Over Riemann Surfaces
The path integral computation of field strength correlation functions for two
dimensional Yang-Mills theories over Riemann surfaces is studied. The
calculation is carried out by abelianization, which leads to correlators that
are topological. They are nontrivial as a result of the topological
obstructions to the abelianization. It is shown in the large N limit on the
sphere that the correlators undergo second order phase transitions at the
critical point. Our results are applied to a computation of contractible Wilson
loops.Comment: final version to appear in Int. Jour. Mod. Phys. A, minor
corrections, added a few comments on Wilson loops and non-abelian Stokes
theore
Rational Terms in Theories with Matter
We study rational remainders associated with gluon amplitudes in gauge
theories coupled to matter in arbitrary representations. We find that these
terms depend on only a small number of invariants of the matter-representation
called indices. In particular, rational remainders can depend on the second and
fourth order indices only. Using this, we find an infinite class of
non-supersymmetric theories in which rational remainders vanish for gluon
amplitudes. This class includes all the "next-to-simplest" quantum field
theories of arXiv:0910.0930. This provides new examples of amplitudes in which
rational remainders vanish even though naive power counting would suggest their
presence.Comment: 10+4 pages. (v2) typos corrected, references adde
Noise and Inertia-Induced Inhomogeneity in the Distribution of Small Particles in Fluid Flows
The dynamics of small spherical neutrally buoyant particulate impurities
immersed in a two-dimensional fluid flow are known to lead to particle
accumulation in the regions of the flow in which rotation dominates over shear,
provided that the Stokes number of the particles is sufficiently small. If the
flow is viewed as a Hamiltonian dynamical system, it can be seen that the
accumulations occur in the nonchaotic parts of the phase space: the
Kolmogorov--Arnold--Moser tori. This has suggested a generalization of these
dynamics to Hamiltonian maps, dubbed a bailout embedding. In this paper we use
a bailout embedding of the standard map to mimic the dynamics of impurities
subject not only to drag but also to fluctuating forces modelled as white
noise. We find that the generation of inhomogeneities associated with the
separation of particle from fluid trajectories is enhanced by the presence of
noise, so that they appear in much broader ranges of the Stokes number than
those allowing spontaneous separation
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