561 research outputs found
Traveling Wave Fronts and Localized Traveling Wave Convection in Binary Fluid Mixtures
Nonlinear fronts between spatially extended traveling wave convection (TW)
and quiescent fluid and spatially localized traveling waves (LTWs) are
investigated in quantitative detail in the bistable regime of binary fluid
mixtures heated from below. A finite-difference method is used to solve the
full hydrodynamic field equations in a vertical cross section of the layer
perpendicular to the convection roll axes. Results are presented for
ethanol-water parameters with several strongly negative separation ratios where
TW solutions bifurcate subcritically. Fronts and LTWs are compared with each
other and similarities and differences are elucidated. Phase propagation out of
the quiescent fluid into the convective structure entails a unique selection of
the latter while fronts and interfaces where the phase moves into the quiescent
state behave differently. Interpretations of various experimental observations
are suggested.Comment: 46 pages, 11 figures. Accepted for publication in Phys. Rev.
Technologische Alternativen zum herkömmlichen Einsatz von Pökelstoffen in Öko-Fleischwaren
On the basis of literature research and opinions of meat processors and other experts, this paper discusses alternatives to the currently permitted use of curing agents in the processing of organic meat. These alternatives include (i) the reduction of addition of nitrite to levels sufficient for the desired sensory properties, (ii) the in situ bacterial formation of nitrite from nitrate naturally present in added vegetable preparations, and (iii) not making use of the beneficial effects of nitrite on the colour and aroma of the product at all. Measures to be taken to compensate for the effects of nitrite, as well as problems in implementation of technological alternatives are discussed
Interaction-free measurements by quantum Zeno stabilisation of ultracold atoms
Quantum mechanics predicts that our physical reality is influenced by events
that can potentially happen but factually do not occur. Interaction-free
measurements (IFMs) exploit this counterintuitive influence to detect the
presence of an object without requiring any interaction with it. Here we
propose and realize an IFM concept based on an unstable many-particle system.
In our experiments, we employ an ultracold gas in an unstable spin
configuration which can undergo a rapid decay. The object - realized by a laser
beam - prevents this decay due to the indirect quantum Zeno effect and thus,
its presence can be detected without interacting with a single atom. Contrary
to existing proposals, our IFM does not require single-particle sources and is
only weakly affected by losses and decoherence. We demonstrate confidence
levels of 90%, well beyond previous optical experiments.Comment: manuscript with 5 figures, 3 supplementary figure, 1 supplementary
not
Spontaneous breaking of spatial and spin symmetry in spinor condensates
Parametric amplification of quantum fluctuations constitutes a fundamental
mechanism for spontaneous symmetry breaking. In our experiments, a spinor
condensate acts as a parametric amplifier of spin modes, resulting in a twofold
spontaneous breaking of spatial and spin symmetry in the amplified clouds. Our
experiments permit a precise analysis of the amplification in specific spatial
Bessel-like modes, allowing for the detailed understanding of the double
symmetry breaking. On resonances that create vortex-antivortex superpositions,
we show that the cylindrical spatial symmetry is spontaneously broken, but
phase squeezing prevents spin-symmetry breaking. If, however, nondegenerate
spin modes contribute to the amplification, quantum interferences lead to
spin-dependent density profiles and hence spontaneously-formed patterns in the
longitudinal magnetization.Comment: 5 pages, 4 figure
A New Derivation of the CPT and Spin-Statistics Theorems in Non-Commutative Field Theories
We propose an alternative axiomatic description for non-commutative field
theories (NCFT) based on some ideas by Soloviev to nonlocal quantum fields. The
local commutativity axiom is replaced by the weaker condition that the fields
commute at sufficiently large spatial separations, called asymptotic
commutativity, formulated in terms of the theory of analytic functionals. The
question of a possible violation of the CPT and Spin-Statistics theorems caused
by nonlocality of the commutation relations
is investigated. In spite of this
inherent nonlocality, we show that the modification aforementioned is
sufficient to ensure the validity of these theorems for NCFT. We restrict
ourselves to the simplest model of a scalar field in the case of only
space-space non-commutativity.Comment: The title is new, and the analysis in the manuscript has been made
more precise. This revised version is to be published in J.Math.Phy
PCT, spin and statistics, and analytic wave front set
A new, more general derivation of the spin-statistics and PCT theorems is
presented. It uses the notion of the analytic wave front set of
(ultra)distributions and, in contrast to the usual approach, covers nonlocal
quantum fields. The fields are defined as generalized functions with test
functions of compact support in momentum space. The vacuum expectation values
are thereby admitted to be arbitrarily singular in their space-time dependence.
The local commutativity condition is replaced by an asymptotic commutativity
condition, which develops generalizations of the microcausality axiom
previously proposed.Comment: LaTeX, 23 pages, no figures. This version is identical to the
original published paper, but with corrected typos and slight improvements in
the exposition. The proof of Theorem 5 stated in the paper has been published
in J. Math. Phys. 45 (2004) 1944-195
Non-Localizability and Asymptotic Commutativity
The mathematical formalism commonly used in treating nonlocal highly singular
interactions is revised. The notion of support cone is introduced which
replaces that of support for nonlocalizable distributions. Such support cones
are proven to exist for distributions defined on the Gelfand-Shilov spaces
, where . This result leads to a refinement of previous
generalizations of the local commutativity condition to nonlocal quantum
fields. For string propagators, a new derivation of a representation similar to
that of K\"{a}llen-Lehmann is proposed. It is applicable to any initial and
final string configurations and manifests exponential growth of spectral
densities intrinsic in nonlocalizable theories.Comment: This version is identical to the initial one whose ps and pdf files
were unavailable, with few corrections of misprint
Convection in nanofluids with a particle-concentration-dependent thermal conductivity
Thermal convection in nanofluids is investigated by means of a continuum
model for binary-fluid mixtures, with a thermal conductivity depending on the
local concentration of colloidal particles. The applied temperature difference
between the upper and the lower boundary leads via the Soret effect to a
variation of the colloid concentration and therefore to a spatially varying
heat conductivity. An increasing difference between the heat conductivity of
the mixture near the colder and the warmer boundary results in a shift of the
onset of convection to higher values of the Rayleigh number for positive values
of the separation ratio psi>0 and to smaller values in the range psi<0. Beyond
some critical difference of the thermal conductivity between the two
boundaries, we find an oscillatory onset of convection not only for psi<0, but
also within a finite range of psi>0. This range can be extended by increasing
the difference in the thermal conductivity and it is bounded by two
codimension-2 bifurcations.Comment: 13 pages, 11 figures; submitted to Physical Review
Effects of the low frequencies of noise on On-Off intermittency
A bifurcating system subject to multiplicative noise can exhibit on-off
intermittency close to the instability threshold. For a canonical system, we
discuss the dependence of this intermittency on the Power Spectrum Density
(PSD) of the noise. Our study is based on the calculation of the Probability
Density Function (PDF) of the unstable variable. We derive analytical results
for some particular types of noises and interpret them in the framework of
on-off intermittency. Besides, we perform a cumulant expansion for a random
noise with arbitrary power spectrum density and show that the intermittent
regime is controlled by the ratio between the departure from the threshold and
the value of the PSD of the noise at zero frequency. Our results are in
agreement with numerical simulations performed with two types of random
perturbations: colored Gaussian noise and deterministic fluctuations of a
chaotic variable. Extensions of this study to another, more complex, system are
presented and the underlying mechanisms are discussed.Comment: 13pages, 13 figure
Adiabatic reduction near a bifurcation in stochastically modulated systems
We re-examine the procedure of adiabatic elimination of fast relaxing
variables near a bifurcation point when some of the parameters of the system
are stochastically modulated. Approximate stationary solutions of the
Fokker-Planck equation are obtained near threshold for the pitchfork and
transcritical bifurcations. Stochastic resonance between fast variables and
random modulation may shift the effective bifurcation point by an amount
proportional to the intensity of the fluctuations. We also find that
fluctuations of the fast variables above threshold are not always Gaussian and
centered around the (deterministic) center manifold as was previously believed.
Numerical solutions obtained for a few illustrative examples support these
conclusions.Comment: RevTeX, 19 pages and 16 figure
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