134 research outputs found
Dimensional Crossover of Weak Localization in a Magnetic Field
We study the dimensional crossover of weak localization in strongly
anisotropic systems. This crossover from three-dimensional behavior to an
effective lower dimensional system is triggered by increasing temperature if
the phase coherence length gets shorter than the lattice spacing . A similar
effect occurs in a magnetic field if the magnetic length becomes shorter
than , where \D_{||}/D_\perp is the ratio of the
diffusion coefficients parallel and perpendicular to the planes or chains.
depends on the direction of the magnetic field, e.g. or
1/2 for a magnetic field parallel or perpendicular to the planes in a quasi
two-dimensional system. We show that even in the limit of large magnetic field,
weak localization is not fully suppressed in a lattice system. Experimental
implications are discussed in detail.Comment: RevTeX, 11 pages, 4 figures; three references added and discusse
Quantum Hall ferromagnets, cooperative transport anisotropy, and the random field Ising model
We discuss the behaviour of a quantum Hall system when two Landau levels with
opposite spin and combined filling factor near unity are brought into energetic
coincidence using an in-plane component of magnetic field. We focus on the
interpretation of recent experiments under these conditions [Zeitler et al,
Phys. Rev. Lett. 86, 866 (2001); Pan et al, Phys. Rev. B 64, 121305 (2001)], in
which a large resistance anisotropy develops at low temperatures. Modelling the
systems involved as Ising quantum Hall ferromagnets, we suggest that this
transport anisotropy reflects domain formation induced by a random field
arising from isotropic sample surface roughness.Comment: 4 pages, submitted to Physical Review
Spin and Charge Structure Factor of the 2-d Hubbard Model
The spin and charge structure factors are calculated for the Hubbard model on
the square lattice near half-filling using a spin-rotation invariant six-slave
boson representation. The charge structure factor shows a broad maximum at the
zone corner and is found to decrease monotonically with increasing interaction
strength and electron density and increasing temperature. The spin structure
factor develops with increasing interaction two incommensurate peaks at the
zone boundary and along the zone diagonal. Comparison with results of Quantum
Monte Carlo and variational calculations is carried out and the agreement is
found to be good. The limitations of an RPA-type approach are pointed out.Comment: 18 pages, revtex, 13 postscript figures, submitted to Phys. Rev.
Multiple light scattering in anisotropic random media
In the last decade Diffusing Wave Spectroscopy (DWS) has emerged as a
powerful tool to study turbid media. In this article we develop the formalism
to describe light diffusion in general anisotropic turbid media. We give
explicit formulas to calculate the diffusion tensor and the dynamic absorption
coefficient, measured in DWS experiments. We apply our theory to uniaxial
systems, namely nematic liquid crystals, where light is scattered from thermal
fluctuations of the local optical axis, called director. We perform a detailed
analysis of the two essential diffusion constants, parallel and perpendicular
to the director, in terms of Frank elastic constants, dielectric anisotropy,
and applied magnetic field. We also point out the relevance of our results to
different liquid crystalline systems, such as discotic nematics, smectic-A
phases, and polymer liquid crystals. Finally, we show that the dynamic
absorption coefficient is the angular average over the inverse viscosity, which
governs the dynamics of director fluctuations.Comment: 23 pages, 12 ps figures, to be published in Phys. Rev.
Kondo effect in quantum dots
We review mechanisms of low-temperature electronic transport through a
quantum dot weakly coupled to two conducting leads. Transport in this case is
dominated by electron-electron interaction. At temperatures moderately lower
than the charging energy of the dot, the linear conductance is suppressed by
the Coulomb blockade. Upon further lowering of the temperature, however, the
conductance may start to increase again due to the Kondo effect. We concentrate
on lateral quantum dot systems and discuss the conductance in a broad
temperature range, which includes the Kondo regime
Plastisol Foaming Process. Decomposition of the Foaming Agent, Polymer Behavior in the Corresponding Temperature Range and Resulting Foam Properties
The decomposition of azodicarbonamide, used as foaming agent in PVC - plasticizer (1/1) plastisols was studied by DSC. Nineteen different plasticizers, all belonging to the ester family, two being polymeric (polyadipates), were compared. The temperature of maximum decomposition rate (in anisothermal regime at 5 K min-1 scanning rate), ranges between 434 and 452 K. The heat of decomposition ranges between 8.7 and 12.5 J g -1. Some trends of variation of these parameters appear significant and are discussed in terms of solvent (matrix) and viscosity effects on the decomposition reactions. The shear modulus at 1 Hz frequency was determined at the temperature of maximum rate of foaming agent decomposition, and differs significantly from a sample to another. The foam density was determined at ambient temperature and the volume fraction of bubbles was used as criterion to judge the efficiency of the foaming process. The results reveal the existence of an optimal shear modulus of the order of 2 kPa that corresponds roughly to plasticizer molar masses of the order of 450 ± 50 g mol-1. Heavier plasticizers, especially polymeric ones are too difficult to deform. Lighter plasticizers such as diethyl phthalate (DEP) deform too easily and presumably facilitate bubble collapse
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
D* Production in Deep Inelastic Scattering at HERA
This paper presents measurements of D^{*\pm} production in deep inelastic
scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The
data have been taken with the ZEUS detector at HERA. The decay channel
(+ c.c.) has been used in the study. The
cross section for inclusive D^{*\pm} production with
and is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region
{ GeV and }. Differential cross
sections as functions of p_T(D^{*\pm}), and are
compared with next-to-leading order QCD calculations based on the photon-gluon
fusion production mechanism. After an extrapolation of the cross section to the
full kinematic region in p_T(D^{*\pm}) and (D^{*\pm}), the charm
contribution to the proton structure function is
determined for Bjorken between 2 10 and 5 10.Comment: 17 pages including 4 figure
Scientific Guidance on the data required for the risk assessment of flavourings to be used in or on foods
Following a request from the European Commission, EFSA developed a new scientific guidance to assist applicants in the preparation of applications for the authorisation of flavourings to be used in or on foods. This guidance applies to applications for a new authorisation as well as for a modification of an existing authorisation of a food flavouring, submitted under Regulation (EC) No 1331/2008. It defines the scientific data required for the evaluation of those food flavourings for which an evaluation and approval is required according to Article 9 of Regulation (EC) No 1334/2008. This applies to flavouring substances, flavouring preparations, thermal process flavourings, flavour precursors, other flavourings and source materials, as defined in Article 3 of Regulation (EC) No 1334/2008. Information to be provided in all applications relates to: (a) the characterisation of the food flavouring, including the description of its identity, manufacturing process, chemical composition, specifications, stability and reaction and fate in foods; (b) the proposed uses and use levels and the assessment of the dietary exposure and (c) the safety data, including information on the genotoxic potential of the food flavouring, toxicological data other than genotoxicity and information on the safety for the environment. For the toxicological studies, a tiered approach is applied, for which the testing requirements, key issues and triggers are described. Applicants should generate the data requested in each section to support the safety assessment of the food flavouring. Based on the submitted data, EFSA will assess the safety of the food flavouring and conclude whether or not it presents risks to human health and to the environment, if applicable, under the proposed conditions of use
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