12,569 research outputs found
Relaxation dynamics of multi-level tunneling systems
A quantum mechanical treatment of an asymmetric double-well potential (DWP)
interacting with a heat bath is presented for circumstances where the
contribution of higher vibrational levels to the relaxation dynamics cannot be
excluded from consideration. The deep quantum limit characterized by a discrete
energy spectrum near the barrier top is considered. The investigation is
motivated by simulations on a computer glass which show that the considered
parameter regime is ``typical'' for DWPs being responsible for the relaxation
peak of sound absorption in glasses. Relaxation dynamics resembling the
spatial- and energy-diffusion-controlled limit of the classical Kramers'
problem, and Arrhenius-like behavior is found under specific conditions.Comment: 23 pages, RevTex, 2 figures can be received from the Authors upon
reques
Kinetics of helium bubble formation in nuclear materials
The formation and growth of helium bubbles due to self-irradiation in
plutonium has been modelled by a discrete kinetic equations for the number
densities of bubbles having atoms. Analysis of these equations shows that
the bubble size distribution function can be approximated by a composite of:
(i) the solution of partial differential equations describing the continuum
limit of the theory but corrected to take into account the effects of
discreteness, and (ii) a local expansion about the advancing leading edge of
the distribution function in size space. Both approximations contribute to the
memory term in a close integrodifferential equation for the monomer
concentration of single helium atoms.
The present boundary layer theory for discrete equations is compared to the
numerical solution of the full kinetic model and to previous approximation of
Schaldach and Wolfer involving a truncated system of moment equations.Comment: 24 pages, 6 figures, to appear in Physica
Direct observation of superconducting vortex clusters pinned by a periodic array of magnetic dots in ferromagnetic/superconducting hybrid structures
Strong pinning of superconducting flux quanta by a square array of 1
m-sized ferromagnetic dots in a magnetic-vortex state was visualized by
low-temperature magnetic force microscopy (LT-MFM). A direct correlation of the
superconducting flux lines with the positions of the dots was derived. The
force that the MFM tip exerts on the individual vortex in the depinning process
was used to estimate the spatial modulation of the pinning potential. It was
found, that the superconducting vortices which are preferably located on top of
the Py dots experience about 15 times stronger pinning forces as compared to
the pinning force in the pure Nb film. The strong pinning exceeds the repulsive
interaction between the superconducting vortices and allows the vortex clusters
to be located at each dot. Our microscopic studies are consistent with global
magnetoresistace measurements on these hybrid structures.Comment: 4 pages, 4 figure
Assignment of grammatical functions in discourse context and word-order ambiguity resolution
In German, noun phrases (NPs) can be ambiguously case-marked as nominative (Subject) or accusative (Object). When readers encounter such ambiguous NPs in scrambled word orders they can experience processing difficulties (e.g., Hemforth, 1993; Knöferle, Crocker, Scheepers, & Pickering, 2001). However, what if preceding discourse context provides information for establishing the grammatical function of NPs? This information could influence readers’ interpretation of subsequent NPs. In principle, both the processing of sentences with scrambled and canonical word orders could profit from such contextual focus
Domain structure of epitaxial Co films with perpendicular anisotropy
Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared
by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single
crystal substrates or with an intermediate Ruthenium buffer layer. The crystal
structure and epitaxial growth relation was studied by XRD, pole figure
measurements and reciprocal space mapping. Detailed VSM analysis shows that the
perpendicular anisotropy of these highly textured Co films reaches the
magnetocrystalline anisotropy of hcp-Co single crystal material. Films were
prepared with thickness t of 20 nm < t < 100 nm to study the crossover from
in-plane magnetization to out-of-plane magnetization in detail. The analysis of
the periodic domain pattern observed by magnetic force microscopy allows to
determine the critical minimum thickness below which the domains adopt a pure
in-plane orientation. Above the critical thickness the width of the stripe
domains is evaluated as a function of the film thickness and compared with
domain theory. Especially the discrepancies at smallest film thicknesses show
that the system is in an intermediate state between in-plane and out-of-plane
domains, which is not described by existing analytical domain models
Tunneling dynamics of side chains and defects in proteins, polymer glasses, and OH-doped network glasses
Simulations on a Lennard-Jones computer glass are performed to study effects
arising from defects in glasses at low temperatures. The numerical analysis
reveals that already a low concentration of defects may dramatically change the
low temperature properties by giving rise to extrinsic double-well potentials
(DWP's). The main characteristics of these extrinsic DWP's are (i) high barrier
heights, (ii) high probability that a defect is indeed connected with an
extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv)
smaller deformation potential coupling to phonons. Designing an extension of
the Standard Tunneling Model (STM) which parametrizes this picture and
comparing with ultrasound experiments on the wet network glass -BO
shows that effects of OH-impurities are accurately accounted for. This model is
then applied to organic polymer glasses and proteins. It is suggested that side
groups may act similarly like doped impurities inasmuch as extrinsic DWP's are
induced, which possess a distribution of barriers peaked around a high barrier
height. This compares with the structurlessly distributed barrier heights of
the intrinsic DWP's, which are associated with the backbone dynamics. It is
shown that this picture is consistent with elastic measurements on polymers,
and can explain anomalous nonlogarithmic line broadening recently observed in
hole burning experiments in PMMA.Comment: 34 pages, Revtex, 9 eps-figures, accepted for publication in J. Chem.
Phy
Random matrix theory for CPA: Generalization of Wegner's --orbital model
We introduce a generalization of Wegner's -orbital model for the
description of randomly disordered systems by replacing his ensemble of
Gaussian random matrices by an ensemble of randomly rotated matrices. We
calculate the one- and two-particle Green's functions and the conductivity
exactly in the limit . Our solution solves the CPA-equation of the
-Anderson model for arbitrarily distributed disorder. We show how the
Lloyd model is included in our model.Comment: 3 pages, Rev-Te
Wave trains, self-oscillations and synchronization in discrete media
We study wave propagation in networks of coupled cells which can behave as
excitable or self-oscillatory media. For excitable media, an asymptotic
construction of wave trains is presented. This construction predicts their
shape and speed, as well as the critical coupling and the critical separation
of time scales for propagation failure. It describes stable wave train
generation by repeated firing at a boundary. In self-oscillatory media, wave
trains persist but synchronization phenomena arise. An equation describing the
evolution of the oscillator phases is derived.Comment: to appear in Physica D: Nonlinear Phenomen
SPARC CCMVal (2010), SPARC Report on the Evaluation of Chemistry-Climate Models: Chapter 5: Transport
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