4,705 research outputs found
Analytic calculation of anomalous scaling in random shell models for a passive scalar
An exact non-perturbative calculation of the fourth-order anomalous
correction to the scaling behaviour of a random shell-model for passive scalars
is presented. Importance of ultraviolet (UV) and infrared (IR) boundary
conditions on the inertial scaling properties are determined. We find that
anomalous behaviour is given by the null-space of the inertial operator and we
prove strong UV and IR independence of the anomalous exponent. A limiting case
where diffusive behaviour can influence inertial properties is also presented.Comment: 3 pages, 1 figure, revised versio
Natural convection with mixed insulating and conducting boundary conditions: low and high Rayleigh numbers regimes
We investigate the stability and dynamics of natural convection in two
dimensions, subject to inhomogeneous boundary conditions. In particular, we
consider a Rayleigh-B\`enard (RB) cell, where the horizontal top boundary
contains a periodic sequence of alternating thermal insulating and conducting
patches, and we study the effects of the heterogeneous pattern on the global
heat exchange, both at low and high Rayleigh numbers. At low Rayleigh numbers,
we determine numerically the transition from a regime characterized by the
presence of small convective cells localized at the inhomogeneous boundary to
the onset of bulk convective rolls spanning the entire domain. Such a
transition is also controlled analytically in the limit when the boundary
pattern length is small compared with the cell vertical size. At higher
Rayleigh number, we use numerical simulations based on a lattice Boltzmann
method to assess the impact of boundary inhomogeneities on the fully turbulent
regime up to
Magnetization Reversal in Elongated Fe Nanoparticles
Magnetization reversal of individual, isolated high-aspect-ratio Fe
nanoparticles with diameters comparable to the magnetic exchange length is
studied by high-sensitivity submicron Hall magnetometry. For a Fe nanoparticle
with diameter of 5 nm, the magnetization reversal is found to be an incoherent
process with localized nucleation assisted by thermal activation, even though
the particle has a single-domain static state. For a larger elongated Fe
nanoparticle with a diameter greater than 10 nm, the inhomogeneous magnetic
structure of the particle plays important role in the reversal process.Comment: 6 pages, 6 figures, to appear in Phys. Rev. B (2005
Stabilization of controlled diffusions via Zubov's method
We consider a controlled stochastic system which is exponentially stabilizable in probability
near an attractor. Our aim is to characterize the set of points which can be driven
by a suitable control to the attractor with either positive probability or with probability
one. This will be done by associating to the stochastic system a suitable control problem
and the corresponding Zubov equation. We then show that this approach can be used
as a basis for numerical computations of these sets
Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals
We have investigated the influence of Fe-excess on the electrical transport
and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both
compositions exhibit resistively determined superconducting transitions (Tc)
with an onset temperature of about 15 K. From the width of the superconducting
transition and the magnitude of the lower critical field Hc1, it is inferred
that excess of Fe suppresses superconductivity. The linear and non-linear
responses of the ac-susceptibility show that the superconducting state for
these compositions is inhomogeneous. A possible origin of this phase separation
is a magnetic coupling between Fe-excess occupying interstitial sites in the
chalcogen planes and those in the Fe-square lattice. The temperature derivative
of the resistivity drho/dT in the temperature range Tc < T < Ta with Ta being
the temperature of a magnetic anomaly, changes from positive to negative with
increasing Fe. A log 1/T divergence of the resistivity above Tc in the sample
with higher amount of Fe suggests a disorder driven electronic localization.Comment: 7 page
A precursor state to unconventional superconductivity in CeIrIn
We present sensitive measurements of the Hall effect and magnetoresistance in
CeIrIn down to temperatures of 50 mK and magnetic fields up to 15 T. The
presence of a low temperature coherent Kondo state is established. Deviations
from Kohler's rule and a quadratic temperature dependence of the cotangent of
the Hall angle are reminiscent of properties observed in the high temperature
superconducting cuprates. The most striking observation pertains to the
presence of a \textit{precursor} state--characterized by a change in the Hall
mobility--that appears to precede the superconductivity in this material, in
similarity to the pseudogap in the cuprate high superconductors.Comment: 4 figure
B cell hyperresponsiveness and expansion of mature follicular B cells but not of marginal zone B cells in NFATc2/c3 double-deficient mice
Marginal zone (MZ) B cells and peritoneal B-I cells provide a first defense system of thymus-independent Ab responses against foreign pathogens and therefore share a number of functional properties. Recently, development of B-1a cells was shown to be controlled by the transcription factor NFATc1. We show here that mice deficient for NFATc2 and c3 display a distinct lower representation of MZ B cells, which is correlated with a reduced capturing of trinitrophenyl-Ficoll. In contrast, mature follicular B cells from NFATc2/c3(-/-) mice are strongly increased in number. NFATc2/c3-/- B cells exhibit a marked increase in BCR-induced intracellular Ca(2+) mobilization and proliferation. However, trinitrophenyl-Ficoll-specific IgM and IgG3 responses of NFATc2/c3-deficient mice are intact, and chimeric mice reconstituted with NFATc2/3-deficient B cells show a normal number of MZ B cells and normal BCR responses. These observations suggest that the strongly elevated Th2 cytokine milieu in NFATc2/c3-deficient mice leads to a hyperactivation of mature, follicular B cells, whereas MZ B cells are less responsive to these signals
Transmission resonance spectroscopy in the third minimum of 232Pa
The fission probability of 232Pa was measured as a function of the excitation
energy in order to search for hyperdeformed (HD) transmission resonances using
the (d,pf) transfer reaction on a radioactive 231Pa target. The experiment was
performed at the Tandem accelerator of the Maier-Leibnitz Laboratory (MLL) at
Garching using the 231Pa(d,pf) reaction at a bombarding energy of E=12 MeV and
with an energy resolution of dE=5.5 keV. Two groups of transmission resonances
have been observed at excitation energies of E=5.7 and 5.9 MeV. The fine
structure of the resonance group at E=5.7 MeV could be interpreted as
overlapping rotational bands with a rotational parameter characteristic to a HD
nuclear shape. The fission barrier parameters of 232Pa have been determined by
fitting TALYS 1.2 nuclear reaction code calculations to the overall structure
of the fission probability. From the average level spacing of the J=4 states,
the excitation energy of the ground state of the 3rd minimum has been deduced
to be E(III)=5.05 MeV.Comment: 6 pages, 8 figure
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