9,624 research outputs found
Polariton lasing in high-quality Selenide-based micropillars in the strong coupling regime
We have designed and fabricated all-epitaxial ZnSe-based optical micropillars
exhibiting the strong coupling regime between the excitonic transition and the
confined optical cavity modes. At cryogenic temperatures, under non-resonant
pulsed optical excitation, we demonstrate single transverse mode polariton
lasing operation in the micropillars. Owing to the high quality factors of
these microstructures, the lasing threshold remains low even in micropillars of
the smallest diameter. We show that this feature can be traced back to a
sidewall roughness grain size below 3 nm, and to suppressed in-plane polariton
escape.Comment: 5 pages, 3 figure
Active contractility in actomyosin networks
Contractile forces are essential for many developmental processes involving
cell shape change and tissue deformation. Recent experiments on reconstituted
actomyosin networks, the major component of the contractile machinery, have
shown that active contractility occurs above a threshold motor concentration
and within a window of crosslink concentration. We present a microscopic
dynamic model that incorporates two essential aspects of actomyosin
self-organization: the asymmetric load response of individual actin filaments
and the correlated motor-driven events mimicking myosin-induced filament
sliding. Using computer simulations we examine how the concentration and
susceptibility of motors contribute to their collective behavior and interplay
with the network connectivity to regulate macroscopic contractility. Our model
is shown to capture the formation and dynamics of contractile structures and
agree with the observed dependence of active contractility on microscopic
parameters including the contractility onset. Cooperative action of
load-resisting motors in a force-percolating structure integrates local
contraction/buckling events into a global contractile state via an active
coarsening process, in contrast to the flow transition driven by uncorrelated
kicks of susceptible motors.Comment: 15 pages, 4 main figures, 4 supplementary figure
Homöopathie an der Universität: Ist eine Integration möglich?
Background and Objective:The Dr. von Hauner Children's Hospital Munich has conducted a pilot project for 4 years, aiming at the integration of homeopathic concomitant treatment into daily clinical practice. This study was carried out to clarify whether the project is successful and accepted by physicians, nurses, and parents. Methods: Questionnaires, all standardized and anonymous, were handed out to 137 physicians, 212 nurses and paramedics, and 1,048 parents of children treated at the hospital. Results: 69% of the physicians appreciated concomitant homeopathic treatments, more than 60% believed the integration of homeopathy is suggestive, 75% saw a need for clinical research in homeopathy. 25% of the staff now had a more positive view towards homeopathy. 63% of the parents appreciated concomitant treatment, but only 19% are willing to participate in double-blind studies. Conclusions: Integration of homeopathy in a university hospital is possible. Therapeutic work should be accompanied by scientific research
Spontaneous waves in muscle fibres
Mechanical oscillations are important for many cellular processes, e.g. the
beating of cilia and flagella or the sensation of sound by hair cells. These
dynamic states originate from spontaneous oscillations of molecular motors. A
particularly clear example of such oscillations has been observed in muscle
fibers under non-physiological conditions. In that case, motor oscillations
lead to contraction waves along the fiber. By a macroscopic analysis of muscle
fiber dynamics we find that the spontaneous waves involve non-hydrodynamic
modes. A simple microscopic model of sarcomere dynamics highlights mechanical
aspects of the motor dynamics and fits with the experimental observations.Comment: 14 pages, 9 figure
Morphogen Transport in Epithelia
We present a general theoretical framework to discuss mechanisms of morphogen
transport and gradient formation in a cell layer. Trafficking events on the
cellular scale lead to transport on larger scales. We discuss in particular the
case of transcytosis where morphogens undergo repeated rounds of
internalization into cells and recycling. Based on a description on the
cellular scale, we derive effective nonlinear transport equations in one and
two dimensions which are valid on larger scales. We derive analytic expressions
for the concentration dependence of the effective diffusion coefficient and the
effective degradation rate. We discuss the effects of a directional bias on
morphogen transport and those of the coupling of the morphogen and receptor
kinetics. Furthermore, we discuss general properties of cellular transport
processes such as the robustness of gradients and relate our results to recent
experiments on the morphogen Decapentaplegic (Dpp) that acts in the fruit fly
Drosophila
Self-organization and Mechanical Properties of Active Filament Bundles
A phenomenological description for active bundles of polar filaments is
presented. The activity of the bundle results from crosslinks, that induce
relative displacements between the aligned filaments. Our generic description
is based on momentum conservation within the bundle. By specifying the internal
forces, a simple minimal model for the bundle dynamics is obtained, capturing
generic dynamic behaviors. In particular, contracted states as well as solitary
and oscillatory waves appear through dynamic instabilities. The introduction of
filament adhesion leads to self-organized persistent filament transport.
Furthermore, calculating the tension, homogeneous bundles are shown to be able
to actively contract and to perform work against external forces. Our
description is motivated by dynamic phenomena in the cytoskeleton and could
apply to stress-fibers and self-organization phenomena during cell-locomotion.Comment: 19 pages, 10 figure
Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing
Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process
The 10Be contents of SNC meteorites
Several authors have explored the possibility that the Shergottites, Nakhlites, and Chassigny (SNC) came from Mars. The spallogenic gas contents of the SNC meteorites have been used to: constrain the sizes of the SNC's during the last few million years; to establish groupings independent of the geochemical ones; and to estimate the likelihood of certain entries in the catalog of all conceivable passages from Mars to Earth. The particular shielding dependence of Be-10 makes the isotope a good probe of the irradiation conditions experienced by the SNC meteorites. The Be-10 contents of nine members of the group were measured using the technique of accelerator mass spectrometry. The Be-10 contents of Nakhla, Governador Valadares, Chassigny, and probably Lafayette, about 20 dpm/kg, exceed the values expected from irradiation of the surface of a large body. The Be-10 data therfore do not support scenario III of Bogard et al., one in which most of the Be-10 in the SNC meteorites would have formed on the Martian surface; they resemble rather the Be-10 contents found in many ordinary chondrites subjected to 4 Pi exposures. The uncertainties of the Be-10 contents lead to appreciable errors in the Be-10 ages, t(1) = -1/lambda ln(1 Be-10/Be-10). Nonetheless, the Be-10 ages are consistent with the Ne-21 ages calculated assuming conventional, small-body production rates and short terrestrial ages for the finds. It is believed that this concordance strengthens the case for at least 3 different irradiation ages for the SNC meteorites. Given the similar half-thicknesses of the Be-10 and Ne-21 production rates, the ratios of the Be-10 and Ne-21 contents do not appear consistent with common ages for any of the groups. In view of the general agreement between the Be-10 and Ne-21 ages it does not seem useful at this time to construct multiple-stage irradiation histories for the SNC meteorites
Routes towards Anderson-Like localization of Bose-Einstein condensates in disordered optical lattices
We investigate, both experimentally and theoretically, possible routes
towards Anderson-like localization of Bose-Einstein condensates in disordered
potentials. The dependence of this quantum interference effect on the nonlinear
interactions and the shape of the disorder potential is investigated.
Experiments with an optical lattice and a superimposed disordered potential
reveal the lack of Anderson localization. A theoretical analysis shows that
this absence is due to the large length scale of the disorder potential as well
as its screening by the nonlinear interactions. Further analysis shows that
incommensurable superlattices should allow for the observation of the
cross-over from the nonlinear screening regime to the Anderson localized case
within realistic experimental parameters.Comment: 4 pages to appear in Phys. Rev. Let
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