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