Spontaneous rotating vortex rings in a parametrically driven polariton fluid

We present the theoretical prediction of spontaneous rotating vortex rings in a parametrically driven quantum fluid of polaritons -- coherent superpositions of coupled quantum well excitons and microcavity photons. These rings arise not only in the absence of any rotating drive, but also in the absence of a trapping potential, in a model known to map quantitatively to experiments. We begin by proposing a novel parametric pumping scheme for polaritons, with circular symmetry and radial currents, and characterize the resulting nonequilibrium condensate. We show that the system is unstable to spontaneous breaking of circular symmetry via a modulational instability, following which a vortex ring with large net angular momentum emerges, rotating in one of two topologically distinct states. Such rings are robust and carry distinctive experimental signatures, and so they could find applications in the new generation of polaritonic devices.Comment: 6 pages, 4 figure

Coordination and organometallic precursors of group 10 and 11 Focused electron beam induced deposition of metals and insight gained from chemical vapour deposition, atomic layer deposition, and fundamental surface and gas phase studies

Nanostructured materials made from group 10 Ni, Pd, Pt and group 11 Cu, Ag, Au elements have outstanding technological relevance in microelectronics, nano optics, catalysis, and energy conversion. Processes that allow for the easy and reliable fabrication of such nanostructures are heavily sought after. Focused electron beam induced deposition FEBID is the only direct write technique that can fabricate nanostructures with arbitrary shape and dimensions down to the sub 10 nm regime. However, the complex chemistry of FEBID involving electron induced dissociation processes of metalorganic precursors molecules, surface kinetics, and thermal effects is poorly understood and far from being optimized. Here, we review in a comparative manner the performance and the underlying chemical reactions of surface deposition processes, namely, chemical vapour deposition CVD , atomic layer deposition ALD , and FEBID itself. The knowledge gained in CVD and ALD as related surface deposition techniques will help us to understand the spatially selective chemistry occurring in FEBID. Fundamental surface and gas phase studies provide insight to electron induced chemistry and desorption of precursor fragments. Specific emphasis is put on the type of the ligands and their different behaviour under thermal, surface related, and electron induced processes. The comprehensive overview of the current state of FEBID for group 10 and 11 metals includes reactive environments and purification approaches as these may provide valuable information on the design of novel precursors. The evaluation of the precursor and process performance is extended to include W, Co, Fe, Ru, Rh, and Ir to represent a general guide towards future developments in FEBID. These may not only rely on the design of novel compounds but also on optimized deposition strategies inspired by ALD and CV

The effect of tumour necrosis factor-α (TNF-α) muteins on human neutrophils in vitro

Tumour necrosis factor-α (TNF-α) has been implicated as an important inflammatory mediator. In vitro, TNF-α is reported to activate human polymorphonuclear neutrophils (PMN), inducing responses such as phagocytic activity, degranulation and oxidative metabolism. Biological responses to TNF-α are initiated by its binding to specific cell surface receptors, and various studies have shown that the major TNF receptor species on PMN is the 75 kDa receptor. To verify the suggestion that the receptor binding domain includes the region close to the N-terminus of the TNF-α molecule, four TNF-α derivatives termed muteins were constructed, using a synthetic cDNA fragment substituting the N-terminal 3–7 selected hydrophilic or hydrophobic amino acids in the original TNF-α genomic DNA. Binding of muteins to PMN was assessed using monoclonal antibodies recognizing either the 55 kDa (p55) or the 75 kDa (p75) TNF receptor subtypes. Blocking by muteins of anti-p75 antibody binding to PMN was as expected from their N-terminal amino acid composition and hydrophilic properties. Hydrophilic muteins competed well with anti-TNF receptor antibodies for binding to the p75 receptor. In contrast, hydrophobic muteins were unable to block anti-p75 binding. Similarly, degranulation, chemiluminescence or enhancement of the PMN response to specific stimuli by the muteins correlated with the hydrophilic properties of the muteins. The significance of these observations in relation to the molecular structure of TNF-α is discussed

New detectors for the kaon and hypernuclear experiments with KaoS at MAMI and with PANDA at GSI

The KaoS spectrometer at the Mainz Microtron MAMI, Germany, is perceived as the ideal candidate for a dedicated spectrometer in kaon and hypernuclei electroproduction. KaoS will be equipped with new read-out electronics, a completely new focal plane detector package consisting of scintillating fibres, and a new trigger system. First prototypes of the fibre detectors and the associated new front-end electronics are shown in this contribution. The Mainz hypernuclei research program will complement the hypernuclear experiments at the planned FAIR facility at GSI, Germany. At the proposed antiproton storage ring the spectroscopy of double Lambda hypernuclei is one of the four main topics which will be addressed by the PANDA Collaboration. The experiments require the operation of high purity germanium (HPGe) detectors in high magnetic fields (B= 1T) in the presence of a large hadronic background. The performance of high resolution Ge detectors in such an environment has been investigated.Comment: Presentation at International Symposium on the Development of Detectors for Particle, Astroparticle and Synchrotron Radiation Experiments, Stanford, Ca (SNIC06), 6 pages, LaTeX, 11 eps figure

Polariton condensation and lasing in optical microcavities - the decoherence driven crossover

We explore the behaviour of a system which consists of a photon mode dipole coupled to a medium of two-level oscillators in a microcavity in the presence of decoherence. We consider two types of decoherence processes which are analogous to magnetic and non-magnetic impurities in superconductors. We study different phases of this system as the decoherence strength and the excitation density is changed. For a low decoherence we obtain a polariton condensate with comparable excitonic and photonic parts at low densities and a BCS-like state with bigger photon component due to the fermionic phase space filling effect at high densities. In both cases there is a large gap in the density of states. As the decoherence is increased the gap is broadened and suppressed, resulting in a gapless condensate and finally a suppression of the coherence in a low density regime and a laser at high density limit. A crossover between these regimes is studied in a self-consistent way analogous to the Abrikosov and Gor'kov theory of gapless superconductivity.Comment: 17 pages, 8 figures, submitted to PR

Density dependent composition of InAs quantum dots extracted from grazing incidence x-ray diffraction measurements.

Epitaxial InAs quantum dots grown on GaAs substrate are being used in several applications ranging from quantum communications to solar cells. The growth mechanism of these dots also helps us to explore fundamental aspects of self-organized processes. Here we show that composition and strain profile of the quantum dots can be tuned by controlling in-plane density of the dots over the substrate with the help of substrate-temperature profile. The compositional profile extracted from grazing incidence x-ray measurements show substantial amount of inter-diffusion of Ga and In within the QD as a function of height in the low-density region giving rise to higher variation of lattice parameters. The QDs grown with high in-plane density show much less spread in lattice parameter giving almost flat density of In over the entire height of an average QD and much narrower photoluminescence (PL) line. The results have been verified with three different amounts of In deposition giving systematic variation of the In composition as a function of average quantum dot height and average energy of PL emission.The authors would like to acknowledge the support of Department of Science and Technology (DST) for carrying out synchrotron experiments at Petra III, DESY, Germany through the DST-DESY project and the EPSRC, UK for financial support.This is the final version of the article. It first appeared from NPG via http://dx.doi.org/10.1038/srep1573

Gas assisted silver deposition with a focused electron beam

Focused electron beam induced deposition FEBID is a flexible direct write method to obtain defined structures with a high lateral resolution. In order to use this technique in application fields like plasmonics, suitable precursors which allow the deposition of desired materials have to be identified. Well known for its plasmonic properties, silver represents an interesting candidate for FEBID. For this purpose the carboxylate complex silver I pentafluoropropionate AgO2CC2F5 was used for the first time in FEBID and resulted in deposits with high silver contents up to 76 at. . As verified by TEM investigations, the deposited material is composed of pure silver crystallites in a carbon matrix. It showed good electrical properties and a strong Raman signal enhancement. Interestingly, silver crystal growth presents a strong dependency on electron dose and precursor refreshment

Many-body physics of a quantum fluid of exciton-polaritons in a semiconductor microcavity

Some recent results concerning nonlinear optics in semiconductor microcavities are reviewed from the point of view of the many-body physics of an interacting photon gas. Analogies with systems of cold atoms at thermal equilibrium are drawn, and the peculiar behaviours due to the non-equilibrium regime pointed out. The richness of the predicted behaviours shows the potentialities of optical systems for the study of the physics of quantum fluids.Comment: Proceedings of QFS2006 conference to appear on JLT

Performance of HPGe Detectors in High Magnetic Fields

A new generation of high-resolution hypernuclear gamma$-spectroscopy experiments with high-purity germanium detectors (HPGe) are presently designed at the FINUDA spectrometer at DAPhiNE, the Frascati phi-factory, and at PANDA, the antiproton proton hadron spectrometer at the future FAIR facility. Both, the FINUDA and PANDA spectrometers are built around the target region covering a large solid angle. To maximise the detection efficiency the HPGe detectors have to be located near the target, and therefore they have to be operated in strong magnetic fields B ~ 1 T. The performance of HPGe detectors in such an environment has not been well investigated so far. In the present work VEGA and EUROBALL Cluster HPGe detectors were tested in the field provided by the ALADiN magnet at GSI. No significant degradation of the energy resolution was found, and a change in the rise time distribution of the pulses from preamplifiers was observed. A correlation between rise time and pulse height was observed and is used to correct the measured energy, recovering the energy resolution almost completely. Moreover, no problems in the electronics due to the magnetic field were observed.Comment: submitted to Nucl. Instrum. Meth. Phys. Res. A, LaTeX, 19 pages, 9 figure

High throughput high content reverse genetics visual screens of ciliogenesis and cilia maintenance

Cilia are small, hair-like structures occurring on the apical surface of most of vertebrate cells. Defects in cilia cause a range of developmental phenotypes grouped into conditions called ciliopathies. Our aim is to dissect the structure and function of cilia and signalling pathways mediated by this organelle. To evaluate this, we are performing a high-throughput siRNA screen using siRNA pools (from the Dharmacon mouse genome siRNA library) targeting over 19,000 separate transcripts and immunofluorescence staining of ciliated mIMCD3 (transformed mouse inner medullary collecting duct) cells to determine cilia number, length and morphology. Secondary datasets from the screen will include measurements of cell size and morphology, nuclear morphology and cell cycle profiles. We have successfully set up a facility for high-throughput high-content imaging, optimized a reverse transfection protocol and validated a series of positive and negative controls. We are currently completing the analysis of candidate hits and expect to obtain several hundred positive hits from the whole screen. We will present the first dataset from this screen with a discussion of prioritization strategies for the validation of the most relevant and interesting candidate hits