Slow synaptic transmission in frog sympathetic ganglia

Bullfrog ganglia contain two classes of neurone, B and C cells, which receive different inputs and exhibit different slow synaptic potentials. B cells, to which most effort has been directed, possess slow and late slow EPSPs. The sEPSP reflects a muscarinic action of acetylcholine released from boutons on B cells, whereas the late sEPSP is caused by a peptide (similar to teleost LHRH) released from boutons on C cells. During either sEPSP there is a selective reduction in two slow potassium conductances, designated 'M' and 'AHP'. The M conductance is voltage dependent and the AHP conductance is calcium dependent. Normally they act synergistically to prevent repetitive firing of action potentials during maintained stimuli. Computer stimulation of the interactions of these conductances with the other five voltage-dependent conductances present in the membrane allows a complete reconstruction of the effects of slow synaptic transmission on electrical behaviour

Gain spectroscopy of a type-II VECSEL chip

Using optical pump-white light probe spectroscopy the gain dynamics is investigated for a VECSEL chip which is based on a type-II heterostructure. The active region the chip consists of a GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs multiple quantum well. For this structure, a fully microscopic theory predicts a modal room temperature gain at a wavelength of 1170 nm, which is confirmed by experimental spectra. The results show a gain buildup on the type-II chip which is delayed relative to that of a type-I chip. This slower gain dynamics is attributed to a diminished cooling rate arising from reduced electron-hole scattering.Comment: 4 pages, 4 figure

Observation of Coulomb-Assisted Dipole-Forbidden Intraexciton Transitions in Semiconductors

We use terahertz pulses to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum-well sample. Monitoring the excitonic photoluminescence, we observe transient quenching of the $1s$ exciton emission, which we attribute to the terahertz-induced $1s$-to-$2p$ excitation. Simultaneously, a pronounced enhancement of the $2s$-exciton emission is observed, despite the $1s$-to-$2s$ transition being dipole forbidden. A microscopic many-body theory explains the experimental observations as a Coulomb-scattering mixing of the 2$s$ and 2$p$ states, yielding an effective terahertz transition between the 1$s$ and 2$s$ populations.Comment: 5 pages, 3 figure

Pilot Experiments with Electrodialysis and Ozonation for the Production of a Fertilizer from Urine

Pilot tests were performed with a process combination of electrodialysis and ozonation for the removal of micropollutants and the concentration of nutrients in urine. In continuous and batch experiments, maximum concentration factors up to 3.5 and 4.1 were obtained, respectively. The desalination capacity did not decrease significantly during continuous operation periods of several weeks. Membrane cleaning after 195 days resulted in approximately 35% increase in desalination rate. The Yeast Estrogen Screen (YES), a bioassay that selectively detects oestrogenic compounds, confirmed that about 90% of the oestrogenic activity was removed by electrodialysis. HPLC analysis showed that ibuprofen was removed to a high extent, while other micropollutants were below the detection limit. In view of the fact that ibuprofen is among the most rapidly transported micropollutants in electrodialysis processes, this result indicates that electrodialysis provides an effective barrier for micropollutants. Standardised plant growth tests were performed in the field with the salt solution resulting from the treatment by electrodialysis and subsequent ozonation. The results show that the plant height is comparable to synthetic fertilisers, but the crop yield is slightly lower. The latter is probably caused by volatilisation losses during field application, which can be prevented by improved application technologies

Probing the plateau-insulator quantum phase transition in the quantum Hall regime

We report quantum Hall experiments on the plateau-insulator transition in a low mobility In_{.53} Ga_{.47} As/InP heterostructure. The data for the longitudinal resistance \rho_{xx} follow an exponential law and we extract a critical exponent \kappa= .55 \pm .05 which is slightly different from the established value \kappa = .42 \pm .04 for the plateau transitions. Upon correction for inhomogeneity effects, which cause the critical conductance \sigma_{xx}^* to depend marginally on temperature, our data indicate that the plateau-plateau and plateau- insulator transitions are in the same universality class.Comment: 4 pages, 4 figures (.eps

Renormalization group improved black hole space-time in large extra dimensions

By taking into account a running of the gravitational coupling constant with an ultra violet fixed point, an improvement of classical black hole space-times in extra dimensions is studied. It is found that the thermodynamic properties in this framework allow for an effective description of the black hole evaporation process. Phenomenological consequences of this approach are discussed and the LHC discovery potential is estimated.Comment: 13 pages, 6 figure

Optical absorption of non-interacting tight-binding electrons in a Peierls-distorted chain at half band-filling

In this first of three articles on the optical absorption of electrons in half-filled Peierls-distorted chains we present analytical results for non-interacting tight-binding electrons. We carefully derive explicit expressions for the current operator, the dipole transition matrix elements, and the optical absorption for electrons with a cosine dispersion relation of band width $W$ and dimerization parameter $\delta$. New correction (``$\eta$''-)terms to the current operator are identified. A broad band-to-band transition is found in the frequency range $W\delta < \omega < W$ whose shape is determined by the joint density of states for the upper and lower Peierls subbands and the strong momentum dependence of the transition matrix elements.Comment: 17 pages REVTEX 3.0, 2 postscript figures; hardcopy versions before May 96 are obsolete; accepted for publication in The Philosophical Magazine

Stellar science from a blue wavelength range - A possible design for the blue arm of 4MOST

From stellar spectra, a variety of physical properties of stars can be derived. In particular, the chemical composition of stellar atmospheres can be inferred from absorption line analyses. These provide key information on large scales, such as the formation of our Galaxy, down to the small-scale nucleosynthesis processes that take place in stars and supernovae. By extending the observed wavelength range toward bluer wavelengths, we optimize such studies to also include critical absorption lines in metal-poor stars, and allow for studies of heavy elements (Z>38) whose formation processes remain poorly constrained. In this context, spectrographs optimized for observing blue wavelength ranges are essential, since many absorption lines at redder wavelengths are too weak to be detected in metal-poor stars. This means that some elements cannot be studied in the visual-redder regions, and important scientific tracers and science cases are lost. The present era of large public surveys will target millions of stars. Here we describe the requirements driving the design of the forthcoming survey instrument 4MOST, a multi-object spectrograph commissioned for the ESO VISTA 4m-telescope. We focus here on high-density, wide-area survey of stars and the science that can be achieved with high-resolution stellar spectroscopy. Scientific and technical requirements that governed the design are described along with a thorough line blending analysis. For the high-resolution spectrograph, we find that a sampling of >2.5 (pixels per resolving element), spectral resolution of 18000 or higher, and a wavelength range covering 393-436 nm, is the most well-balanced solution for the instrument. A spectrograph with these characteristics will enable accurate abundance analysis (+/-0.1 dex) in the blue and allow us to confront the outlined scientific questions. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A

Photoluminescence and Terahertz Emission from Femtosecond Laser-Induced Plasma Channels

Luminescence as a mechanism for terahertz emission from femtosecond laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb scattering between electrons and ions is shown to lead to luminescence even for a spatially homogeneous plasma. The spectral features introduced by the rod geometry of laser-induced plasma channels in air are discussed on the basis of a generalized mode-function analysis.Comment: 4 pages with 2 figures