Cooperative quantum jumps for three dipole-interacting atoms

We investigate the effect of the dipole-dipole interaction on the quantum jump statistics of three atoms. This is done for three-level systems in a V configuration and in what may be called a D configuration. The transition rates between the four different intensity periods are calculated in closed form. Cooperative effects are shown to increase by a factor of 2 compared to two of either three-level systems. This results in transition rates that are, for distances of about one wavelength of the strong transition, up to 100% higher than for independent systems. In addition the double and triple jump rates are calculated from the transition rates. In this case cooperative effects of up to 170% for distances of about one wavelength and still up to 15% around 10 wavelengths are found. Nevertheless, for the parameters of an experiment with Hg+ ions the effects are negligible, in agreement with the experimental data. For three Ba+ ions this seems to indicate that the large cooperative effects observed experimentally cannot be explained by the dipole-dipole interaction.Comment: 9 pages, 9 figures. Revised version, to be published in PR

Distribution of Damped Lyman-alpha Absorbers in a Lambda Cold Dark Matter Universe

We present the results of a numerical study of a galactic wind model and its implications on the properties of damped Lyman-alpha absorbers (DLAs) using cosmological hydrodynamic simulations. We vary both the wind strength and the internal parameters of the the wind model in a series of cosmological SPH simulations that include radiative cooling and heating by a UV background, star formation, and feedback from supernovae and galactic winds. To test our simulations, we examine the DLA `rate-of-incidence' as a function of halo mass, galaxy apparent magnitude, and impact parameter. We find that the statistical distribution of DLAs does not depend on the exact values of internal numerical parameters that control the decoupling of hydrodynamic forces when the gas is ejected from starforming regions. The DLA rate-of-incidence in our simulations at z=3 is dominated by the faint galaxies with apparent magnitude R_AB < 25.5. However, interestingly in a `strong wind' run, the differential distribution of DLA sight-lines is peaked at Mhalo = 10^{12} Msun/h (R_AB~27), and the mean DLA halo mass is Mmean=10^{12.4} Msun/h (R_AB ~ 26). These mass-scales are much larger than those if we ignore winds, because galactic wind feedback suppresses the DLA cross section in low-mass halos and increases the relative contribution to the DLA incidence from more massive halos. The DLAs in our simulations are more compact than the present-day disk galaxies, and the impact parameter distribution is very narrow unless we limit the search for the host galaxy to only bright LBGs. The comoving number density of DLAs is higher than that of LBGs down to R_AB=30 mag if the physical radius of each DLA is smaller than 5 kpc/h_70. We discuss conflicts between current simulations and observations, and potential problems with simulations based on the CDM model.Comment: 37 pages, 11 figures. Accepted to ApJ. Additional numerical tests of the internal parameters of the galactic wind model are presente

TTX, cations and spider venom modify avian muscle tone in vitro

Agents that reduce skeletal muscle tone may have a number of useful clinical applications, e.g., for muscle spasticity and other muscle disorders. Recently, we reported that the venoms of two species of Australian theraphosid (Araneae, Theraphosidae) spiders (Coremiocnemis tropix and Selenotholus foelschei) reduced the baseline tension of chick biventer cervicis nerve-muscle preparation. The purpose of this study was to determine the underlying physiology mediating the change in muscle tension, which was addressed by conducting isometric tension experiments. We found that MgCl2 (20mM), CaCl2 (20mM), tetrodotoxin (1μM) or C. tropix venom (2μl/ml) produced a similar decrease in baseline tension, whereas d-tubocurarine (100μM), gadolinium (1mM), verapamil (10mM), an increase in osmotic pressure by the addition of glucose (40mM), or the presence/absence of electrical stimulation did not produce a significant change in baseline tension. We suggest that mechanosensitive or muscle TTX-sensitive sodium channels are activated during muscle stretch. This may have implications for the treatment of stretch induced muscle damage

Large-scale surface reconstruction energetics of Pt(100) and Au(100) by all-electron DFT

The low-index surfaces of Au and Pt all tend to reconstruct, a fact that is of key importance in many nanostructure, catalytic, and electrochemical applications. Remarkably, some significant questions regarding their structural energies remain even today, in particular for the large-scale quasihexagonal reconstructed (100) surfaces: Rather dissimilar reconstruction energies for Au and Pt in available experiments, and experiment and theory do not match for Pt. We here show by all-electron density-functional theory that only large enough "(5 x N)" approximant supercells capture the qualitative reconstruction energy trend between Au(100) and Pt(100), in contrast to what is often done in the theoretical literature. Their magnitudes are then in fact similar, and closer to the measured value for Pt(100); our calculations achieve excellent agreement with known geometric characteristics and provide direct evidence for the electronic reconstruction driving force.Comment: updated version - also includes EPAPS information as auxiliary file; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

On the temperature dependence of correlation functions in the space like direction in hot QCD

We study the temperature dependence of quark antiquark correlations in the space like direction. In particular, we predict the temperature dependence of space like Bethe-Salpeter amplitudes using recent Lattice gauge data for the space like string potential. We also investigate the effect of the space like string potential on the screening mass and discuss possible corrections which may arise when working with point sources.Comment: 15 pages 8 figures (not included, will be sent on request), (SUNY-NTG-94-3

Pion electromagnetic form factor at finite temperature

Temperature effects on the electromagnetic couplings of pions in hot hadronic matter are studied with an effective chiral Lagrangian. We show that the Ward-Takahashi identity is satisfied at non-zero temperature in the soft pion limit. The in-medium electromagnetic form factor of the pion is obtained in the time-like region and shown to be reduced in magnitude, especially near the vector-meson resonance region. Finally, we discuss the consequences of this medium effect on dilepton production from hot hadronic matter.Comment: 29 pages (LaTex) + 11 figure

Cosmic Renaissance: The First Sources of Light

I review recent progress in understanding the formation of the first stars and quasars. The initial conditions for their emergence are given by the now firmly established model of cosmological structure formation. Numerical simulations of the collapse and fragmentation of primordial gas indicate that the first stars formed at redshifts z ~ 20 - 30, and that they were predominantly very massive, with M_* > 100 M_sun. Important uncertainties, however, remain. Paramount among them is the accretion process, which builds up the final stellar mass by incorporating part of the diffuse, dust-free envelope into the central protostellar core. The first quasars, on the other hand, are predicted to have formed later on, at z ~ 10, in more massive dark matter halos, with total masses, ~ 10^8 M_sun, characteristic of dwarf galaxies.Comment: 16 pages, 7 figures, invited review, to appear in PASP, Feb. 200

The cosmological evolution of metal enrichment in quasar host galaxies

We study the gas metallicity of quasar hosts using cosmological hydrodynamic simulations of the Lambda-cold dark matter model. Galaxy formation in the simulations is coupled with a prescription for black hole activity enabling us to study the evolution of the metal enrichment in quasar hosts and hence explore the relationship between star/spheroid formation and black hole growth/activity. We find a steep radial metallicity gradient in quasar host galaxies, with gas metallicities close to solar values in the outer parts but becoming supersolar in the center. The hosts of the rare bright quasars at z~5-6 have star formation rates of several hundred solar masses per year and halo masses of order ~10^12 solar masses. Already at these redshifts they have supersolar (Z ~2-3 solar) central metallicities, with a mild dependence of metallicity on luminosity, consistent with observed trends. The mean value of metallicity is sensitive to the assumed quasar lifetime, providing a useful new probe of this parameter. We find that lifetimes from 10^7-4x10^7yr are favored by comparison to observational data. In both the models and observations, the rate of evolution of the mean quasar metallicity as a function of redshift is generally flat out to z ~ 4-5. Beyond the observed redshift range and out to redshift z ~ 6-8, we predict a slow decline of the mean central metallicity towards solar and slightly subsolar values as we approach the epoch of the first significant star formation activityComment: submitted to ApJ, 13 pages, 9 figures, emulateapj.sty, corrected typos and added referenc