All-optical generation and detection of sub-picosecond ac spin current pulses in GaAs

Sub-picosecond ac spin current pulses are generated optically in GaAs bulk and quantum wells at room temperature and 90K through quantum interference between one-photon and two-photon absorptions driven by two phase-locked ultrafast laser pulses that are both circularly polarized. The dynamics of the current pulses are detected optically by monitoring in real time and real space nanoscale motion of electrons with high-resolution pump-probe techniques.Comment: 5 pages, 5 figure

Search for cold and hot gas in the ram pressure stripped Virgo dwarf galaxy IC3418

We present IRAM 30m sensitive upper limits on CO emission in the ram pressure stripped dwarf Virgo galaxy IC3418 and in a few positions covering HII regions in its prominent 17 kpc UV/Ha gas-stripped tail. In the central few arcseconds of the galaxy, we report a possible marginal detection of about 1x10^6 M_sun of molecular gas (assuming a Galactic CO-to-H_2 conversion factor) that could correspond to a surviving nuclear gas reservoir. We estimate that there is less molecular gas in the main body of IC3418, by at least a factor of 20, than would be expected from the pre-quenching UV-based star formation rate assuming the typical gas depletion timescale of 2 Gyr. Given the lack of star formation in the main body, we think the H_2-deficiency is real, although some of it may also arise from a higher CO-to-H_2 factor typical in low-metallicity, low-mass galaxies. The presence of HII regions in the tail of IC3418 suggests that there must be some dense gas; however, only upper limits of < 1x10^6 M_sun were found in the three observed points in the outer tail. This yields an upper limit on the molecular gas content of the whole tail < 1x10^7 M_sun, which is an amount similar to the estimates from the observed star formation rate over the tail. We also present strong upper limits on the X-ray emission of the stripped gas in IC3418 from a new Chandra observation. The measured X-ray luminosity of the IC3418 tail is about 280 times lower than that of ESO 137-001, a spiral galaxy in a more distant cluster with a prominent ram pressure stripped tail. Non-detection of any diffuse X-ray emission in the IC3418 tail may be due to a low gas content in the tail associated with its advanced evolutionary state and/or due to a rather low thermal pressure of the surrounding intra-cluster medium.Comment: 15 pages, 11 figures, A&A accepte

Sex differences in deleterious mutational effects in Drosophila melanogaster: combining quantitative and population genetic insights

Fitness effects of deleterious mutations can differ between females and males due to: (i) sex differences in the strength of purifying selection; and (ii) sex differences in ploidy. Although sex differences in fitness effects have important broader implications (e.g., for the evolution of sex and lifespan), few studies have quantified their scope. Those that have belong to one of two distinct empirical traditions: (i) quantitative genetics, which focusses on multi-locus genetic variances in each sex, but is largely agnostic about their genetic basis; and (ii) molecular population genetics, which focusses on comparing autosomal and X-linked polymorphism, but is poorly suited for inferring contemporary sex differences. Here, we combine both traditions to present a comprehensive analysis of female and male adult reproductive fitness among 202 outbred, laboratory-adapted, hemiclonal genomes of Drosophila melanogaster. While we find no clear evidence for sex differences in the strength of purifying selection, sex differences in ploidy generate multiple signals of enhanced purifying selection for X-linked loci. These signals are present in quantitative genetic metrics—i.e., a disproportionate contribution of the X to male (but not female) fitness variation—and population genetic metrics—i.e., steeper regressions of an allele’s average fitness effect on its frequency, and proportionally less nonsynonymous polymorphism on the X than autosomes. Fitting our data to models for both sets of metrics, we infer that deleterious alleles are partially recessive. Given the often-large gap between quantitative and population genetic estimates of evolutionary parameters, our study showcases the benefits of combining genomic and fitness data when estimating such parameters

Power dependence of pure spin current injection by quantum interference

We investigate the power dependence of pure spin current injection in GaAs bulk and quantum-well samples by a quantum interference and control technique. Spin separation is measured as a function of the relative strength of the two transition pathways driven by two laser pulses. By keeping the relaxation time of the current unchanged, we are able to relate the spin separation to the injected average velocity. We find that the average velocity is determined by the relative strength of the two transitions in the same way as in classical interference. Based on this, we conclude that the density of injected pure spin current increases monotonically with the excitation laser intensities. The experimental results are consistent with theoretical calculations based on Fermi's golden rule.Comment: 6 pages, 4 figure

Evidence of anomalous dispersion of the generalized sound velocity in glasses

The dynamic structure factor, S(Q,w), of vitreous silica, has been measured by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4-16.5 nm-1 and up to energies hw=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows to accurately determine the longitudinal current spectra, and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region, and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive dispersion in the generalized sound velocity that, around Q=5 nm-1, varies from 6500 to 9000 m/s: this phenomenon was never experimentally observed in a glass.Comment: 5 pages, 3 figures. To appear in Phys. Rev.

Effect of solids on flow regime transition in three-phase bubble columns

In bubble column reactors there are two principal flow regimes [1,2,3], the homogeneous (HoR) and the heterogeneous (HeR). These reactors have different behaviour in HoR and HeR, thus the dependences of the rates of mass, heat and momentum transfer on the design and operating parameters (such as reactor geometry, gas and liquid flow rates and properties of the contacting phases) are also very different. Therefore, for rational reactor design and operation it is of crucial importance to know the range of parameters over which a certain regime prevails and the regime transition conditions [4]. The effect of the presence of solids on gas holdup in bubble columns has been extensively investigated. Most of the published work reports that the gas holdup decreases with increasing solid concentration [5-14]. However, a favourable effect of solids on gas holdup has been also observed by Douek et al. [15]. In spite of all these efforts, information on the effect of solids on homogeneous-heterogeneous regime transition is very scarce. Krishna et al. [16] found that the transition gas holdup was significantly reduced due to the presence of silica particles. Xie et al. [17] reported that as pulp consistency is increased, the regime transition is delayed. The goal of this work was to examine the influence of solid particles on homogenous regime stability and regime transition in a three phase bubble column. For that, two studies were done, one focused on the regime transition and the other on visualization of bubbleparticle interaction. The regime transition experiments were performed in a cylindrical bubble column of 0.14 m diameter. Air, distilled water and calcium alginate beads (deq=2.1mm) at concentrations up to 30%(v/v) were the phases. The dependence of the gas holdup (e) on the gas flow rate (q) was measured. At low solid loading, the experimental data e(q) show a slight increase of gas holdup with solid concentration, which is not normally observed. At higher solid loading, a significant reduction of gas holdup with solid concentration is observed. This suggests that solid concentration could play a dual role in the homogeneous regime stability: low concentrations stabilize, while high concentrations destabilize. The critical point, where the HoR loses stability and the transition begins, was evaluated using the drift flux plot. The critical values of gas flow rate and voidage were the measures of the homogeneous regime stability. For low solid volume fraction, a stabilizing effect of the presence of solids is shown by an increase in the critical values, while for higher solids volume fraction ( φ s>3%) the critical values decrease, witnessing a destabilizing effect of the solids. The visualization study was performed in a 0.07 m diameter cylindrical bubble column, using a standard and high-speed cameras. This work shows important phenomena that result from the bubble-particle interactions, which partly explains the regime transition results. At low solid loading, bubble-particle collisions lead to reduction of bubble rise velocity that results in higher gas holdup and a delay of the transition. At higher solid loading, the bubble coalescence rate increases with the solid content that decreases the gas holdup and advances the transition.Commission of the European Communities for the Marie Curie Training Site - contract No. HPMT-CT-2000-0074

Effect of surfactants on homogeneous regime stability in bubble columns

Grant Agency of the Czech Republic (GA CR) - Grant No. 104/01/0547.European Commission - Maria Curie Training Sites Project (MCTS)- Contract No. HPMT-CT-2000-00074