Copyright (c) 2013 Syracuse University All rights reserved. http://surface.syr.edu/phy Recent documents in Physics en-us Mon, 28 Jan 2013 01:04:31 PST 3600 http://surface.syr.edu/phy/515 http://surface.syr.edu/phy/515 Tue, 15 Jan 2013 05:19:54 PST We report photocarrier time-of-flight measurements of electron drift mobilities for the p-type CuIn_1-xGa_xSe₂ films incorporated in solar cells. The electron mobilities range from 0.02 to 0.05 cm^2/Vs and are weakly temperature-dependent from 100–300 K. These values are lower than the range of electron Hall mobilities (2-1100 cm²/Vs) reported for n-type polycrystalline thin films and single crystals. We propose that the electron drift mobilities are properties of disorder-induced mobility edges and discuss how this disorder could increase cell efficiencies.

]]> Steluta A. Dinca et al. http://surface.syr.edu/phy/514 http://surface.syr.edu/phy/514 Tue, 28 Feb 2012 06:31:09 PST We calculate the maximum optical absorptance enhancements in thin semiconductor films on metals due to structures that diffuse light and couple it to surface plasmon polaritons. The calculations can be used to estimate plasmonic effects on light-trapping in solar cells. The calculations are based on the statistical distribution of energy in the electromagnetic modes of the structure, which include surface plasmon polariton modes at the metal interface as well as the trapped waveguide modes in the film. The enhancement has the form 4n²+nλ/h (n – film refractive index, λ – optical wavelength, h – film thickness), which is an increase beyond the non-plasmonic “classical” enhancement 4n². Larger resonant enhancements occur for wavelengths near the surface plasmon frequency; these add up to 2 mA/cm² to the photocurrent of a solar cell based on a 500 nm film of crystalline silicon. We also calculated the effects of plasmon dissipation in the metal. Dissipation rates typical of silver reverse the resonant enhancement effect for silicon, but a non-resonant enhancement remains

]]> Eric A. Schiff http://surface.syr.edu/phy/513 http://surface.syr.edu/phy/513 Tue, 13 Dec 2011 06:49:37 PST We report on the first results of experiments to measure the recombination rate of hydrogen on surfaces of astrophysical interest. Our measurements give lower values for the recombination efficiency (sticking probability S x probability of recombination upon H-H encounter \gamma) than model-based estimates. We propose that our results can be reconciled with average estimates of the recombination rate (1/2 n(H) n(g) v(H)A S \gamma) from astronomical observations, if the actual surface of an average grain is rougher, and its area bigger, than the one considered in models.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/512 http://surface.syr.edu/phy/512 Tue, 13 Dec 2011 06:49:34 PST We report on laboratory measurements of molecular hydrogen formation and recombination on an olivine slab as a function of surface temperature under conditions relevant to those encountered in the interstellar medium. On the basis of our experimental evidence, we recognize that there are two main regimes of H coverage that are of astrophysical importance; for each of them we provide an expression giving the production rate of molecular hydrogen in interstellar clouds.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/511 http://surface.syr.edu/phy/511 Tue, 13 Dec 2011 06:49:31 PST An analysis of the kinetics of H2 formation on interstellar dust grains is presented using rate equations. It is shown that semi-empirical expressions that appeared in the literature represent two different physical regimes. In particular, it is shown that the expression given by Hollenbach, Werner and Salpeter [ApJ, 163, 165 (1971)] applies when high flux, or high mobility, of H atoms on the surface of a grain, makes it very unlikely that H atoms evaporate before they meet each other and recombine. The expression of Pirronello et al.\ [ApJ, 483, L131 (1997)] -- deduced on the basis of accurate measurements on realistic dust analogue -- applies to the opposite regime (low coverage and low mobility). The implications of this analysis for the understanding of the processes dominating in the Interstellar Medium are discussed.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/510 http://surface.syr.edu/phy/510 Tue, 13 Dec 2011 06:49:28 PST Recent experimental results about the formation of molecular hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic hydrogen diffusion and desorption, the barrier for molecular hydrogen desorption, and the probability of spontaneous desorption of a hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first and second order kinetic processes within a single model. Using the values of the parameters that fit best the experimental results, the efficiency of hydrogen recombination on olivine and amorphous carbon surfaces is obtained for a range of hydrogen flux and surface temperature pertinent to a wide range of interstellar conditions.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/509 http://surface.syr.edu/phy/509 Tue, 13 Dec 2011 06:49:25 PST Recent experimental results on the formation of molecular hydrogen on astrophysically relevant surfaces under conditions similar to those encountered in the interstellar medium provided useful quantitative information about these processes. Rate equation analysis of experiments on olivine and amorphous carbon surfaces provided the activation energy barriers for the diffusion and desorption processes relevant to hydrogen recombination on these surfaces. However, the suitability of rate equations for the simulation of hydrogen recombination on interstellar grains, where there might be very few atoms on a grain at any given time, has been questioned. To resolve this problem, we introduce a master equation that takes into account both the discrete nature of the H atoms and the fluctuations in the number of atoms on a grain. The hydrogen recombination rate on microscopic grains, as a function of grain size and temperature, is then calculated using the master equation. The results are compared to those obtained from the rate equations and the conditions under which the master equation is required are identified.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/508 http://surface.syr.edu/phy/508 Tue, 13 Dec 2011 06:49:21 PST The results of experiments on the formation of molecular hydrogen on low density and high density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low density ice gives rise to a broader spectrum of energy barriers compared to the high density ice. Inserting these parameters into the rate equation model under steady state conditions we evaluate the production rate of molecular hydrogen on ice-coated interstellar dust grains.

]]> Hagai B. Perets et al. http://surface.syr.edu/phy/507 http://surface.syr.edu/phy/507 Tue, 13 Dec 2011 06:49:18 PST Molecular hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of molecular hydrogen in simulated astrophysical environments. Since the formation of molecular hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/506 http://surface.syr.edu/phy/506 Tue, 13 Dec 2011 06:49:15 PST Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H_{2} formation within a temperature range which is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/505 http://surface.syr.edu/phy/505 Tue, 13 Dec 2011 06:49:11 PST The study of the formation of molecular hydrogen on low temperature surfaces is of interest both because it allows to explore elementary steps in the heterogeneous catalysis of a simple molecule and because of the applications in astrochemistry. Here we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations and the activation energies of the processes leading to molecular hydrogen formation are obtained from the TPD data. We show that a model based on a single isotope provides the correct results for the activation energies for diffusion and desorption of H atoms. These results can thus be used to evaluate the formation rate of H_2 on dust grains under the actual conditions present in interstellar clouds.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/504 http://surface.syr.edu/phy/504 Tue, 13 Dec 2011 06:49:08 PST Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H_2 formation in diffuse interstellar clouds.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/503 http://surface.syr.edu/phy/503 Tue, 13 Dec 2011 06:49:05 PST We study the interaction of atomic and molecular hydrogen with a surface of tholin, a man-made polymer considered to be an analogue of aerosol particles present in Titan's atmosphere, using thermal programmed desorption at low temperatures below 30 K. The results are fitted and analyzed using a fine-grained rate equation model that describes the diffusion, reaction and desorption processes. We obtain the energy barriers for diffusion and desorption of atomic and molecular hydrogen. These barriers are found to be in the range of 30 to 60 meV, indicating that atom/molecule-surface interactions in this temperature range are dominated by weak adsorption forces. The implications of these results for the understanding of the atmospheric chemistry of Titan are discussed.

]]> Gianfranco Vidali et al. http://surface.syr.edu/phy/502 http://surface.syr.edu/phy/502 Tue, 13 Dec 2011 06:49:02 PST We establish the phase diagram of three--dimensional quantum gravity coupled to Ising matter. We find that in the negative curvature phase of the quantum gravity there is no disordered phase for ferromagnetic Ising matter because the coordination number of the sites diverges. In the positive curvature phase of the quantum gravity there is evidence for two spin phases with a first order transition between them.

]]> Simon Catterall et al. http://surface.syr.edu/phy/501 http://surface.syr.edu/phy/501 Tue, 13 Dec 2011 06:48:58 PST We present the results of a numerical simulation aimed at understanding the nature of the `c = 1 barrier' in two dimensional quantum gravity. We study multiple Ising models living on dynamical \phi^3 graphs and analyse the behaviour of moments of the graph loop distribution. We notice a universality at work as the average properties of typical graphs from the ensemble are determined only by the central charge. We further argue that the qualitative nature of these results can be understood from considering the effect of fluctuations about a mean field solution in the Ising sector.

]]> Simon Catterall et al. http://surface.syr.edu/phy/500 http://surface.syr.edu/phy/500 Tue, 13 Dec 2011 06:48:55 PST We employ a nonrelativistic version of QCD (NRQCD) to study heavy quark-antiquark bound states in the lowest approximation without fine structure. We use gluon configurations on a 16^3 by 48 lattice at beta=6.2 from the UKQCD collaboration. For quark masses in the vicinity of the b we obtain bound state masses for S, P and both types of D wave. We also detect signals for two types of hybrids (quark,antiquark,gluon states). The results are sufficiently accurate to confirm that the values of the D wave mass from both lattice D waves coincide indicating that the cubical invariance of the lattice is restored to full rotational invariance at large distance. Our results also show that the S-P splitting is indeed insensitive to variations in the bare quark mass from Ma=1.0 to Ma=1.9.

]]> Simon Catterall et al. http://surface.syr.edu/phy/499 http://surface.syr.edu/phy/499 Tue, 13 Dec 2011 06:48:52 PST We study the XY model on a lattice with fluctuating connectivity. The expectation is that at an appropriate critical point such a system corresponds to a compactified boson coupled to 2d quantum gravity. Our simulations focus, in particular, on the important topological features of the system. The results lend strong support to the two phase structure predicted on the basis of analytical calculations. A careful finite size scaling analysis yields estimates for the critical exponents in the low temperature phase.

]]> Simon Catterall et al. http://surface.syr.edu/phy/498 http://surface.syr.edu/phy/498 Tue, 13 Dec 2011 06:48:49 PST We show how to simulate U(1) gauge fields coupled to three-dimensional quantum gravity and then examine the phase diagram of this system. Quenched mean field theory suggests that a transition separates confined and deconfined phases (for the gauge matter) in both the negative curvature phase and the positive curvature phase of the quantum gravity, but numerical simulations find no evidence for such transitions.

]]> Simon Catterall et al. http://surface.syr.edu/phy/497 http://surface.syr.edu/phy/497 Tue, 13 Dec 2011 06:48:45 PST Following the Non-Relativistic QCD approach we use a gauge invariant smearing method with factorization to measure the excitation energies for a heavy Q\bar{Q} system on a 24^3\times 48 lattice at \beta=6.2. The results come from averaging over an ensemble of 60 QCD configurations. In order to enhance the signal from each configuration we use wall sources for quark propagators. The quark Hamiltonian contains only the simplest non-relativistic kinetic energy term. The results are listed for a range of bare quark masses. The mass splittings are insensitive to this variable though there are a slight trends with increasing quark mass. For an appropriate choice of UV cut-off (a^{-1}=3.2Gev) the mass spectrum compares reasonably well with the experimental values for the spin-averaged energy gaps of the \Upsilon system. We also present results for the DE and DT waves for the lowest bare quark mass. The results are consistent with degeneracy between the two types of D wave. This encourages the idea that even with our simple quark Hamiltonian the departure from rotational invariance is not great.

]]> Simon Catterall et al. http://surface.syr.edu/phy/496 http://surface.syr.edu/phy/496 Tue, 13 Dec 2011 06:48:42 PST We present the results of a high statistics Monte Carlo study of a model for four dimensional euclidean quantum gravity based on summing over triangulations. We show evidence for two phases; in one there is a logarithmic scaling on the mean linear extent with volume, whilst the other exhibits power law behaviour with exponent 1/2. We are able to extract a finite size scaling exponent governing the growth of the susceptibility peak

]]> Simon Catterall et al.