First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen

Using density-functional theory in combination with a thermodynamic formalism we calculate the relative stability of various structural models of the polar O-terminated (000-1)-O surface of ZnO. Model surfaces with different concentrations of oxygen vacancies and hydrogen adatoms are considered. Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2 gas phase we determine a phase diagram of the lowest-energy surface structures. For a wide range of temperatures and pressures we find that hydrogen will be adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At high temperatures and low pressures the hydrogen can be removed and a structure with 1/4 of the surface oxygen atoms missing becomes the most stable one. The clean, defect-free surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, since we find that the dissociative adsorption of molecular hydrogen and water (if also the Zn-terminated surface is present) is energetically very preferable, it is very unlikely that a clean, defect-free (000-1)-O surface can be observed in experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro

Unstable Attractors: Existence and Robustness in Networks of Oscillators With Delayed Pulse Coupling

We consider unstable attractors; Milnor attractors $A$ such that, for some neighbourhood $U$ of $A$, almost all initial conditions leave $U$. Previous research strongly suggests that unstable attractors exist and even occur robustly (i.e. for open sets of parameter values) in a system modelling biological phenomena, namely in globally coupled oscillators with delayed pulse interactions. In the first part of this paper we give a rigorous definition of unstable attractors for general dynamical systems. We classify unstable attractors into two types, depending on whether or not there is a neighbourhood of the attractor that intersects the basin in a set of positive measure. We give examples of both types of unstable attractor; these examples have non-invertible dynamics that collapse certain open sets onto stable manifolds of saddle orbits. In the second part we give the first rigorous demonstration of existence and robust occurrence of unstable attractors in a network of oscillators with delayed pulse coupling. Although such systems are technically hybrid systems of delay differential equations with discontinuous `firing' events, we show that their dynamics reduces to a finite dimensional hybrid system system after a finite time and hence we can discuss Milnor attractors for this reduced finite dimensional system. We prove that for an open set of phase resetting functions there are saddle periodic orbits that are unstable attractors.Comment: 29 pages, 8 figures,submitted to Nonlinearit

The State Be7 in the Core of the Sun and the Solar Neutrino Flux

The exact ionization state of Be7 in the solar core is crucial for the precise prediction of the solar B8 neutrino flux. We therefore examine the effect of pressure ionization on the ionization state of Be7 and all elements with 12 >= Z >= 4. We show that under the conditions prevailing in the solar core, one has to consider the effect of the nearest neighbor on the electronic structure of a given ion. To this goal, we first solve the Schroedinger and then the Kohn-Sham equations for an ion immersed in a dense plasma under conditions for which the mean interparticle distance is smaller than the Debye radius. The question of which boundary conditions should be imposed on the wave function is discussed, examined and found to be crucial. Contrary to previous estimates showing that Beryllium is partially ionized, we find that it is fully ionized. As a consequence, the predicted rate of the Be7 + e- reaction is reduced by 20-30%, depending on the exact solar model. Since Be7 is a trace element, its total production is controlled by the unchanged He4+He3 reaction rate, and its destruction is determined by the rate of electron capture. As the latter rate decreases when the Beryllium is fully ionized (relative to the case of partially ionized Be), the estimate for the abundance of Be7 increases and with it the B8 neutrino flux. The increase in phi_nu(B8) is by about 20-30%. The neutrino flux due to Be7 electron capture remains effectively unchanged because the change in the rate is compensated for by a change in the abundance. Hence the prediction for the ratio of phi_nu(B8) / phi_nu(Be7) changes as well.Comment: 10 pages, 10 figures. Submitted to MNRA

Parental strategies used in the family meal session of Family-Based Treatment adolescent anorexia nervosa : links with treatment outcomes

Examine relationships between parental mealtime strategies used in the family meal session of Family-Based Treatment (FBT) and adolescent outcomes at EOT (session 20). Method: Eighteen families with an adolescent receiving FBT-AN participated. Parental strategies during videoed family meals were assessed using a family mealtime coding system. Change scores were calculated for both adolescent %EBW and EDE scores. Results: Increased use of parental direct and non-direct eating prompts during the family meal was associated with greater adolescent weight gain at EOT. Use of parental mealtime strategies was not associated with any significant change in adolescent eating psychopathology at EOT. Discussion: Parental verbal eating prompts during the family meal may be effective in promoting short-term weight gain. During the family meal session, parents should be encouraged to maintain a direct focus on their adolescent child’s eating behaviour which may assist their child with food consumption and potential weight gain. Further research examining food-based interactions among parents and their adolescent child with AN is needed

Ab initio study of ferroelectric domain walls in PbTiO3

We have investigated the atomistic structure of the 180-degree and 90-degree domain boundaries in the ferroelectric perovskite compound PbTiO3 using a first-principles ultrasoft-pseudopotential approach. For each case we have computed the position, thickness and creation energy of the domain walls, and an estimate of the barrier height for their motion has been obtained. We find both kinds of domain walls to be very narrow with a similar width of the order of one to two lattice constants. The energy of the 90-dergree domain wall is calculated to be 35 mJ/m^2, about a factor of four lower than the energy of its 180-degree counterpart, and only a miniscule barrier for its motion is found. As a surprising feature we detected a small offset of 0.15-0.2 eV in the electrostatic potential across the 90-degree domain wall.Comment: 12 pages, with 9 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/bm_dw/index.htm

Effects of univariate and multivariate bias correction on hydrological impact projections in alpine catchments

Alpine catchments show a high sensitivity to climate variation as they include the elevation range of the snow line. Therefore, the correct representation of climate variables and their interdependence is crucial when describing or predicting hydrological processes. When using climate model simulations in hydrological impact studies, forcing meteorological data are usually downscaled and bias corrected, most often by univariate approaches such as quantile mapping of individual variables, neglecting the relationships that exist between climate variables. In this study we test the hypothesis that the explicit consideration of the relation between air temperature and precipitation will affect hydrological impact modelling in a snow-dominated mountain environment. Glacio-hydrological simulations were performed for two partly glacierized alpine catchments using a recently developed multivariate bias correction method to post-process EURO-CORDEX regional climate model outputs between 1976 and 2099. These simulations were compared to those obtained by using the common univariate quantile mapping for bias correction. As both methods correct each climate variable's distribution in the same way, the marginal distributions of the individual variables show no differences. Yet, regarding the interdependence of precipitation and air temperature, clear differences are notable in the studied catchments. Simultaneous correction based on the multivariate approach led to more precipitation below air temperatures of 0&thinsp;∘C and therefore more simulated snowfall than with the data of the univariate approach. This difference translated to considerable consequences for the hydrological responses of the catchments. The multivariate bias-correction-forced simulations showed distinctly different results for projected snow cover characteristics, snowmelt-driven streamflow components, and expected glacier disappearance dates. In all aspects – the fraction of precipitation above and below 0&thinsp;∘C, the simulated snow water equivalents, glacier volumes, and the streamflow regime – simulations resulting from the multivariate-corrected data corresponded better with reference data than the results of univariate bias correction. Differences in simulated total streamflow due to the different bias correction approaches may be considered negligible given the generally large spread of the projections, but systematic differences in the seasonally delayed streamflow components from snowmelt in particular will matter from a planning perspective. While this study does not allow conclusive evidence that multivariate bias correction approaches are generally preferable, it clearly demonstrates that incorporating or ignoring inter-variable relationships between air temperature and precipitation data can impact the conclusions drawn in hydrological climate change impact studies in snow-dominated environments.</p

Extended Thomas-Fermi approximation to the one-body density matrix

The one-body density matrix is derived within the Extended Thomas-Fermi approximation. This has been done starting from the Wigner-Kirkwood distribution function for a non-local single-particle potential. The links between this new approach to the density matrix with former ones available in the literature are widely discussed. The semiclassical Hartree-Fock energy at Extended Thomas-Fermi level is also obtained in the case of a non-local one-body Hamiltonian. Numerical applications are performed using the Gogny and Brink-Boeker effective interactions. The semiclassical binding energies and root mean square radii are compared with the fully quantal ones and with those obtained using the Strutinsky averaged method.Comment: 27 pages, LateX, and 2 PostScript figures, (submitted to Nucl. Phys. A

Quantum lattice dynamical effects on the single-particle excitations in 1D Mott and Peierls insulators

As a generic model describing quasi-one-dimensional Mott and Peierls insulators, we investigate the Holstein-Hubbard model for half-filled bands using numerical techniques. Combining Lanczos diagonalization with Chebyshev moment expansion we calculate exactly the photoemission and inverse photoemission spectra and use these to establish the phase diagram of the model. While polaronic features emerge only at strong electron-phonon couplings, pronounced phonon signatures, such as multi-quanta band states, can be found in the Mott insulating regime as well. In order to corroborate the Mott to Peierls transition scenario, we determine the spin and charge excitation gaps by a finite-size scaling analysis based on density-matrix renormalization group calculations.Comment: 5 pages, 5 figure

Parental strategies used in the family meal session of family-based treatment for adolescent anorexia nervosa: Links with treatment outcomes

Objective: Examine relationships between parental mealtime strategies used in the family meal session of Family-Based Treatment (FBT) and adolescent outcomes at EOT (session 20). Method: Eighteen families with an adolescent receiving FBT-AN participated. Parental strategies during videoed family meals were assessed using a family mealtime coding system. Change scores were calculated for both adolescent ëW and EDE scores. Results: Increased use of parental direct and non-direct eating prompts during the family meal was associated with greater adolescent weight gain at EOT. Use of parental mealtime strategies was not associated with any significant change in adolescent eating psychopathology at EOT. Discussion: Parental verbal eating prompts during the family meal may be effective in promoting short-term weight gain. During the family meal session, parents should be encouraged to maintain a direct focus on their adolescent child’s eating behaviour which may assist their child with food consumption and potential weight gain. Further research examining food-based interactions among parents and their adolescent child with AN is needed

How do parents of adolescent patients with anorexia nervosa interact with their child at mealtimes? A study of parental strategies used in the family meal session of family-based treatment

Objective: To examine the range and frequency of parental mealtime strategies used during the family meal session of Family-Based Treatment (FBT) for adolescent anorexia nervosa, and to explore the relationships between parental mealtime strategies, mealtime emotional tone and parental 'success' at encouraging adolescent food consumption. Method: Participants were 21 families with a child aged between 12 and 18 years receiving FBT for adolescent anorexia nervosa. Video recordings of the family meal session (FBT session two) were coded using the Family Mealtime Coding System adapted in this study for use with adolescents (FMCS-A) to identify frequency of parental strategies, emotional tone of the meal (measured by adolescent positive and negative vocalisations) and frequency of prompted mouthfuls consumed by the adolescent (measured by the number of mouthfuls consumed by the adolescent immediately following parental interactions). Results: A range of parental mealtime strategies were in use. Those used repeatedly included direct eating prompts, non-direct eating prompts, physical prompts, and providing information or food-related choices. Several parental mealtime strategies (direct and non-direct eating prompts) were found to be consistently associated with the tone of adolescents' vocalisations and the number of mouthfuls consumed in response to a parental prompt. Discussion: Despite associations with negativity from the adolescent, the use of food-related prompts (both verbal and physical) seems to be associated with increased eating. This indicates the potentially important role of parental control of eating. Following replication, these findings might provide a focus for therapists when supporting and coaching parents during the family meal session. © 2014 Wiley Periodicals, Inc