Superfluidity versus localization in bulk 4He at zero temperature

We present a zero-temperature quantum Monte Carlo calculation of liquid $^4$He immersed in an array of confining potentials. These external potentials are centered in the lattice sites of a fcc solid geometry and, by modifying their well depth and range, the system evolves from a liquid phase towards a progressively localized system which mimics a solid phase. The superfluid density decreases with increasing order, reaching a value $\rho_{\rm s}/\rho = 0.079(16)$ when the Lindemann's ratio of the model equals the experimental value for solid $^4$He.Comment: 5 pages,5 figure

The kinetics of homogeneous melting beyond the limit of superheating

Molecular dynamics simulation is used to study the time-scales involved in the homogeneous melting of a superheated crystal. The interaction model used is an embedded-atom model for Fe developed in previous work, and the melting process is simulated in the microcanonical $(N, V, E)$ ensemble. We study periodically repeated systems containing from 96 to 7776 atoms, and the initial system is always the perfect crystal without free surfaces or other defects. For each chosen total energy $E$ and number of atoms $N$, we perform several hundred statistically independent simulations, with each simulation lasting for between 500 ps and 10 ns, in order to gather statistics for the waiting time $\tau_{\rm w}$ before melting occurs. We find that the probability distribution of $\tau_{\rm w}$ is roughly exponential, and that the mean value $<\tau_{\rm w} >$ depends strongly on the excess of the initial steady temperature of the crystal above the superheating limit identified by other researchers. The mean $$ also depends strongly on system size in a way that we have quantified. For very small systems of $\sim 100$ atoms, we observe a persistent alternation between the solid and liquid states, and we explain why this happens. Our results allow us to draw conclusions about the reliability of the recently proposed Z method for determining the melting properties of simulated materials, and to suggest ways of correcting for the errors of the method.Comment: 19 pages, 8 figure

Supersolidity in quantum films adsorbed on graphene and graphite

Using quantum Monte Carlo we have studied the superfluid density of the first layer of $^4$He and H$_2$ adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered $\sqrt3 \times \sqrt3$ phase. The perfect solid phase of H$_2$ shows no superfluid signal whereas $^4$He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in $^4$He without destroying the spatial solid order.Comment: 5 pages, accepted for publication in PR

Reading-related Cognitive Deficits in Spanish Developmental Dyslexia

AbstractSpanish-speaking children learn to read words written in a relatively transparent orthography. Variations in orthographic transparency may shape the manifestation of reading difficulties. This study was intended to help clarify the nature of developmental dyslexia in Spanish. Developmentally Dyslexic children (DD) were compared to a chronological age-matched control group (CA). Measures included rapid automated naming, verbal working memory, phonological short-term memory, and phonemic awareness. Results demonstrated that developmental dyslexics show reading-related cognitive deficits in areas such as naming speed, verbal working memory, phonological short-term memory, and phonemic awareness. Our results are consistent with studies conducted in the Spanish language and in other transparent orthographies

Melting curve and Hugoniot of molybdenum up to 400 GPa by ab initio simulations

We report ab initio calculations of the melting curve and Hugoniot of molybdenum for the pressure range 0-400 GPa, using density functional theory (DFT) in the projector augmented wave (PAW) implementation. We use the ``reference coexistence'' technique to overcome uncertainties inherent in earlier DFT calculations of the melting curve of Mo. Our calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high pressure melting curve from static compression experiments. Our calculated P(V) and T(P) Hugoniot relations agree well with shock measurements. We use calculations of phonon dispersion relations as a function of pressure to eliminate some possible interpretations of the solid-solid phase transition observed in shock experiments on Mo.Comment: 8 pages, 6 figure

Solidification of small para-H2 clusters at zero temperature

We have determined the ground-state energies of para-H$_2$ clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N=55. From there on, all clusters are solid. The ground-state energies in the range N=13--75 are established and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, feature that can help to solve ambiguities in the determination of the specific phase of H$_2$ clusters.Comment: 17 pages, accepted for publication in J. Phys. Chem.

Evaluation of biological control agents for managing cucurbit powdery mildew on greenhouse-grown melon

An evaluation was made of the ability of two mycoparasite-based products AQ10® (Ampelomyces quisqualis) and Mycotal® (Lecanicillium lecanii), as well as three strains of Bacillus subtilis, to manage powdery mildew disease, caused by Podosphaera fusca on melon seedlings maintained under different regimes of relative humidity and on plants grown under greenhouse conditions in Spain. In every case fungal and bacterial biocontrol agents (BCAs) performed better under conditions of high relative humidity (90–95% RH). In greenhouse experiments, the effectiveness of the mycoparasites to manage powdery mildew was absolutely dependent on mineral oil. The strains of B. subtilis provided disease control similar to that achieved with the mycoparasites or the fungicide azoxystrobin. Microscopic analysis showed the ability of these bacterial strains to efficiently colonize leaf surfaces and revealed the occurrence of antagonistic interactions between biological agents and P. fusca structures. These results confirmed the usefulness of these BCAs for managing powdery mildew on greenhouse-grown cucurbits either as single products or as a component of integrated control programmes.Estación Experimental ‘La Mayora’ (CSIC), Algarrobo-Costa, 29750 Málaga, Spain Grupo de Microbiología y Patología Vegetal-Unidad Asociada a CSIC, Departamento de Microbiología, Facultad de Ciencias, Universidad de Málaga, 29071 MálagaPeer reviewe

Proba-3: ESA’s Small Satellites Precise Formation Flying Mission to Study the Sun’s Inner Corona as Never Before

This paper showcases ESA’s Proba-3 mission as a demonstration of how small satellites, in combination with formation flying technology, can achieve relevant scientific goals and perform scientific measurements not possible otherwise, all within a tight cost and programmatic context. The study of the Sun inner corona down to 1.1 solar radius can only be performed by creating in space artificial eclipses with a large distance between a Coronograph instrument and an occulting disk, much bigger than the size of any spacecraft that can fit within a launcher. Proba-3 will achieve these enhanced scientific observations by controlling two small satellites (~1.5 m cubes in the 200-300kg range) as a 150 m long ‘large virtually rigid structure’ by maintaining millimetre and arc second relative precision. In effect the paired satellites will fly as a giant virtual satellite creating an ‘externally occulted’ coronagraph, in which a satellite imager is shielded from glaring sunlight by an occulting disk on the other satellite, forming an artificial eclipse. Precise station keeping for Coronagraphy will be kept for 6 consecutive hours within each 20 hour orbit for a minimum total of 1000 hours of scientific observations over the 2 years of mission lifetime. This will be achieved autonomously, without relying on the ground for active control of the formation. In addition, Proba-3 will practically demonstrate formation flying technologies enabling other future science missions: station-keeping at different relative distances (from 25 m up to 250 m); approaching and separating in precise formation without losing millimetre precision; the capability to repoint the formation as a virtual rigid body away from the Sun and the combination of station keeping, resizing and re-targeting manoeuvres. Proba-3 is at full speed in the assembly, integration and verification phase, with the aim of launching Proba-3 in two years’ time. The paper describes the overall Proba-3 mission concept and detailed design, the different challenges that were overcome in spacecraft design, formation flying metrology and control, and the need to implement novel verification and operation approaches to achieve the world’s first precise formation flying mission

Testing Reactive Probabilistic Processes

We define a testing equivalence in the spirit of De Nicola and Hennessy for reactive probabilistic processes, i.e. for processes where the internal nondeterminism is due to random behaviour. We characterize the testing equivalence in terms of ready-traces. From the characterization it follows that the equivalence is insensitive to the exact moment in time in which an internal probabilistic choice occurs, which is inherent from the original testing equivalence of De Nicola and Hennessy. We also show decidability of the testing equivalence for finite systems for which the complete model may not be known