Hydrogen dissociation on the Mg(0001) surface from quantum Monte Carlo calculations

We have used diffusion Monte Carlo (DMC) simulations to calculate the energy barrier for H$_2$ dissociation on the Mg(0001) surface. The calculations employ pseudopotentials and systematically improvable B-spline basis sets to expand the single particle orbitals used to construct the trial wavefunctions. Extensive tests on system size, time step, and other sources of errors, performed on periodically repeated systems of up to 550 atoms, show that all these errors together can be reduced to $\sim 0.03$ eV. The DMC dissociation barrier is calculated to be $1.18 \pm 0.03$ eV, and is compared to those obtained with density functional theory using various exchange-correlation functionals, with values ranging between 0.44 and 1.07 eV.Comment: 6 pages, 4 figures, to appear in Physical Review

The discovery of a low mass, pre-main-sequence stellar association around gamma Velorum

We report the serendipitous discovery of a population of low mass, pre-main sequence stars (PMS) in the direction of the Wolf-Rayet/O-star binary system gamma^{2} Vel and the Vela OB2 association. We argue that gamma^{2} Vel and the low mass stars are truly associated, are approximately coeval and that both are at distances between 360-490 pc, disagreeing at the 2 sigma level with the recent Hipparcos parallax of gamma^{2} Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the gamma^{2} Vel system, but also offer an exciting opportunity to investigate the influence of high mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.Comment: Monthly Notices of the Royal Astronomical Society, Letters - in pres

Quantum chaos in the mesoscopic device for the Josephson flux qubit

We show that the three-junction SQUID device designed for the Josephson flux qubit can be used to study quantum chaos when operated at high energies. In the parameter region where the system is classically chaotic we analyze the spectral statistics. The nearest neighbor distributions $P(s)$ are well fitted by the Berry Robnik theory employing as free parameters the pure classical measures of the chaotic and regular regions of phase space in the different energy regions. The phase space representation of the wave functions is obtained via the Husimi distributions and the localization of the states on classical structures is analyzed.Comment: Final version, to be published in Phys. Rev. B. References added, introduction and conclusions improve

Hydrogen Dissociation and Diffusion on Ni and Ti -doped Mg(0001) Surfaces

It is well known, both theoretically and experimentally, that alloying MgH$_2$ with transition elements can significantly improve the thermodynamic and kinetic properties for H$_2$ desorption, as well as the H$_2$ intake by Mg bulk. Here we present a density functional theory investigation of hydrogen dissociation and surface diffusion over Ni-doped surface, and compare the findings to previously investigated Ti-doped Mg(0001) and pure Mg(0001) surfaces. Our results show that the energy barrier for hydrogen dissociation on the pure Mg(0001) surface is high, while it is small/null when Ni/Ti are added to the surface as dopants. We find that the binding energy of the two H atoms near the dissociation site is high on Ti, effectively impeding diffusion away from the Ti site. By contrast, we find that on Ni the energy barrier for diffusion is much reduced. Therefore, although both Ti and Ni promote H$_2$ dissociation, only Ni appears to be a good catalyst for Mg hydrogenation, allowing diffusion away from the catalytic sites. Experimental results corroborate these theoretical findings, i.e. faster hydrogenation of the Ni doped Mg sample as opposed to the reference Mg or Ti doped Mg.Comment: 17 pages, 15 figures, to appear in Journal of Chemical Physic

Freeze-thaw durability of recycled concrete from construction and demolition wastes

Road engineering is one of the most accepted applications for concrete including recycled aggregates from construction and demolition wastes as a partial replacement of the natural coarse aggregates. Amongst the durability concerns of such application, the deterioration due to freeze-thaw cycles is one of the most important causes decreasing the life span of concrete in countries with a continental climate. Moreover, the use of de-icing salts, which is a common practice to prevent ice formation on roadways and walkways, increases the superficial degradation of concrete due to frost-salt scaling. Thus, this paper aims to assess the resistance to frost salt with de-icing salts of two recycled concrete mixtures containing a 50% replacement of the conventional gravel by recycled aggregates both of mixed and ceramic nature, i.e. containing ceramic percentages of 34% and 100%, in comparison to a conventional concrete made with siliceous gravel. Therefore, the surface scaling was evaluated based on EN 1339 (2004) on 28 days cured cylinders, exposed to 7, 14, 21 and 28 freeze-thaw cycles in the presence of sodium chloride solution. Given that no airentraining admixture was used in any of the mixtures, the scaling of both conventional and recycled concretes exceeded the 1 kg/m2 limit established by the European standard. Nonetheless, for the casting surface, the recycled concrete with low ceramic content exhibited a similar behaviour to the conventional concrete, whereas the performance of the recycled concrete with high ceramic content was better. However, as expected, trowelled surfaces showed a worse performance and both recycled concretes had a lower freeze-thaw durability than the conventional mixture. In any case, the results suggested that the composition of the recycled aggregates could be used as a factor to limit the differences in performance between recycled and conventional mixtures

PSYCHOSOCIAL RESOURCES AND RISK FACTORS BEFORE AND AFTER CHILDBIRTH: A LONGITUDINAL STUDY AMONG SECOND TIME MOTHERS

ABSTRACT Previous literature stated that the transition to parenthood is marked by many stressful changes. Studies focused on the transition to first time-motherhood as implying a radical overhaul of individuals' hierarchy of values, requiring an experiential adjustment to new environmental challenges, and a redistribution of the limited psychic resources among them in daily life. In spite of the radical changes that are connected to the birth of the second child, literature on second-time motherhood is scarce and under-investigated. Moving from these premises, the study aimed at analysing the psychosocial resources and the risk factors associated to second-time motherhood before and after childbirth. In particular, we focused on mothers' everyday life and quality of experience. The novelty in the present approach was to focus on the joint analysis of well-being, in both its hedonic and eudaimonic components, and ill-being, evaluated in terms of risk of perinatal depression. Participants were 22 women (aged 24-40) recruited at the Obstetric and Gynaecology Unit of Hospital Luigi Sacco of Milan, between November 2011 and January 2014. Data were collected through single-administration questionnaires assessing well-being and ill-being indicators, as well as real-time measures, assessing everyday life and quality of experience. In particular, data were collected trough Experience Sampling Method (ESM), providing on-line information on the experience fluctuation during daily life. For one week participants carried an electronic device sending acoustic randomised signals 6-8 times a day. At signal receipt, participants provided self-reports about ongoing activities and associated experience. Likert-type scales measured emotional, cognitive, and motivational psychological dimensions, including challenges perceived in the activity and related personal skills. Answers to open-ended questions were assigned numeric codes and grouped into categories according to functional criteria. The values of scaled variables were standardised before analysis. Moreover, the relationship between levels of perceived challenges and skills - on the one side - and the quality of experience - on the other side \u2013 was explored through the Experience Fluctuation Model (EFM). Findings obtained from single-administration instruments analysing well- and ill-being indicators showed that women overall reported low levels of depression and psychopathological symptoms, as well as good mental health both during and after pregnancy. Evidence obtained from real-time instruments highlighted that women's daily quality of experience was subject to fluctuations associated with contingent roles and tasks required by motherhood: Along with meaningful sources of optimal experience, the elective investment on children and nuclear family could also be intended as an important source of boredom, relaxation and apathy. Results showed that the complexity and specificity of second-time pregnancy and puerperium transition should encourage health professionals to adopt a more articulated prevention approach, supporting mothers\u2019 healthy psychological adjustment. To the best of our knowledge, this was the first study focusing on the joint analysis of well-being, in both its hedonic and the eudaimonic components, and ill-being, evaluated in terms of risk of perinatal depression

FeO Content of Earth’s Liquid Core

The standard model of Earth’s core evolution has the bulk composition set at formation, with slow cooling beneath a solid mantle providing power for geomagnetic field generation. However, controversy surrounding the incorporation of oxygen, a critical light element, and the rapid cooling rates needed to maintain the early dynamo have called this model into question. The predicted cooling rates imply early core temperatures that far exceed estimates of the lower mantle solidus, suggesting that early core evolution was governed by interaction with a molten lower mantle. Here we develop ab initio techniques to compute the chemical potentials of arbitrary solutes in solution and use them to calculate oxygen partitioning between liquid Fe-O metal and silicate melts at the pressure-temperature (P-T) conditions expected for the early core-mantle system. Our distribution coefficients are compatible with those obtained by extrapolating experimental data at lower P-T values and reveal that oxygen strongly partitions into metal at core conditions via an exothermic reaction. Our results suggest that the bulk of Earth’s core was undersaturated in oxygen compared to the FeO content of the magma ocean during the latter stages of its formation, implying the early creation of a stably stratified oxygen-enriched layer below the core-mantle boundary (CMB). FeO partitioning is accompanied by heat release due to the exothermic reaction. If the reaction occurred at the CMB, this heat sink could have significantly reduced the heat flow driving the core convection and magnetic field generation