2,543 research outputs found
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 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 eV. The DMC dissociation
barrier is calculated to be 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
Assessment of average muscle fiber conduction velocity from surface EMG signals during fatiguing dynamic contractions
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
Interesse legittimo ed esigenze di tutela contenzioso-amministrativa sostanziali nella Chiesa
Growth of Dome-Shaped Carbon Nanoislands on Ir(111): The Intermediate between Carbidic Clusters and Quasi-Free-Standing Graphene (vol 103, 166101, 2009)
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 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 with transition elements can significantly improve the thermodynamic
and kinetic properties for H desorption, as well as the H 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
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 Earthâs Liquid Core
The standard model of Earthâs 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 Earthâs 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
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