1,398 research outputs found
Ballistic conductance of magnetic Co and Ni nanowires with ultrasoft pseudo-potentials
The scattering-based approach for calculating the ballistic conductance of
open quantum systems is generalized to deal with magnetic transition metals as
described by ultrasoft pseudo-potentials. As an application we present
quantum-mechanical conductance calculations for monatomic Co and Ni nanowires
with a magnetization reversal. We find that in both Co and Ni nanowires, at the
Fermi energy, the conductance of electrons is blocked by a magnetization
reversal, while the states (one per spin) are perfectly transmitted.
electrons have a non-vanishing transmission in a small energy window below the
Fermi level. Here, transmission is larger in Ni than in Co.Comment: 9 pages, 6 figures, to appear in PR
Phonons Softening in Tip-Stretched Monatomic Nanowires
It has been shown in recent experiments that electronic transport through a
gold monatomic nanowire is dissipative above a threshold voltage due to
excitation of phonons via the electron-phonon interaction. We address that data
by computing, via density functional theory, the zone boundary longitudinal
phonon frequency of a perfect monatomic nanowire during its mechanical
elongation. The theoretical frequency that we find for an ideally strained
nanowire is not compatible with experiment if a uniformly distributed stretch
is assumed. With the help of a semi-empirical Au-Au potential, we model the
realistic nanowire stretching as exerted by two tips. In this model we see that
strain tends to concentrate in the junctions, so that the mean strain of the
nanowire is roughly one half of the ideal value. With this reduced strain, the
calculated phonon softening is in much better agreement with experiment.Comment: 9 pages,3 figures, Surface Science, in pres
First-Principles Wannier Functions of Silicon and Gallium Arsenide
We present a self-consistent, real-space calculation of the Wannier functions
of Si and GaAs within density functional theory. We minimize the total energy
functional with respect to orbitals which behave as Wannier functions under
crystal translations and, at the minimum, are orthogonal. The Wannier functions
are used to calculate the total energy, lattice constant, bulk modulus, and the
frequency of the zone-center TO phonon of the two semiconductors with the
accuracy required nowadays in ab-initio calculations. Furthermore, the centers
of the Wannier functions are used to compute the macroscopic polarization of Si
and GaAs in zero electric field. The effective charges of GaAs, obtained by
finite differentiation of the polarization, agree with the results of linear
response theory.Comment: 12 pages, 2 PostScript figures, RevTeX, to appear in Physical Review
Selective d-state Conduction Blocking in Nickel Nanocontacts
The lowest conductance step for a Ni nanocontact is anomalously small in
comparison with the large expected number of conducting channels. We present
electronic structure calculations for an extremely idealized Ni nanobridge
consisting of just a monatomic nanowire. Our calculations show that no less
than eight single spin bands cross the Fermi level in a nonmagnetic Ni
monatomic wire, dropping marginally to seven in the more stable, fully
ferromagnetic state. However, when we build in the wire a magnetization
reversal, or domain wall, by forcing the net magnetization to be zero, we
suddenly find that d electrons selectively cease to propagate across the wall.
s electron propagation remains, and can account for the small observed
conductance steps.Comment: 9 pages, 4 figures, Surface Science, to appea
Heavy Metal Pollutions: State of the Art and Innovation in Phytoremediation
Mineral nutrition of plants greatly depends on both environmental conditions, particularly of soils, and the genetic background of the plant itself. Being sessile, plants adopted a range of strategies for sensing and responding to nutrient availability to optimize development and growth, as well as to protect their metabolisms from heavy metal toxicity. Such mechanisms, together with the soil environment, meaning the soil microorganisms and their interaction with plant roots, have been extensively studied with the goal of exploiting them to reclaim polluted lands; this approach, defined phytoremediation, will be the subject of this review. The main aspects and innovations in this field are considered, in particular with respect to the selection of efficient plant genotypes, the application of improved cultural strategies, and the symbiotic interaction with soil microorganisms, to manage heavy metal polluted soils
The Puzzling Stability of Monatomic Gold Wires
We have examined theoretically the spontaneous thinning process of
tip-suspended nanowires, and subsequently studied the structure and stability
of the monatomic gold wires recently observed by Transmission Electron
Microscopy (TEM). The methods used include thermodynamics, classical many-body
force simulations, Local Density (LDA) and Generalized Gradient (GGA)
electronic structure calculations as well as ab-initio simulations including
the two tips. The wire thinning is well explained in terms of a thermodynamic
tip suction driving migration of surface atoms from the wire to the tips. For
the same reason the monatomic wire becomes progressively stretched.
Surprisingly, however, all calculations so far indicate that the stretched
monatomic gold wire should be unstable against breaking, contrary to the
apparent experimental stability. The possible reasons for the observed
stability are discussed.Comment: 4 figure
Hair dehydroepiandrosterone sulfate as biomarker of employees’ well-being? A longitudinal investigation of support, resilience, and work engagement during COVID-19 pandemic
Introduction: Building on the motivational process of the job demands-resources (JD-R) theory, in the current research we investigated the longitudinal association between supervisor support/resilience as job/personal resources, work engagement (WE) and hair dehydroepiandrosterone sulfate, or DHEA(S), as a possible biomarker of employees’ well-being. Methods: In the context of the COVID-19 pandemic, 122 workers completed two self-report questionnaires (i.e., psychological data): the former at Time 1 (T1) and the latter three months afterwards, at Time 2 (T2). Participants also collected a strand of hair (i.e., biological data) at T2. Results: Results from path analysis showed that both SS and resilience at T1 were positively related to WE at T2, which, in its turn, was positively related to hair DHEA(S) at T2. Both SS and resilience at T1 had a positive indirect effect on hair DHEA(S) at T2 through WE at T2, which fully mediated the association between job/personal resources and hair DHEA(S). Discussion: Overall, results are consistent with the motivational process of the JD-R. Furthermore, this study provides preliminary evidence for the role of hair DHEA(S) as a biomarker of WE, a type of work-related subjective well-being that plays a central role in the motivational process of the JD-R, leading to favorable personal and organizational outcomes. Finally, the article outlines practical implications for organizations and professionals to foster WE within the workplace
A First-Principles Approach to Insulators in Finite Electric Fields
We describe a method for computing the response of an insulator to a static,
homogeneous electric field. It consists of iteratively minimizing an electric
enthalpy functional expressed in terms of occupied Bloch-like states on a
uniform grid of k points. The functional has equivalent local minima below a
critical field E_c that depends inversely on the density of k points; the
disappearance of the minima at E_c signals the onset of Zener breakdown. We
illustrate the procedure by computing the piezoelectric and nonlinear
dielectric susceptibility tensors of III-V semiconductors.Comment: 4 pages, with 1 postscript figure embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/is_ef/index.htm
Evaluating the use of amber in palaeoatmospheric reconstructions: The carbon-isotope variability of modern and Cretaceous conifer resins.
Stable carbon-isotope geochemistry of fossilized tree resin (amber) potentially could be a very useful tool to infer the composition of past atmospheres. To test the reliability of amber as a proxy for the atmosphere, we studied the variability of modern resin d13C at both local and global scales. An amber d13C curve was then built for the Cretaceous, a period of abundant resin production, and interpreted in light of data from modern resins. Our data show that hardening changes the pristine d13C value by causing a 13C-depletion in solid resin when compared to fresh liquid-viscous resin, probably due to the loss of 13C-enriched volatiles. Modern resin d13C values vary as a function of physiological and environmental parameters in ways that are similar to those described for leaves and wood. Resin d13C varies between plant species and localities, within the same tree and between different plant tissues by up to 6Âż, and in general increases with increasing altitudes of the plant-growing site. We show that, as is the case with modern resin, Cretaceous amber d13C has a high variability, generally higher than that of other fossil material. Despite the high natural variability, amber shows a negative 2.5-3Âż d13C trend from the middle Early Cretaceous to the Maastrichtian that parallels published terrestrial d13C records. This trend mirrors changes in the atmospheric d13C calculated from the d13C and d18O of benthic foraminiferal tests, although the magnitude of the shift is larger in plant material than in the atmosphere. Increasing mean annual precipitation and pO2 could have enhanced plant carbon-isotope fractionation during the Late Cretaceous, whereas changing pCO2 levels seem to have had no effect on plant carbon-isotope fractionation. The results of this study suggest that amber is a powerful fossil plant material for palaeoenvironmental and palaeoclimatic reconstructions. Improvement of the resolution of the existing data coupled with more detailed information about botanical source and environmental growing conditions of the fossil plant material will probably allow a more faithful interpretation of amber d13C records and a wider understanding of the composition of the past atmosphere
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