461 research outputs found
First-principles investigation of spin polarized conductance in atomic carbon wire
We analyze spin-dependent energetics and conductance for one dimensional (1D)
atomic carbon wires consisting of terminal magnetic (Co) and interior
nonmagnetic (C) atoms sandwiched between gold electrodes, obtained employing
first-principles gradient corrected density functional theory and Landauer's
formalism for conductance. Wires containing an even number of interior carbon
atoms are found to be acetylenic with sigma-pi bonding patterns, while cumulene
structures are seen in wires containing odd number of interior carbon atoms, as
a result of strong pi-conjugation. Ground states of carbon wires containing up
to 13 C atoms are found to have anti-parallel spin configurations of the two
terminal Co atoms, while the 14 C wire has a parallel Co spin configuration in
the ground state. The stability of the anti-ferromagnetic state in the wires is
ascribed to a super-exchange effect. For the cumulenic wires this effect is
constant for all wire lengths. For the acetylenic wires, the super-exchange
effect diminishes as the wire length increases, going to zero for the atomic
wire containing 14 carbon atoms. Conductance calculations at the zero bias
limit show spin-valve behavior, with the parallel Co spin configuration state
giving higher conductance than the corresponding anti-parallel state, and a
non-monotonic variation of conductance with the length of the wires for both
spin configurations.Comment: revtex, 6 pages, 5 figure
Gauging the spectator equations
We show how to derive relativistic, unitary, gauge invariant, and charge
conserving three-dimensional scattering equations for a system of hadrons
interacting with an electromagnetic field. In the method proposed, the
spectator equations describing the strong interactions of the hadrons are
gauged using our recently introduced gauging of equations method. A key
ingredient in our model is the on-mass-shell particle propagator. We discuss
how to gauge this on-mass-shell propagator so that both the Ward-Takahashi and
Ward identities are satisfied. We then demonstrate our gauging procedure by
deriving the gauge-invariant three-dimensional expression for the deuteron
photodisintegration amplitude within the spectator approach.Comment: 17 pages, REVTeX, epsf, 1 Postscript figur
Basic Psychological Needs, Suicidal Ideation, and Risk for Suicidal Behavior in Young Adults
Associations between the satisfaction of basic psychological needs of autonomy, competence, and relatedness with current suicidal ideation and risk for suicidal behavior were examined. Two logistic regressions were conducted with a cross-sectional database of 440 university students to examine the association of need satisfaction with suicidal ideation and risk for suicidal behavior, while controlling for demographics and depressive symptoms. Suicidal ideation was reported by 15% of participants and 18% were found to be at risk for suicidal behavior. A one standard deviation increase in need satisfaction reduced the odds of suicidal ideation by 53%, OR (95% CI) = 0.47 (0.33–0.67), and the odds of being at risk for suicidal behavior by 50%, OR (95% CI) = 0.50 (0.37–0.69). Young adults whose basic psychological needs are met may be less likely to consider suicide and engage in suicidal behavior. Prospective research is needed to confirm these associations
Gauging the three-nucleon spectator equation
We derive relativistic three-dimensional integral equations describing the
interaction of the three-nucleon system with an external electromagnetic field.
Our equations are unitary, gauge invariant, and they conserve charge. This has
been achieved by applying the recently introduced gauging of equations method
to the three-nucleon spectator equations where spectator nucleons are always on
mass shell. As a result, the external photon is attached to all possible places
in the strong interaction model, so that current and charge conservation are
implemented in the theoretically correct fashion. Explicit expressions are
given for the three-nucleon bound state electromagnetic current, as well as the
transition currents for the scattering processes
\gamma He3 -> NNN, Nd -> \gamma Nd, and \gamma He3 -> Nd. As a result, a
unified covariant three-dimensional description of the NNN-\gamma NNN system is
achieved.Comment: 23 pages, REVTeX, epsf, 4 Postscript figure
Theoretical study of the stable states of small carbon clusters Cn (n = 2-10)
Both even- and odd-numbered neutral carbon clusters Cn (n = 2-10) are
systematically studied using the energy minimization method and the modified
Brenner potential for the carbon-carbon interactions. Many stable
configurations were found and several new isomers are predicted. For the lowest
energy stable configurations we obtained their binding energies and bond
lengths. We found that for n < 6 the linear isomer is the most stable one while
for n > 5 the monocyclic isomer becomes the most stable. The latter was found
to be regular for all studied clusters. The dependence of the binding energy
for linear and cyclic clusters versus the cluster size n (n = 2-10) is found to
be in good agreement with several previous calculations, in particular with ab
initio calculations as well as with experimental data for n = 2-5.Comment: Submitted to Phys. Rev.
Nonperturbative study of generalized ladder graphs in a \phi^2\chi theory
The Feynman-Schwinger representation is used to construct scalar-scalar bound
states for the set of all ladder and crossed-ladder graphs in a \phi^2\chi
theory in (3+1) dimensions. The results are compared to those of the usual
Bethe-Salpeter equation in the ladder approximation and of several
quasi-potential equations. Particularly for large couplings, the ladder
predictions are seen to underestimate the binding energy significantly as
compared to the generalized ladder case, whereas the solutions of the
quasi-potential equations provide a better correspondence. Results for the
calculated bound state wave functions are also presented.Comment: 5 pages revtex, 3 Postscripts figures, uses epsf.sty, accepted for
publication in Physical Review Letter
Relativistic Effects in the Electromagnetic Current at GeV Energies
We employ a recent approach to the non-relativistic reduction of the
electromagnetic current operator in calculations of electronuclear reactions.
In contrast to the traditional scheme, where approximations are made for the
transferred momentum, transferred energy and initial momentum of the struck
nucleon in obtaining an on-shell inspired form for the current, we treat the
problem exactly for the transferred energy and transferred momentum. We
calculate response functions for the reaction at CEBAF (TJNAF)
energies and find large relativistic corrections. We also show that in Plane
Wave Impulse Approximation, it is always possible to use the full operator, and
we present a comparison of such a limiting case with the results incorporating
relativistic effects to the first order in the initial momentum of the struck
nucleon.Comment: 31 pages, 8 figures, Revte
Relativistic bound-state equations in three dimensions
Firstly, a systematic procedure is derived for obtaining three-dimensional
bound-state equations from four-dimensional ones. Unlike ``quasi-potential
approaches'' this procedure does not involve the use of delta-function
constraints on the relative four-momentum. In the absence of negative-energy
states, the kernels of the three-dimensional equations derived by this
technique may be represented as sums of time-ordered perturbation theory
diagrams. Consequently, such equations have two major advantages over
quasi-potential equations: they may easily be written down in any Lorentz
frame, and they include the meson-retardation effects present in the original
four-dimensional equation. Secondly, a simple four-dimensional equation with
the correct one-body limit is obtained by a reorganization of the generalized
ladder Bethe-Salpeter kernel. Thirdly, our approach to deriving
three-dimensional equations is applied to this four-dimensional equation, thus
yielding a retarded interaction for use in the three-dimensional bound-state
equation of Wallace and Mandelzweig. The resulting three-dimensional equation
has the correct one-body limit and may be systematically improved upon. The
quality of the three-dimensional equation, and our general technique for
deriving such equations, is then tested by calculating bound-state properties
in a scalar field theory using six different bound-state equations. It is found
that equations obtained using the method espoused here approximate the wave
functions obtained from their parent four-dimensional equations significantly
better than the corresponding quasi-potential equations do.Comment: 28 pages, RevTeX, 6 figures attached as postscript files. Accepted
for publication in Phys. Rev. C. Minor changes from original version do not
affect argument or conclusion
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