2,076 research outputs found
Reactivity difference between protolytic forms of some macrocyclic chromium(III) complexes in ligand substitution and electron transfer processes
The review provides insight into the mechanism of ligand substitution and electron transfer (from chromium( III) to iron(III)) by comparison of the reactivity of some tetraazamacrocyclic chromium(III) complexes in the conjugate acid-base forms. Use of two geometrical isomers made possible to estimate the influence of geometry and protolytic reactions in trans and cis position towards the leaving group on the rate enhancement. Studies on the reaction rates in different media demonstrated the role played by outer sphere interactions in a monodentate ligand substitution
FRW Universe Models in Conformally Flat Spacetime Coordinates. III: Universe models with positive spatial curvature
We deduce general expressions for the line element of universe models with
positive spatial curvature described by conformally flat spacetime coordinates.
Models with dust, radiation and vacuum energy are exhibited. Discussing the
existence of particle horizons we show that there is continual annihilation of
space, matter and energy in a dust and radiation dominated universe, and
continual creation in a LIVE domined universe when conformal time is used in
Friedmann-Robertson-Walker models with positive spatial curvature. A general
procedure is given for finding coordinates to be used in Penrose diagrams. We
also calculate the age and the redshift of some universe models using conformal
time.Comment: 22 pages, 9 figure
FRW Universe Models in Conformally Flat Spacetime Coordinates. II: Universe models with negative and vanishing spatial curvature
We deduce general expressions for the line element of universe models with
negative and vanishing spatial curvature described by conformally flat
spacetime coordinates. The empty Milne universe model and models with dust,
radiation and vacuum energy are exhibited. Discussing the existence of particle
horizons we show that there is continual creation of space, matter and energy
when conformal time is used in Friedmann-Robertson-Walker models with negative
spatial curvature.Comment: 25 pages, 12 figure
The Significance of the Schott Energy in the Electrodynamics of Charged Particles and their Fields
Computation of local exchange coefficients in strongly interacting one-dimensional few-body systems: local density approximation and exact results
One-dimensional multi-component Fermi or Bose systems with strong zero-range
interactions can be described in terms of local exchange coefficients and
mapping the problem into a spin model is thus possible. For arbitrary external
confining potentials the local exchanges are given by highly non-trivial
geometric factors that depend solely on the geometry of the confinement through
the single-particle eigenstates of the external potential. To obtain accurate
effective Hamiltonians to describe such systems one needs to be able to compute
these geometric factors with high precision which is difficult due to the
computational complexity of the high-dimensional integrals involved. An
approach using the local density approximation would therefore be a most
welcome approximation due to its simplicity. Here we assess the accuracy of the
local density approximation by going beyond the simple harmonic oscillator that
has been the focus of previous studies and consider some double-wells of
current experimental interest. We find that the local density approximation
works quite well as long as the potentials resemble harmonic wells but break
down for larger barriers. In order to explore the consequences of applying the
local density approximation in a concrete setup we consider quantum state
transfer in the effective spin models that one obtains. Here we find that even
minute deviations in the local exchange coefficients between the exact and the
local density approximation can induce large deviations in the fidelity of
state transfer for four, five, and six particles.Comment: 12 pages, 7 figures, 1 table, final versio
The Gluonic Field of a Heavy Quark in Conformal Field Theories at Strong Coupling
We determine the gluonic field configuration sourced by a heavy quark
undergoing arbitrary motion in N=4 super-Yang-Mills at strong coupling and
large number of colors. More specifically, we compute the expectation value of
the operator tr[F^2+...] in the presence of such a quark, by means of the
AdS/CFT correspondence. Our results for this observable show that signals
propagate without temporal broadening, just as was found for the expectation
value of the energy density in recent work by Hatta et al. We attempt to shed
some additional light on the origin of this feature, and propose a different
interpretation for its physical significance. As an application of our general
results, we examine when the quark undergoes oscillatory motion,
uniform circular motion, and uniform acceleration. Via the AdS/CFT
correspondence, all of our results are pertinent to any conformal field theory
in 3+1 dimensions with a dual gravity formulation.Comment: 1+38 pages, 16 eps figures; v2: completed affiliation; v3: corrected
typo, version to appear in JHE
The twin paradox and Mach's principle
The problem of absolute motion in the context of the twin paradox is
discussed. It is shown that the various versions of the clock paradox feature
some aspects which Mach might have been appreciated. However, the ultimate
cause of the behavior of the clocks must be attributed to the autonomous status
of spacetime, thereby proving the relational program advocated by Mach as
impracticable.Comment: Latex2e, 11 pages, 6 figures, 33 references, no tables. Accepted for
publication in The European Physical Journal PLUS (EPJ PLUS
Semiclassical limit for Dirac particles interacting with a gravitational field
The behavior of spin-1/2 particle in a weak static gravitational field is
considered. The Dirac Hamiltonian is diagonalized by the Foldy-Wouthuysen
transformation providing also the simple form for the momentum and spin
polarization operators. The operator equations of momentum and spin motion are
derived for a first time. Their semiclassical limit is analyzed. The dipole
spin-gravity coupling in the previously found (another) Hamiltonian does not
lead to any observable effects. The general agreement between the quantum and
classical analysis is established, contrary to several recent claims. The
expression for gravitational Stern-Gerlach force is derived. The helicity
evolution in the gravitational field and corresponding accelerated frame
coincides, being the manifestation of the equivalence principle.Comment: 8 pages, no figures; version to appear in Physical Review
Estimating oceanic primary production using vertical irradiance and chlorophyll profiles from ocean gliders in the North Atlantic
An autonomous underwater vehicle (Seaglider) has been used to estimate marine primary production (PP) using a combination of irradiance and fluorescence vertical profiles. This method provides estimates for depth-resolved and temporally evolving PP on fine spatial scales in the absence of ship-based calibrations. We describe techniques to correct for known issues associated with long autonomous deployments such as sensor calibration drift and fluorescence quenching. Comparisons were made between the Seaglider, stable isotope (13C), and satellite estimates of PP. The Seaglider-based PP estimates were comparable to both satellite estimates and stable isotope measurements
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