1,569 research outputs found
Ambiguities in the partial-wave analysis of pseudoscalar-meson photoproduction
Ambiguities in pseudoscalar-meson photoproduction, arising from incomplete
experimental data, have analogs in pion-nucleon scattering. Amplitude
ambiguities have important implications for the problems of amplitude
extraction and resonance identification in partial-wave analysis. The effect of
these ambiguities on observables is described. We compare our results with
those found in earlier studies.Comment: 12 pages of text. No figure
Flow in a slowly-tapering channel with oscillating walls
The flow of a fluid in a channel with walls inclined at an angle to each other is investigated at arbitrary Reynolds number. The flow is driven by an oscillatory motion of the wall incorporating a time-periodic displacement perpendicular to the channel centreline. The gap between the walls varies linearly with distance along the channel and is a prescribed periodic function of time. An approximate solution is constructed assuming that the angle of inclination of the walls is small. At leading order the flow corresponds to that in a channel with parallel, vertically oscillating walls examined by Hall and Papageorgiou \cite{HP}. A careful study of the governing partial differential system for the first order approximation controlling the tapering flow due to the wall inclination is conducted. It is found that as the Reynolds number is increased from zero the tapering flow loses symmetry and undergoes exponential growth in time. The loss of symmetry occurs at a lower Reynolds number than the symmetry-breaking for the parallel-wall flow. A window of asymmetric, time-periodic solutions is found at higher Reynolds number, and these are reached via a quasiperiodic transient from a given set of initial conditions. Beyond this window stability is again lost to exponentially growing solutions as the Reynolds number is increased
Comparison of quantum mechanical and classical trajectory calculations of cross sections for ion-atom impact ionization of negative - and positive -ions for heavy ion fusion applications
Stripping cross sections in nitrogen have been calculated using the classical
trajectory approximation and the Born approximation of quantum mechanics for
the outer shell electrons of 3.2GeV I and Cs ions. A large
difference in cross section, up to a factor of six, calculated in quantum
mechanics and classical mechanics, has been obtained. Because at such high
velocities the Born approximation is well validated, the classical trajectory
approach fails to correctly predict the stripping cross sections at high
energies for electron orbitals with low ionization potential.Comment: submitted to Phys. Rev.
Unscreened Hartree-Fock calculations for metallic Fe, Co, Ni, and Cu from ab-initio Hamiltonians
Unscreened Hartree-Fock approximation (HFA) calculations for metallic Fe, Co,
Ni, and Cu are presented, by using a quantum-chemical approach. We believe that
these are the first HFA results to have been done for crystalline 3d transition
metals. Our approach uses a linearized muffin-tin orbital calculation to
determine Bloch functions for the Hartree one-particle Hamiltonian, and from
these obtains maximally localized Wannier functions, using a method proposed by
Marzari and Vanderbilt. Within this Wannier basis all relevant one-particle and
two-particle Coulomb matrix elements are calculated. The resulting
second-quantized multi-band Hamiltonian with ab-initio parameters is studied
within the simplest many-body approximation, namely the unscreened,
self-consistent HFA, which takes into account exact exchange and is free of
self-interactions. Although the d-bands sit considerably lower within HFA than
within the local (spin) density approximation L(S)DA, the exchange splitting
and magnetic moments for ferromagnetic Fe, Co, and Ni are only slightly larger
in HFA than what is obtained either experimentally or within LSDA. The HFA
total energies are lower than the corresponding LSDA calculations. We believe
that this same approach can be easily extended to include more sophisticated
ab-initio many-body treatments of the electronic structure of solids.Comment: 11 papes, 7 figures, 5 table
Charge and Orbital Ordering in Pr_{0.5} Ca_{0.5} MnO_3 Studied by ^{17}O NMR
The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for the
first time by ^{17}O NMR. This local probe is sensitive to spin, charge and
orbital correlations. Two transitions exist in this system: the charge and
orbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition at
T_N = 170 K. Both are clearly seen in the NMR spectra measured in a magnetic
field of 7T. Above T_{CO} there exists only one NMR line with a large isotropic
shift, whose temperature dependence is in accordance with the presence of
ferromagnetic (FM) correlations. This line splits into two parts below T_{CO},
which are attributed to different types of oxygen in the charge/orbital ordered
state. The interplay of FM and AF spin correlations of Mn ions in the charge
ordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the hole
hopping motion that is slowed down with decreasing temperature. The developing
fine structure of the spectra evidences, that there still exist
charge-disordered regions at T_{CO} > T > T_N and that the static (t >
10^{-6}s) orbital order is established only on approaching T_N. The CE-type
magnetic correlations develop gradually below T_{CO}, so that at first the AF
correlations between checkerboard ab-layers appear, and only at lower
temperature - CE correlations within the ab-planes
Surface state engineering of molecule-molecule interactions
Engineering the electronic structure of organics through interface
manipulation, particularly the interface dipole and the barriers to charge
carrier injection, is of essential importance to improved organic devices. This
requires the meticulous fabrication of desired organic structures by precisely
controlling the interactions between molecules. The well-known principles of
organic coordination chemistry cannot be applied without proper consideration
of extra molecular hybridization, charge transer and dipole formation at the
interfaces. Here we identify the interplay between energy level alignment,
charge transfer, surface dipole and charge pillow effect and show how these
effects collectively determine the net force between adsorbed porphyrin 2H-TPP
on Cu(111). We show that the forces between supported porphyrins can be altered
by controlling the amount of charge transferred across the interface accurately
through the relative alignment of molecular electronic levels with respect to
the Shockley surface state of the metal substrate, and hence govern the
self-assembly of the molecules
Pinch Technique and the Batalin-Vilkovisky formalism
In this paper we take the first step towards a non-diagrammatic formulation
of the Pinch Technique. In particular we proceed into a systematic
identification of the parts of the one-loop and two-loop Feynman diagrams that
are exchanged during the pinching process in terms of unphysical ghost Green's
functions; the latter appear in the standard Slavnov-Taylor identity satisfied
by the tree-level and one-loop three-gluon vertex. This identification allows
for the consistent generalization of the intrinsic pinch technique to two
loops, through the collective treatment of entire sets of diagrams, instead of
the laborious algebraic manipulation of individual graphs, and sets up the
stage for the generalization of the method to all orders. We show that the task
of comparing the effective Green's functions obtained by the Pinch Technique
with those computed in the background field method Feynman gauge is
significantly facilitated when employing the powerful quantization framework of
Batalin and Vilkovisky. This formalism allows for the derivation of a set of
useful non-linear identities, which express the Background Field Method Green's
functions in terms of the conventional (quantum) ones and auxiliary Green's
functions involving the background source and the gluonic anti-field; these
latter Green's functions are subsequently related by means of a Schwinger-Dyson
type of equation to the ghost Green's functions appearing in the aforementioned
Slavnov-Taylor identity.Comment: 45 pages, uses axodraw; typos corrected, one figure changed, final
version to appear in Phys.Rev.
Modeling DNA Structure, Elasticity and Deformations at the Base-pair Level
We present a generic model for DNA at the base-pair level. We use a variant
of the Gay-Berne potential to represent the stacking energy between neighboring
base-pairs. The sugar-phosphate backbones are taken into account by semi-rigid
harmonic springs with a non-zero spring length. The competition of these two
interactions and the introduction of a simple geometrical constraint leads to a
stacked right-handed B-DNA-like conformation. The mapping of the presented
model to the Marko-Siggia and the Stack-of-Plates model enables us to optimize
the free model parameters so as to reproduce the experimentally known
observables such as persistence lengths, mean and mean squared base-pair step
parameters. For the optimized model parameters we measured the critical force
where the transition from B- to S-DNA occurs to be approximately . We
observe an overstretched S-DNA conformation with highly inclined bases that
partially preserves the stacking of successive base-pairs.Comment: 15 pages, 25 figures. submitted to PR
Oblique triangular antiferromagnetic phase in CsCuCoCl
The spin-1/2 stacked triangular antiferromagnet CsCuCoCl with
undergoes two phase transitions at zero field. The
low-temperature phase is produced by the small amount of Co doping. In
order to investigate the magnetic structures of the two ordered phases, the
neutron elastic scattering experiments have been carried out for the sample
with . It is found that the intermediate phase is identical to
the ordered phase of CsCuCl, and that the low-temperature phase is an
oblique triangular antiferromagnetic phase in which the spins form a triangular
structure in a plane tilted from the basal plane. The tilting angle which is
42 at K decreases with increasing temperature, and becomes
zero at K. An off-diagonal exchange term is proposed as the
origin of the oblique phase.Comment: 6 pages, 7 figure
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