507 research outputs found
Exact one- and two-particle excitation spectra of acute-angle helimagnets above their saturation magnetic field
The two-magnon problem for the frustrated XXZ spin-1/2 Heisenberg Hamiltonian
and external magnetic fields exceeding the saturation field Bs is considered.
We show that the problem can be exactly mapped onto an effective tight-binding
impurity problem. It allows to obtain explicit exact expressions for the
two-magnon Green's functions for arbitrary dimension and number of
interactions. We apply this theory to a quasi-one dimensional helimagnet with
ferromagnetic nearest neighbor J1 < 0 and antiferromagnetic next-nearest
neighbor J2 > 0 interactions. An outstanding feature of the excitation spectrum
is the existence of two-magnon bound states. This leads to deviations of the
saturation field Bs from its classical value Bs(classical) which coincides with
the one-magnon instability. For the refined frustration ratio |J2/J1|> 0.374661
the minimum of the two-magnon spectrum occurs at the boundary of the Brillouin
zone. Based on the two-magnon approach, we propose general analytic expressions
for the saturation field Bs, confirming known previous results for
one-dimensional isotropic systems, but explore also the role of interchain and
long-ranged intrachain interactions as well as of the exchange anisotropy.Comment: 21 pages, 6 Figures. submitted to Phys. Rev.
Measurement of the elastic scattering cross section of neutrons from argon and neon
Background: The most significant source of background in direct dark matter
searches are neutrons that scatter elastically from nuclei in the detector's
sensitive volume. Experimental data for the elastic scattering cross section of
neutrons from argon and neon, which are target materials of interest to the
dark matter community, were previously unavailable. Purpose: Measure the
differential cross section for elastic scattering of neutrons from argon and
neon in the energy range relevant to backgrounds from (alpha,n) reactions in
direct dark matter searches. Method: Cross-section data were taken at the
Triangle Universities Nuclear Laboratory (TUNL) using the neutron
time-of-flight technique. These data were fit using the spherical optical
model. Results: The differential cross section for elastic scatting of neutrons
from neon at 5.0 and 8.0 MeV and argon at 6.0 MeV was measured. Optical-model
parameters for the elastic scattering reactions were determined from the best
fit to these data. The total elastic scattering cross section for neon was
found to differ by 6% at 5.0 MeV and 13% at 8.0 MeV from global optical-model
predictions. Compared to a local optical-model for 40Ar, the elastic scattering
cross section was found to differ from the data by 8% at 6.0 MeV. Conclusions:
These new data are important for improving Monte-Carlo simulations and
background estimates for direct dark matter searches and for benchmarking
optical models of neutron elastic scattering from these nuclei
Measurements at low energies of the polarization-transfer coefficient Kyy' for the reaction 3H(p,n)3He at 0 degrees
Measurements of the transverse polarization coefficient Kyy' for the reaction
3H(p,n)3He are reported for outgoing neutron energies of 1.94, 5.21, and 5.81
MeV. This reaction is important both as a source of polarized neutrons for
nuclear physics experiments, and as a test of theoretical descriptions of the
nuclear four-body system. Comparison is made to previous measurements,
confirming the 3H(p,n)3He reaction can be used as a polarized neutron source
with the polarization known to an accuracy of approximately 5%. Comparison to
R-matrix theory suggests that the sign of the 3F3 phase-shift parameter is
incorrect. Changing the sign of this parameter dramatically improves the
agreement between theory and experiment.Comment: 12 pages, RevTeX, 5 eps figures, submitted to Phys. Rev.
Effects of the magnetic moment interaction between nucleons on observables in the 3N continuum
The influence of the magnetic moment interaction of nucleons on
nucleon-deuteron elastic scattering and breakup cross sections and on elastic
scattering polarization observables has been studied. Among the numerous
elastic scattering observables only the vector analyzing powers were found to
show a significant effect, and of opposite sign for the proton-deuteron and
neutron-deuteron systems. This finding results in an even larger discrepancy
than the one previously established between neutron-deuteron data and
theoretical calculations. For the breakup reaction the largest effect was found
for the final-state-interaction cross sections. The consequences of this
observation on previous determinations of the ^1S_0 scattering lengths from
breakup data are discussed.Comment: 24 pages, 6 ps figures, 1 png figur
Neutron-proton analyzing power at 12 MeV and inconsistencies in parametrizations of nucleon-nucleon data
We present the most accurate and complete data set for the analyzing power
Ay(theta) in neutron-proton scattering. The experimental data were corrected
for the effects of multiple scattering, both in the center detector and in the
neutron detectors. The final data at En = 12.0 MeV deviate considerably from
the predictions of nucleon-nucleon phase-shift analyses and potential models.
The impact of the new data on the value of the charged pion-nucleon coupling
constant is discussed in a model study.Comment: Six pages, four figures, one table, to be published in Physics
Letters
Composition of LHB Comets and Their Influence on the Early Earth Atmosphere Composition
Two main processes were responsible for the composition of this atmosphere: chemical evolution of the volatile fraction of the accretion material forming the planet and the delivery of gasses to the planetary surface by impactors during the late heavy bombardment (LHB). The amount and composition of the volatile fraction influences the outgassing of the Earth mantle during the last planetary formation period. A very weakened form of outgassing activity can still be observed today by examining the composition of volcanic gasses. An enlightenment of the second process is based on the sparse records of the LHB impactors resulting from the composition of meteorites, observed cometary comas, and the impact material found on the Moon. However, for an assessment of the influence of the outgassing on the one hand and the LHB event on the other, one has to supplement the observations with numerical simulations of the formation of volatiles and their incorporation into the accretion material which is the precursors of planetary matter, comets and asteroids. These simulations are performed with a combined hydrodynamic-chemical model of the solar nebula (SN). We calculate the chemical composition of the gas and dust phase of the SN. From these data, we draw conclusions on the upper limits of the water content and the amount of carbon and nitrogen rich volatiles incorporated later into the accretion material. Knowing these limits we determine the portion of major gas compounds delivered during the LHB and compare it with the related quantities of the outgassed species
Earth-like Habitats in Planetary Systems
Understanding the concept of habitability is related to an evolutionary
knowledge of the particular planet-in-question. Additional indications
so-called "systemic aspects" of the planetary system as a whole governs a
particular planet's claim on habitability. Here we focus on such systemic
aspects and discuss their relevance to the formation of an 'Earth-like'
habitable planet. We summarize our results obtained by lunar sample work and
numerical models within the framework of the Research Alliance "Planetary
Evolution and Life". We consider various scenarios which simulate the dynamical
evolution of the Solar System and discuss the likelihood of forming an
Earth-like world orbiting another star. Our model approach is constrained by
observations of the modern Solar System and the knowledge of its history.
Results suggest that the long-term presence of terrestrial planets is
jeopardized due to gravitational interactions if giant planets are present. But
habitability of inner rocky planets may be supported in those planetary systems
hosting giant planets.
Gravitational interactions within a complex multiple-body structure including
giant planets may supply terrestrial planets with materials which formed in the
colder region of the proto-planetary disk. During these processes, water, the
prime requisite for habitability, is delivered to the inner system. This may
occur either during the main accretion phase of terrestrial planets or via
impacts during a post-accretion bombardment. Results for both processes are
summarized and discussed with reference to the lunar crater record.
Starting from a scenario involving migration of the giant planets this
contribution discusses the delivery of water to Earth, the modification of
atmospheres by impacts in a planetary system context and the likelihood of the
existence of extrasolar Earth-like habitable worlds.Comment: 36 Pages, 6 figures, 2014, Special Issue in Planetary and Space
Science on the Helmholtz Research Alliance on Planetary Evolution and Lif
Effects of hole-doping on the magnetic ground state and excitations in the edge-sharing CuO chains of CaYCuO
Neutron scattering experiments were performed on the undoped and hole-doped
CaYCuO, which consists of ferromagnetic edge-sharing
CuO chains. It was previously reported that in the undoped
CaYCuO there is an anomalous broadening of spin-wave
excitations along the chain, which is caused mainly by the antiferromagnetic
interchain interactions [Matsuda , Phys. Rev. B 63, 180403(R)
(2001)]. A systematic study of temperature and hole concentration dependencies
of the magnetic excitations shows that the magnetic excitations are softened
and broadened with increasing temperature or doping holes irrespective of
direction. The broadening is larger at higher . A characteristic feature is
that hole-doping is much more effective to broaden the excitations along the
chain. It is also suggested that the intrachain interaction does not change so
much with increasing temperature or doping although the anisotropic interaction
and the interchain interaction are reduced. In the spin-glass phase (=1.5)
and nearly disordered phase (=1.67) the magnetic excitations are much
broadened in energy and . It is suggested that the spin-glass phase
originates from the antiferromagnetic clusters, which are caused by the hole
disproportionation.Comment: 8 pages, submitted to Phys. Rev.
The electric dipole response of Se above 4 MeV
The dipole response of Se in the energy range 4 to 9 MeV has been
analyzed using a polarized photon scattering
technique, performed at the High Intensity -Ray Source facility, to
complement previous work performed using unpolarized photons. The results of
this work offer both an enhanced sensitivity scan of the dipole response and an
unambiguous determination of the parities of the observed J=1 states. The
dipole response is found to be dominated by excitations, and can
reasonably be attributed to a pygmy dipole resonance. Evidence is presented to
suggest that a significant amount of directly unobserved excitation strength is
present in the region, due to unobserved branching transitions in the decays of
resonantly excited states. The dipole response of the region is underestimated
when considering only ground state decay branches. We investigate the electric
dipole response theoretically, performing calculations in a 3D cartesian-basis
time-dependent Skyrme-Hartree-Fock framework.Comment: 20 pages, 18 figures, to be submitted to PR
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