24,759 research outputs found
Comment on "138La-138Ce-136Ce nuclear cosmochronometer of the supernova neutrino process"
The nuclear chosmochronometer suggested by Hayakawa et al. [Phys. Rev.C 77,
065802 (2008)] based on the 138La-138Ce-136Ce abundance ratio in presolar
grains would be affected by the existence of a hitherto unknown low-energy 1+
state in 138La. Results of a recent high-resolution study of the 138Ba(3He,t)
reaction under kinematics selectively populating 1+ states in 138La through
Gamow-Teller transitions provides strong evidence against the existence of such
a hypothetical state.Comment: Comment on Phys. Rev. C 77, 065802 (2008), submittted to Phys. Rev.
Thermodynamics of the frustrated - Heisenberg ferromagnet on the body-centered cubic lattice with arbitrary spin
We use the spin-rotation-invariant Green's function method as well as the
high-temperature expansion to discuss the thermodynamic properties of the
frustrated spin- - Heisenberg magnet on the body-centered
cubic lattice. We consider ferromagnetic nearest-neighbor bonds and
antiferromagnetic next-nearest-neighbor bonds and arbitrary spin
. We find that the transition point between the ferromagnetic ground
state and the antiferromagnetic one is nearly independent of the spin ,
i.e., it is very close to the classical transition point . At finite temperatures we focus on the parameter regime
with a ferromagnetic ground-state. We calculate the Curie
temperature and derive an empirical formula describing the
influence of the frustration parameter and spin on . We find
that the Curie temperature monotonically decreases with increasing frustration
, where very close to the -curve exhibits a
fast decay which is well described by a logarithmic term
. To characterize the magnetic ordering
below and above , we calculate the spin-spin correlation functions
, the spontaneous
magnetization, the uniform static susceptibility as well as the
correlation length . Moreover, we discuss the specific heat and the
temperature dependence of the excitation spectrum. As approaching the
transition point some unusual features were found, such as negative
spin-spin correlations at temperatures above even though the ground state
is ferromagnetic or an increase of the spin stiffness with growing temperature.Comment: 19 pages, 10 figures, version as in EPJ
Properties of the first excited state of 9Be derived from (gamma,n) and (e,e') reactions
Properties of the first excited state of the nucleus 9Be are discussed based
on recent (e,e') and (gamma,n) experiments. The parameters of an R-matrix
analysis of different data sets are consistent with a resonance rather than a
virtual state predicted by some model calculations. The energy and the width of
the resonance are deduced. Their values are rather similar for all data sets,
and the energy proves to be negative. It is argued that the disagreement
between the extracted B(E1) values may stem from different ways of integration
of the resonance. If corrected, fair agreement between the (e,e') and one of
the (gamma,n) data sets is found. A recent (gamma,n) experiment at the HIgS
facility exhibits larger cross sections close to the neutron threshold which
remain to be explained.Comment: 5 pages, accepted fro publication in Phys. Rev.
Multiple Scales in the Fine Structure of the Isoscalar Giant Quadrupole Resonance in ^{208}Pb
The fine structure of the isoscalar giant quadrupole resonance in ^{208}Pb,
observed in high-resolution (p,p') and (e,e') experiments, is studied using the
entropy index method. In a novel way, it enables to determine the number of
scales present in the spectra and their magnitude. We find intermediate scales
of fluctuations around 1.1 MeV, 460 keV and 125 keV for an excitation energy
region 0 - 12 MeV. A comparison with scales extracted from second RPA
calculations, which are in good agreement with experiment, shows that they
arise from the internal mixing of collective motion with two particle-two hole
components of the nuclear wavefunction.Comment: 14 pages including 6 figures (to be published in Phys. Lett. B
An HST/COS legacy survey of intervening SiIII absorption in the extended gaseous halos of low-redshift galaxies
Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas
inside and outside of galaxies. It can be observed in the local Universe in
ultraviolet (UV) absorption against bright extragalactic background sources. We
here present an extensive study of intervening SiIII-selected absorbers and
their relation to the circumgalactic medium (CGM) of galaxies at low redshift
(z<=0.1), based on the analysis of UV absorption spectra along 303
extragalactic lines of sight obtained with the Cosmic Origins Spectrograph
(COS) on board the Hubble Space Telescope (HST). Along a total redshift path of
Dz=24 we identify 69 intervening SiIII systems that all show associated
absorption from other low and high ions. We derive a bias-corrected number
density of dN/dz(SiIII)=2.5 for absorbers with column densities log
N(SiIII)>12.2. We develop a geometrical model for the absorption-cross section
of the CGM around the local galaxy population and find excellent agreement
between the model predictions and the observations. We further compare
redshifts and positions of the absorbers with that of ~64,000 galaxies using
archival galaxy-survey data. For the majority of the absorbers we identify
possible host galaxies within 300 km/s of the absorbers and derive impact
parameters rho<200 kpc, demonstrating that the spatial distributions of SiIII
absorbers and galaxies are highly correlated. Our study indicates that the
majority of SiIII-selected absorbers in our sample trace the CGM of nearby
galaxies within their virial radii at a typical covering fraction of ~70 per
cent. From a detailed ionization model we estimate that diffuse gas in the CGM
around galaxies, as traced by SiIII, contains substantially more baryonic mass
than their neutral interstellar medium.Comment: 32 pages, 17 figures; final version accepted for publication in A&
Absorption Line Studies in the Halo
Significant progress has been made over the last few years to explore the
gaseous halo of the Milky Way by way of absorption spectroscopy. I review
recent results on absorption line studies in the halo using various
instruments, such as the Far Ultraviolet Spectroscopic Explorer, the Space
Telescope Imaging Spectrograph, and others. The new studies imply that the
infall of low-metallicity gas, the interaction with the Magellanic Clouds, and
the Galactic Fountain are responsible for the phenomenon of the intermediate-
and high-velocity clouds in the halo. New measurements of highly-ionized gas in
the vicinity of the Milky Way indicate that these clouds are embedded in a
corona of hot gas that extends deep into the intergalactic space.Comment: 7 pages, 1 figure; Invited review at the conference "How does the
Galaxy work ?", Granada/Spain, June 200
Resonance parameters of the first 1/2+ state in 9Be and astrophysical implications
Spectra of the 9Be(e,e') reaction have been measured at the S-DALINAC at an
electron energy E_0 = 73 MeV and scattering angles of 93{\deg} and 141{\deg}
with high energy resolution up to excitation energies E_x = 8 MeV. The
astrophysically relevant resonance parameters of the first excited 1/2+ state
of 9Be have been extracted in a one-level approximation of R-matrix theory
resulting in a resonance energy E_R = 1.748(6) MeV and width Gamma_R = 274(8)
keV in good agreement with the latest 9Be(gamma,n) experiment but with
considerably improved uncertainties. However, the reduced B(E1) transition
strength deduced from an extrapolation of the (e,e') data to the photon point
is a factor of two smaller. Implications of the new results for a possible
production of 12C in neutron-rich astrophysical scenarios are discussed.Comment: 8 pages, 7 figures, accepted for publication in Phys. Rev.
Exact eigenstates of highly frustrated spin lattices probed in high fields
Strongly frustrated antiferromagnets such as the magnetic molecule
{Mo72Fe30}, the kagome, or the pyrochlore lattice exhibit a variety of
fascinating properties like low-lying singlets, magnetization plateaus as well
as magnetization jumps. During recent years exact many-body eigenstates could
be constructed for several of these spin systems. These states become ground
states in high magnetic fields, and they also lead to exotic behavior. A key
concept to an understanding of these properties is provided by independent
localized magnons. The energy eigenvalue of these n-magnon states scales
linearly with the number n of independent magnons and thus with the total
magnetic quantum number M=Ns-n. In an applied field this results in a giant
magnetization jump which constitutes a new macroscopic quantum effect. It will
be demonstrated that this behavior is accompanied by a massive degeneracy, an
extensive (T=0)-entropy, and thus a large magnetocaloric effect at the
saturation field. The connection to flat band ferromagnetism will be outlined.Comment: 4 pages, submitted to the proceedings of the Yamada Conference LX on
Research in High Magnetic Fields, August 16-19, 2006 Sendai, Japa
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