7,499 research outputs found
Pair-transfer probability in open- and closed-shell Sn isotopes
Approximations made to estimate two-nucleon transfer probabilities in
ground-state to ground-state transitions and physical interpretation of these
probabilities are discussed. Probabilities are often calculated by
approximating both ground states, of the initial nucleus A and of the final
nucleus A\pm 2 by the same quasiparticle vacuum. We analyze two improvements of
this approach. First, the effect of using two different ground states with
average numbers of particles A and A\pm2 is quantified. Second, by using
projection techniques, the role of particle number restoration is analyzed. Our
analysis shows that the improved treatment plays a role close to magicity,
leading to an enhancement of the pair-transfer probability. In mid-shell
regions, part of the error made by approximating the initial and final ground
states by a single vacuum is compensated by projecting onto good particle
number. Surface effects are analyzed by using pairing interactions with a
different volume-to-surface mixing. Finally, a simple expression of the
pair-transfer probability is given in terms of occupation probabilities in the
canonical basis. We show that, in the canonical basis formulation, surface
effects which are visible in the transfer probability are related to the
fragmentation of single-particle occupancies close to the Fermi energy. This
provides a complementary interpretation with respect to the standard
quasiparticle representation where surface effects are generated by the
integrated radial profiles of the contributing wave functions.Comment: 12 pages, 7 figure
Magnetoresistance and collective Coulomb blockade in super-lattices of ferromagnetic CoFe nanoparticles
We report on transport properties of millimetric super-lattices of CoFe
nanoparticles surrounded by organic ligands. R(T)s follow R(T) =
R_0.exp(T/T_0)^0.5 with T_0 ranging from 13 to 256 K. At low temperature I(V)s
follow I=K[(V-V_T)/V_T]^ksi with ksi ranging 3.5 to 5.2. I(V) superpose on a
universal curve when shifted by a voltage proportional to the temperature.
Between 1.8 and 10 K a high-field magnetoresistance with large amplitude and a
strong voltage-dependence is observed. Its amplitude only depends on the
magnetic field/temperature ratio. Its origin is attributed to the presence of
paramagnetic states present at the surface or between the nanoparticles. Below
1.8 K, this high-field magnetoresistance abruptly disappears and inverse
tunnelling magnetoresistance is observed, the amplitude of which does not
exceed 1%. At this low temperature, some samples display in their I(V)
characteristics abrupt and hysteretic transitions between the Coulomb blockade
regime and the conductive regime. The increase of the current during these
transitions can be as high as a factor 30. The electrical noise increases when
the sample is near the transition. The application of a magnetic field
decreases the voltage at which these transitions occur so magnetic-field
induced transitions are also observed. Depending on the applied voltage, the
temperature and the amplitude of the magnetic field, the magnetic-field induced
transitions are either reversible or irreversible. These abrupt and hysteretic
transitions are also observed in resistance-temperature measurements. They
could be the soliton avalanches predicted by Sverdlov et al. [Phys. Rev. B 64,
041302 (R), 2001] or could also be interpreted as a true phase transition
between a Coulomb glass phase to a liquid phase of electrons
Transport Model Simulations of Projectile Fragmentation Reactions at 140 MeV/nucleon
The collisions in four different reaction systems using Ca and
Ni isotope beams and a Be target have been simulated using the Heavy
Ion Phase Space Exploration and the Antisymmetrized Molecular Dynamics models.
The present study mainly focuses on the model predictions for the excitation
energies of the hot fragments and the cross sections of the final fragments
produced in these reactions. The effects of various factors influencing the
final fragment cross sections, such as the choice of the statistical decay code
and its parameters have been explored. The predicted fragment cross sections
are compared to the projectile fragmentation cross sections measured with the
A1900 mass separator. At MeV, reaction dynamics can significantly
modify the detection efficiencies for the fragments and make them different
from the efficiencies applied to the measured data reported in the previous
work. The effects of efficiency corrections on the validation of event
generator codes are discussed in the context of the two models.Comment: 28 pages, 13 figure
Magnetic hyperthermia in single-domain monodisperse FeCo nanoparticles: Evidences for Stoner-Wohlfarth behaviour and large losses
We report on hyperthermia measurements on a colloidal solution of 15 nm
monodisperse FeCo nanoparticles (NPs). Losses as a function of the magnetic
field display a sharp increase followed by a plateau, which is what is expected
for losses of ferromagnetic single-domain NPs. The frequency dependence of the
coercive field is deduced from hyperthermia measurement and is in quantitative
agreement with a simple model of non-interacting NPs. The measured losses (1.5
mJ/g) compare to the highest of the literature, though the saturation
magnetization of the NPs is well below the bulk one.Comment: 14 pages, 3 figure
Kondo Screening and Magnetic Ordering in Frustrated UNi4B
UNi4B exhibits unusual properties and, in particular, a unique
antiferromagnetic arrangement involving only 2/3 of the U sites. Based on the
low temperature behavior of this compound, we propose that the remaining 1/3 U
sites are nonmagnetic due to the Kondo effect. We derive a model in which the
coexistence of magnetic and nonmagnetic U sites is the consequence of the
competition between frustration of the crystallographic structure and
instability of the 5f moments.Comment: 4 pages, 2 figure
Lexicographical polytopes
Within a fixed integer box of Rn, lexicographical polytopes are the convex hulls of the integer points that are lexicographically between two given integer points. We provide their descriptions by means of linear inequalities
Fermi Surface of The One-dimensional Kondo Lattice Model
We show a strong indication of the existence of a large Fermi surface in the
one-dimensional Kondo lattice model. The characteristic wave vector of the
model is found to be , being the density of the
conduction electrons. This result is at first obtained for a variant of the
model that includes an antiferromagnetic Heisenberg interaction between
the local moments. It is then directly observed in the conventional Kondo
lattice , in the narrow range of Kondo couplings where the long
distance properties of the model are numerically accessible.Comment: 11 pages, 6 figure
Particle-Number Restoration within the Energy Density Functional Formalism
We give a detailed analysis of the origin of spurious divergences and finite
steps that have been recently identified in particle-number restoration
calculations within the nuclear energy density functional framework. We isolate
two distinct levels of spurious contributions to the energy. The first one is
encoded in the definition of the basic energy density functional itself whereas
the second one relates to the canonical procedure followed to extend the use of
the energy density functional to multi-reference calculations. The first level
of spuriosity relates to the long-known self-interaction problem and to the
newly discussed self-pairing interaction process which might appear when
describing paired systems with energy functional methods using auxiliary
reference states of Bogoliubov or BCS type. A minimal correction to the second
level of spuriosity to the multi-reference nuclear energy density functional
proposed in [D. Lacroix, T. Duguet, M. Bender, arXiv:0809.2041] is shown to
remove completely the anomalies encountered in particle-number restored
calculations. In particular, it restores sum-rules over (positive) particle
numbers that are to be fulfilled by the particle-number-restored formalism. The
correction is found to be on the order of several hundreds of keVs up to about
1 MeV in realistic calculations, which is small compared to the total binding
energy, but often accounts for a substantial percentage of the energy gain from
particle-number restoration and is on the same energy scale as the excitations
one addresses with multi-reference energy density functional methods.Comment: 37 pages, 14 figures, accepted for publication in PR
Emission of gamma rays shifted from resonant absorption by electron-nuclear double transitions in ^{151}Eu^{2+}:CaF_2
We show that the emission of a gamma-ray photon by a nucleus can be
influenced by a microwave magnetic field acting on the atomic electrons. We
study theoretically these electron-nuclear double transitions (ENDTs) for
^{151}Eu nuclei in a CaF_2 lattice at low temperature, in the presence of a
static magnetic field and of a microwave magnetic field. The ENDTs acquire a
significant intensity for certain resonance frequencies. The ENDTs are of
interest for the identification of the position of the lines in complex
M\"{o}ssbauer spectra.Comment: 8 pages; 3 Postscript figures: Fig. 1, Fig. 2(a), Fig. 2(b
Ratings of Overall Olfactory Function
The aim of this study was to investigate the accuracy of self-reported ratings of olfactory function in 83 healthy subjects. Such ratings were compared with quantitative measures of olfactory function, as well as with ratings of nasal patency. In experiment 1 subjects rated olfactory function and nasal patency before olfactory testing, whereas in experiment 2 the reverse was the case. No feedback regarding test results were provided until after completion of the testing. The principal findings were: (i) when ratings preceded measurements of olfactory function, there was no significant correlation between the two parameters. However, ratings of olfactory function correlated significantly with ratings of nasal airway patency. (ii) In contrast, when measurements of olfactory function preceded the ratings, this constellation switched. Now ratings of olfactory function correlated significantly with measured olfactory function, whereas there was no significant correlation between ratings of nasal airway patency and ratings of olfactory function. In conclusion, these data suggest that ratings of olfactory function are unreliable in healthy, untrained subjects. The ratings seem to reflect changes of nasal airway patency to a larger degree than measurable olfactory function. The results further indicate that this is mainly due to the limited attention the sense of smell receives in daily lif
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