3,043 research outputs found
Two-Step Contribution to Intermediate Energy (p,p') and (p,n) Reactions
We calculate the two-step contribution to (p,p') and (p,n) reactions at
intermediate energy. We describe the motion of the incident nucleon with plane
wave and compare the contribution from the two-step processes with that from
the one-step processes. To describe the two-step processes, we extende the
response functions into the nondiagonal ones with respect to the momentum
transfer q.
We performed a numerical calculation for the cross sections of the C,
Ca(p,p') scatterings and the spin longitudinal and the spin transverse
cross sections of the C,Ca(p,n) reactions at 346 MeV and 494 MeV.
We found that the two-step contribution is appreciable in comparison with the
one-step processes in higher energy transfer region for the spin longitudinal
and the spin transverse (p,n) reactions. We also found that the two-step
processes give larger contribution to the spin transverse (p,n) reaction than
to the spin longitudinal reaction. This finding is very encouraging to
interpret the discrepancy between the DWIA calculation and the experimental
results of the spin longitudinal and the spin transverse cross sections.Comment: LaTeX, 18 pages, 11 Postscript file
Unified Analysis of Spin Isospin Responses of Nuclei
We investigated the Gamow-Teller (GT) strength distribution, especially the
quenching with respect to the GT sum rule, and the enhancement of the pionic
responses in the quasielasic scattering region, in the same theoretical
framework. That is the continuum random phase approximation with the pi+rho+g'
model interaction, incorporated with distorted wave impulse approximation and
two-step calculations. From this analysis we searched the Landau-Migdal
parameters, g'NN and g'ND, through the comparison with the experimental data of
the GT strength distribution obtained at 300 MeV and the spin-longitudinal
(pionic) cross sections IDq of (p,n) at 350 and 500 MeV. This comprehensive and
sophisticated study gave a common set of g'NN=0.6-0.7 and g'ND=0.2-0.4, for
both low and high momentum transfers.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
Quasi-Elastic Reaction and Spin Response Functions of the Deuteron
We calculated response functions of the deuteron for charge exchange
processes, including the final state interaction between two protons. Using
them we evaluated the double differential cross section and polarization
observables of by means of plane wave impulse
approximation with an optimal factorization. Calculation well reproduced the
shape of the energy spectra of the cross section, though somewhat overestimated
the magnitude. It also reproduced the spin observables well.Comment: 19 pages of LaTeX (4 figures not included, hard copy available upon
request), UT-Komaba 93-2
Long-range ferromagnetic correlations between Anderson impurities in a semiconductor host
We study the two-impurity Anderson model for a semiconductor host using the
quantum Monte Carlo technique. We find that the impurity spins exhibit
ferromagnetic correlations with a range which can be much more enhanced than in
a half-filled metallic band. In particular, the range is longest when the Fermi
level is located above the top of the valence band and decreases as the
impurity bound state becomes occupied. Comparisons with the photoemission and
optical absorption experiments suggest that this model captures the basic
electronic structure of Ga_{1-x}Mn_xAs, the prototypical dilute magnetic
semiconductor (DMS). These numerical results might also be useful for
synthesizing DMS or dilute-oxide ferromagnets with higher Curie temperatures.Comment: 4 pages, 4 figure
Possible d0 ferromagnetism in MgO doped with nitrogen
We study the possibility of d0 ferromagnetism in the compound
MgO doped with nitrogen (N). The Haldane-Anderson impurity model is
formulated within the tight-binding approximation for determining the host
band-structure and the impurity-host hybridization. Using the quantum Monte
Carlo technique, we observe a finite local moment for an N impurity, and
long-range ferromagnetic correlations between two N impurities. The
ferromagnetic correlations are strongly influenced by the impurity bound state.
When the ferromagnetic correlation between a pair of impurities is mapped onto
the isotropic Heisenberg model for two spin-1/2 particles, the effective
exchange constant J12 is found to increase with increasing temperature. Similar
temperature dependence of J12 is also obtained in other diluted magnetic
semiconductors, such as zincblende ZnO doped with Mn. The temperature
dependence of J12 suggests that the mapping of the full Hamiltonian onto the
spin Hamiltonian cannot fully describe the magnetic correlations for the
diluted magnetic semiconductors at least in the limit of low impurity spin.Comment: 8 pages, 8 figure
Pionic Modes Studied by Quasielastic (\vec{p}, \vec{n}) Reactions
It has long been expected that the pionic modes show some collective
phenomena such as the pion condensation in the high density nuclear matter and
its precursor phenomena in the ordinary nuclei. Here we show an evidence of the
precursor observed in the isovector spin longitudinal cross sections ID_q of
the quasielastic 12C, 40Ca (\vec{p}, \vec{n}) reactions at T_p = 346 and 494MeV
with the momentum transfer q = 1.7fm-1. Another aim of this report is to
evaluate the Landau-Migdal parameters g'_{NN}, g'_{N\Delta} and
g'_{\Delta\Delta} at the large momentum region from the above reactions. We
obtained g'_{NN} \approx 0.6-0.7, g'_{N\Delta} \approx 0.3-0.4. The results are
consistent with those at the small momentum region, which are obtained from the
Gamov-Teller strength distribution.Comment: 6 pages, 4 figures, proceedings for 7th International Spring Seminar
on Nuclear Physics "Challenges of Nuclear Structure" at Maiori, Ital
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