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 $^{12}$C, $^{40}$Ca(p,p') scatterings and the spin longitudinal and the spin transverse cross sections of the $^{12}$C,$^{40}$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 d(p⃗,n⃗)2pd(\vec{p},\vec{n})2p 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 $d(\vec{p},\vec{n})2p$ 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