51,144 research outputs found
Carrier States and Ferromagnetism in Diluted Magnetic Semiconductors
Applying the dynamical coherent potential approximation to a simple model, we
have systematically studied the carrier states in Mn-type diluted
magnetic semiconductors (DMS's). The model calculation was performed for three
typical cases of DMS's: The cases with strong and moderate exchange
interactions in the absence of nonmagnetic potentials, and the case with strong
attractive nonmagnetic potentials in addition to moderate exchange interaction.
When the exchange interaction is sufficiently strong, magnetic impurity bands
split from the host band. Carriers in the magnetic impurity band mainly stay at
magnetic sites, and coupling between the carrier spin and the localized spin is
very strong. The hopping of the carriers among the magnetic sites causes
ferromagnetism through a {\it double-exchange (DE)-like} mechanism. We have
investigated the condition for the DE-like mechanism to operate in DMS's. The
result reveals that the nonmagnetic attractive potential at the magnetic site
assists the formation of the magnetic impurity band and makes the DE-like
mechanism operative by substantially enhancing the effect of the exchange
interaction. Using conventional parameters we have studied the carrier states
in GaMnAs. The result shows that the ferromagnetism is caused
through the DE-like mechanism by the carriers in the bandtail originating from
the impurity states.Comment: 20 pages, 14 figure
Nuclear Astrophysics
Nuclear astrophysics is that branch of astrophysics which helps understanding
some of the many facets of the Universe through the knowledge of the microcosm
of the atomic nucleus. In the last decades much advance has been made in
nuclear astrophysics thanks to the sometimes spectacular progress in the
modelling of the structure and evolution of the stars, in the quality and
diversity of the astronomical observations, as well as in the experimental and
theoretical understanding of the atomic nucleus and of its spontaneous or
induced transformations. Developments in other sub-fields of physics and
chemistry have also contributed to that advance. Many long-standing problems
remain to be solved, however, and the theoretical understanding of a large
variety of observational facts needs to be put on safer grounds. In addition,
new questions are continuously emerging, and new facts endanger old ideas. This
review shows that astrophysics has been, and still is, highly demanding to
nuclear physics in both its experimental and theoretical components. On top of
the fact that large varieties of nuclei have to be dealt with, these nuclei are
immersed in highly unusual environments which may have a significant impact on
their static properties, the diversity of their transmutation modes, and on the
probabilities of these modes. In order to have a chance of solving some of the
problems nuclear astrophysics is facing, the astrophysicists and nuclear
physicists are obviously bound to put their competence in common, and have
sometimes to benefit from the help of other fields of physics, like particle
physics, plasma physics or solid-state physics.Comment: LaTeX2e with iopart.cls, 84 pages, 19 figures (graphicx package), 374
updated references. Published in Reports on Progress in Physics, vol.62, pp.
395-464 (1999
Modified Spin Wave Analysis of Low Temperature Properties of Spin-1/2 Frustrated Ferromagnetic Ladder
Low temperature properties of the spin-1/2 frustrated ladder with
ferromagnetic rungs and legs, and two different antiferromagnetic next nearest
neighbor interaction are investigated using the modified spin wave
approximation in the region with ferromagnetic ground state. The temperature
dependence of the magnetic susceptibility and magnetic structure factors is
calculated. The results are consistent with the numerical exact diagonalization
results in the intermediate temperature range. Below this temperature range,
the finite size effect is significant in the numerical diagonalization results,
while the modified spin wave approximation gives more reliable results. The low
temperature properties near the limit of the stability of the ferromagnetic
ground state are also discussed.Comment: 9 pages, 8 figure
Integrable Magnetic Model of Two Chains Coupled by Four-Body Interactions
An exact solution for an XXZ chain with four-body interactions is obtained
and its phase diagram is determined. The model can be reduced to two chains
coupled by four-body interactions, and it is shown that the ground state of the
two-chain model is magnetized in part. Furthermore, a twisted four-body
correlation function of the anti-ferromagnetic Heisenberg chain is obtained.Comment: 7 pages, LaTeX, to be published in J. Phys. Soc. Jpn., rederived the
mode
Thermodynamical Bethe Ansatz and Condensed Matter
The basics of the thermodynamic Bethe ansatz equation are given. The simplest
case is repulsive delta function bosons, the thermodynamic equation contains
only one unknown function. We also treat the XXX model with spin 1/2 and the
XXZ model and the XYZ model. This method is very useful for the investigation
of the low temperature thermodynamics of solvable systems.Comment: 52 pages, 6 figures, latex, lamuphys.st
Effective field theory for spinor dipolar Bose Einstein condensates
We show that the effective theory of long wavelength low energy behavior of a
dipolar Bose-Einstein condensate(BEC) with large dipole moments (treated as a
classical spin) can be modeled using an extended Non-linear sigma model (NLSM)
like energy functional with an additional non-local term that represents long
ranged anisotropic dipole-dipole interaction. Minimizing this effective energy
functional we calculate the density and spin-profile of the dipolar
Bose-Einstein condensate in the mean-field regime for various trapping
geometries. The resulting configurations show strong intertwining between the
spin and mass density of the condensate, transfer between spin and orbital
angular momentum in the form of Einstein-de Hass effect, and novel topological
properties. We have also described the theoretical framework in which the
collective excitations around these mean field solutions can be studied and
discuss some examples qualitatively.Comment: Latex + 3 eps figures, accepted for publication in a special issue of
EPJB on "Novel Quantum Phases and Mesoscopic Physics in Quantum Gases
Altitude variations of cosmic-ray soft and hard components observed by airborne detectors
The altitude variations of cosmic-ray total and hard components were measured up to 12,000 m on board a jet liner over Japan island on December, 1981. Observed results together with soft component are presented comparing with the model calculations through the atmosphere by applying the hadronic cascade
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