61,828 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
The Free Energy and the Scaling Function of the Ferromagnetic Heisenberg Chain in a Magnetic Field
A nonlinear susceptibilities (the third derivative of a magnetization
by a magnetic field ) of the =1/2 ferromagnetic Heisenberg chain and the
classical Heisenberg chain are calculated at low temperatures In both
chains the nonlinear susceptibilities diverge as and a linear
susceptibilities diverge as The arbitrary spin Heisenberg
ferromagnet has a scaling relation between and
The scaling function
=(2/3)-(44/135) + O() is common to all values of spin
Comment: 16 pages (revtex 2.0) + 6 PS figures upon reques
On the location of two blow up points on an annulus for the mean field equation
We consider the mean field equation on two-dimensional annular domains, and
prove that if and are two blow up points of a blowing-up solution
sequence of the equation, then we must have .Comment: To appear in CRA
Universal low-temperature properties of quantum and classical ferromagnetic chains
We identify the critical theory controlling the universal, low temperature,
macroscopic properties of both quantum and classical ferromagnetic chains. The
theory is the quantum mechanics of a single rotor. The mapping leads to an
efficient method for computing scaling functions to high accuracy.Comment: 4 pages, 2 tables and 3 Postscript 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
Trans-Magnetosonic Accretion in a Black Hole Magnetosphere
We present the critical conditions for hot trans-fast magnetohydrodynamical
(MHD) flows in a stationary and axisymmetric black-hole magnetosphere. To
accrete onto the black hole, the MHD flow injected from a plasma source with
low velocity must pass through the fast magnetosonic point after passing
through the ``inner'' or ``outer'' Alfven point. We find that a trans-fast MHD
accretion solution related to the inner Alfven point is invalid when the
hydrodynamical effects on the MHD flow dominate at the magnetosonic point,
while the other accretion solution related to the outer Alfven point is invalid
when the total angular momentum of the MHD flow is seriously large. When both
regimes of the accretion solutions are valid in the black hole magnetosphere,
we can expect the transition between the two regimes. The variety of these
solutions would be important in many highly energetic astrophysical situations.Comment: 27 pages, 12 figures, accepted to Ap
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