230,158 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 $A_{1-x}$Mn$_xB$-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 Ga$_{1-x}$Mn$_x$As. 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

### Role of Large Gluonic Excitation Energy for Narrow Width of Penta-Quark Baryons in QCD String Theory

We study the narrow decay width of low-lying penta-quark baryons in the QCD
string theoryin terms of gluonic excitations. In the QCD string theory, the
penta-quark baryon decays via a gluonic-excited state of a baryon and meson
system, where a pair of Y-shaped junction and anti-junction is created. Since
lattice QCD shows that the lowest gluonic-excitation energy takes a large value
of about 1 GeV, the decay of the penta-quark baryon near the threshold is
considered as a quantum tunneling process via a highly-excited state (a
gluonic-excited state) in the QCD string theory. This mechanism strongly
suppresses the decay and leads to an extremely narrow decay width of the
penta-quark system.Comment: Talk given at International Conference on the Structure of Baryons
(Baryons 04) October 25 - 29, 2004, Ecole Polytechnique, Palaiseau, Franc

### Takahashi Integral Equation and High-Temperature Expansion of the Heisenberg Chain

Recently a new integral equation describing the thermodynamics of the 1D
Heisenberg model was discovered by Takahashi. Using the integral equation we
have succeeded in obtaining the high temperature expansion of the specific heat
and the magnetic susceptibility up to O((J/T)^{100}). This is much higher than
those obtained so far by the standard methods such as the linked-cluster
algorithm. Our results will be useful to examine various approximation methods
to extrapolate the high temperature expansion to the low temperature region.Comment: 5 pages, 4 figures, 2 table

### Universal correlations of one-dimensional interacting electrons in the gas phase

We consider dynamical correlation functions of short range interacting
electrons in one dimension at finite temperature. Below a critical value of the
chemical potential there is no Fermi surface anymore, and the system can no
longer be described as a Luttinger liquid. Its low temperature thermodynamics
is that of an ideal gas. We identify the impenetrable electron gas model as a
universal model for the gas phase and present exact and explicit expressions
for the asymptotics of correlation functions at small temperatures, in the
presence of a magnetic field.Comment: 4 pages, Revte

### Anisotropic radiation field and trapped photons around the Kerr black hole

Aims. In order to understand the anisotropic properties of local radiation
field in the curved spacetime around a rotating black hole, we investigate the
appearance of a black hole seen by an observer located near the black hole.
When the black hole is in front of a source of illumination the black hole cast
shadow in the illumination. Accordingly, the appearance of the black hole is
called the black hole shadow.
Methods. We first analytically describe the shape of the shadow in terms of
constants of motion for a photon seen by the observer in the locally
non-rotating reference frame (LNRF). Then, we newly derive the useful equation
for the solid angle of the shadow. In a third step, we can easily plot the
apparent image of the black hole shadow. Finally, we also calculate the ratio
of the photon trapped by the hole and the escape photon to the distant region
for photons emitted near the black hole.
Results. From the shape and the size of the black hole shadow, we can
understand the signatures of the curved spacetime; i.e., the mass and spin of
the black hole. Our equations for the solid angle of the shadow has technical
advantages in calculating the photon trapping ratio. That is, this equation is
computationally very easy, and gives extremely precise results. This is because
this equation is described by the one-parameter integration with given values
of the spin and location for the black hole considered. After this, the solid
angle can be obtained without numerical calculations of the null geodesics for
photons.Comment: Accepted for publication in A&

### Estimating a War of Attrition: The Case of the U.S. Movie Theater Industry

This paper provides a tractable empirical framework to analyze firm behavior in a dynamic oligopoly when demand is declining over time. I modify Fudenberg and Tirole (1986).s model of exit in a duopoly with incomplete information to a model that can be used in an oligopoly, and combine this with an auxiliary entry model to address the initial conditions problem. I estimate this model with panel data on the U.S. movie theater industry from 1949 to 1955, using variations in TV diffusion rates across households, market structure before the exit game starts, and other market characteristics to identify the parameters in the theater’s payoff function and the distribution of unobservable fixed costs. Using the estimated model, I measure strategic delays in the exit process due to oligopolistic competition and incomplete information. The delay in exit that arises from strategic interaction is 2.7 years on average. Out of these years, 3.7% of this delay is accounted for by incomplete information, while the remaining 96.3% is explained by oligopolistic competition

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