7,558 research outputs found
Slow-string limit and "antiferromagnetic" state in AdS/CFT
We discuss a slow-moving limit of a rigid circular equal-spin solution on R x
S^3. We suggest that the solution with the winding number equal to the total
spin approximates the quantum string state dual to the maximal-dimension
``antiferromagnetic'' state of the SU(2) spin chain on the gauge theory side.
An expansion of the string action near this solution leads to a weakly coupled
system of a sine-Gordon model and a free field. We show that a similar
effective Hamiltonian appears in a certain continuum limit from the half-filled
Hubbard model that was recently suggested to describe the all-order dilatation
operator of the dual gauge theory in the SU(2) sector. We also discuss some
other slow-string solutions with one spin component in AdS_5 and one in S^5.Comment: 32 pages, Latex v2: one footnote and references adde
Bosonic sector of the two-dimensional Hubbard model studied within a two-pole approximation
The charge and spin dynamics of the two-dimensional Hubbard model in the
paramagnetic phase is first studied by means of the two-pole approximation
within the framework of the Composite Operator Method. The fully
self-consistent scheme requires: no decoupling, the fulfillment of both Pauli
principle and hydrodynamics constraints, the simultaneous solution of fermionic
and bosonic sectors and a very rich momentum dependence of the response
functions. The temperature and momentum dependencies, as well as the dependency
on the Coulomb repulsion strength and the filling, of the calculated charge and
spin susceptibilities and correlation functions are in very good agreement with
the numerical calculations present in the literature
Mott-Hubbard insulators for systems with orbital degeneracy
We study how the electron hopping reduces the Mott-Hubbard band gap in the
limit of a large Coulomb interaction U and as a function of the orbital
degeneracy N. The results support the conclusion that the hopping contribution
grows as roughly \sqrt{N}W, where W is the one-particle band width, but in
certain models a crossover to a \sim NW behavior is found for a sufficiently
large N.Comment: 7 pages, revtex, 6 figures more information at
http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
Assisted hopping and interaction effects in impurity models
We study, using Numerical Renormalization Group methods, the generalization
of the Anderson impurity model where the hopping depends on the filling of the
impurity. We show that the model, for sufficiently large values of the assisted
hopping term, shows a regime where local pairing correlations are enhanced.
These correlations involve pairs fluctuating between on site and nearest
neighbor positions
A Study of the Antiferromagnetic Phase in the Hubbard Model by means of the Composite Operator Method
We have investigated the antiferromagnetic phase of the 2D, the 3D and the
extended Hubbard models on a bipartite cubic lattice by means of the Composite
Operator Method within a two-pole approximation. This approach yields a fully
self-consistent treatment of the antiferromagnetic state that respects the
symmetry properties of both the model and the algebra. The complete phase
diagram, as regards the antiferromagnetic and the paramagnetic phases, has been
drawn. We firstly reported, within a pole approximation, three kinds of
transitions at half-filling: Mott-Hubbard, Mott-Heisenberg and Heisenberg. We
have also found a metal-insulator transition, driven by doping, within the
antiferromagnetic phase. This latter is restricted to a very small region near
half filling and has, in contrast to what has been found by similar approaches,
a finite critical Coulomb interaction as lower bound at half filling. Finally,
it is worth noting that our antiferromagnetic gap has two independent
components: one due to the antiferromagnetic correlations and another coming
from the Mott-Hubbard mechanism.Comment: 20 pages, 37 figures, RevTeX, submitted to Phys. Rev.
Tone-activated, remote, alert communication system
Pocket sized transmitter, frequency modulated by crystal derived tones, with integral loop antenna provides police with easy operating alert signal communicator which uses patrol car radio to relay signal. Communication channels are time shared by several patrol units
Metal-Insulator transition in the Generalized Hubbard model
We present the exact ground-state wave function and energy of the generalized
Hubbard model, subjected to the condition that the number of double occupied
sites is conserved, for a wide, physically relevant range of parameters. For
one hole and one double occupied site the existence of the ferromagnetic
ground-state is proved which allow one to determine the critical value of the
on-site repulsion corresponding to the point of metal-insulator transition. For
the one dimensional model the exact solution for special values of the
parameters is obtained.Comment: 20 pages, LaTex. Mod.Phys.Lett.B 7 (1993) 1397; Journal of Physics:
Condensed Matter (to appear
Turbulent dynamo with advective magnetic helicity flux
Many astrophysical bodies harbor magnetic fields that are thought to be
sustained by a dynamo process. However, it has been argued that the production
of large-scale magnetic fields by mean-field dynamo action is strongly
suppressed at large magnetic Reynolds numbers owing to the conservation of
magnetic helicity. This phenomenon is known as {\it catastrophic quenching}.
Advection of magnetic fields by stellar and galactic winds toward the outer
boundaries and away from the dynamo is expected to alleviate such quenching.
Here we explore the relative roles played by advective and turbulent--diffusive
fluxes of magnetic helicity in the dynamo. In particular, we study how the
dynamo is affected by advection. We do this by performing direct numerical
simulations of a turbulent dynamo of type driven by forced
turbulence in a Cartesian domain in the presence of a flow away from the
equator where helicity changes sign. Our results indicate that in the presence
of advection, the dynamo, otherwise stationary, becomes oscillatory. We confirm
an earlier result for turbulent--diffusive magnetic helicity fluxes that for
small magnetic Reynolds numbers (\Rm\lesssim 100...200, based on the
wavenumber of the energy-carrying eddies) the magnetic helicity flux scales
less strongly with magnetic Reynolds number (\Rm^{-1/2}) than the term
describing magnetic helicity destruction by resistivity (\Rm^{-1}). Our new
results now suggest that for larger \Rm the former becomes approximately
independent of \Rm, while the latter falls off more slowly. We show for the
first time that both for weak and stronger winds, the magnetic helicity flux
term becomes comparable to the resistive term for \Rm\gtrsim 1000, which is
necessary for alleviating catastrophic quenching.Comment: 9 pages, 9 figures, accepted for publication in MNRA
Theoretical Radii of Transiting Giant Planets: The Case of OGLE-TR-56b
We calculate radius versus age trajectories for the photometrically-selected
transiting extrasolar giant planet, OGLE-TR-56b, and find agreement between
theory and observation, without introducing an ad hoc extra source of heat in
its core. The fact that the radius of HD209458b seems larger than the radii of
the recently discovered OGLE family of extremely close-in transiting planets
suggests that HD209458b is anomalous. Nevertheless, our good fit to OGLE-TR-56b
bolsters the notion that the generic dependence of transit radii on stellar
irradiation, mass, and age is, to within error bars, now quantitatively
understood.Comment: 11 pages, 1 figure, submitted to the Astrophysical Journa
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