982 research outputs found
Magnetic phase diagram of the Hubbard model with next-nearest-neighbour hopping
We calculate the magnetic phase diagram of the Hubbard model for a Bethe
lattice with nearest neighbour (NN) hopping and next nearest neighbour
(NNN) hopping in the limit of infinite coordination. We use the amplitude
of the NNN hopping to tune the density of states (DOS) of the
non-interacting system from a situation with particle-hole symmetry to an
asymmetric one with van-Hove singularities at the lower ()
respectively upper () band edge for large enough . For
this strongly asymmetric situation we find rather extended parameter regions
with ferromagnetic states and regions with antiferromagnetic states.Comment: 13 pages, 7 figure
Supersymmetric quantum cosmology for Bianchi class A models
The canonical theory of (N=1) supergravity, with a matrix representation for
the gravitino covector-spinor, is applied to the Bianchi class A spatially
homogeneous cosmologies. The full Lorentz constraint and its implications for
the wave function of the universe are analyzed in detail. We found that in this
model no physical states other than the trivial "rest frame" type occur.Comment: 10 pages, Revte
On a class of invariant coframe operators with application to gravity
Let a differential 4D-manifold with a smooth coframe field be given. Consider
the operators on it that are linear in the second order derivatives or
quadratic in the first order derivatives of the coframe, both with coefficients
that depend on the coframe variables. The paper exhibits the class of operators
that are invariant under a general change of coordinates, and, also, invariant
under the global SO(1,3)-transformation of the coframe. A general class of
field equations is constructed. We display two subclasses in it. The subclass
of field equations that are derivable from action principles by free variations
and the subclass of field equations for which spherical-symmetric solutions,
Minkowskian at infinity exist. Then, for the spherical-symmetric solutions, the
resulting metric is computed. Invoking the Geodesic Postulate, we find all the
equations that are experimentally (by the 3 classical tests) indistinguishable
from Einstein field equations. This family includes, of course, also Einstein
equations. Moreover, it is shown, explicitly, how to exhibit it. The basic tool
employed in the paper is an invariant formulation reminiscent of Cartan's
structural equations. The article sheds light on the possibilities and
limitations of the coframe gravity. It may also serve as a general procedure to
derive covariant field equations
Hidden Consequence of Active Local Lorentz Invariance
In this paper we investigate a hidden consequence of the hypothesis that
Lagrangians and field equations must be invariant under active local Lorentz
transformations. We show that this hypothesis implies in an equivalence between
spacetime structures with several curvature and torsion possibilities.Comment: Some misprints appearing in the published version have been correcte
Nonclassical effects in a driven atoms/cavity system in the presence of arbitrary driving field and dephasing
We investigate the photon statistics of light transmitted from a driven
optical cavity containing one or two atoms interacting with a single mode of
the cavity field. We treat arbitrary driving fields with emphasis on departure
from previous weak field results. In addition effects of dephasing due to
atomic transit through the cavity mode are included using two different models.
We find that both models show the nonclassical correlations are quite sensitive
to dephasing. The effect of multiple atoms on the system dynamics is
investigated by placing two atoms in the cavity mode at different positions,
therefore having different coupling strengths.Comment: 8 pages, 10 figures, minor typographical errors corrected, submitted
to Phys Rev
Reionization Constraints on the Contribution of Primordial Compact Objects to Dark Matter
Many lines of evidence suggest that nonbaryonic dark matter constitutes
roughly 30% of the critical closure density, but the composition of this dark
matter is unknown. One class of candidates for the dark matter is compact
objects formed in the early universe, with typical masses M between 0.1 and 1
solar masses to correspond to the mass scale of objects found with microlensing
observing projects. Specific candidates of this type include black holes formed
at the epoch of the QCD phase transition, quark stars, and boson stars. Here we
show that accretion onto these objects produces substantial ionization in the
early universe, with an optical depth to Thomson scattering out to z=1100 of
approximately tau=2-4 [f_CO\epsilon_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where
\epsilon_{-1} is the accretion efficiency \epsilon\equiv L/{\dot M}c^2 divided
by 0.1 and f_CO is the fraction of matter in the compact objects. The current
upper limit to the scattering optical depth, based on the anisotropy of the
microwave background, is approximately 0.4. Therefore, if accretion onto these
objects is relatively efficient, they cannot be the main component of
nonbaryonic dark matter.Comment: 12 pages including one figure, uses aaspp4, submitted to Ap
A statistical model for the intrinsically broad superconducting to normal transition in quasi-two-dimensional crystalline organic metals
Although quasi-two-dimensional organic superconductors such as
-(BEDT-TTF)Cu(NCS) seem to be very clean systems, with apparent
quasiparticle mean-free paths of several thousand \AA, the superconducting
transition is intrinsically broad (e.g K wide for K).
We propose that this is due to the extreme anisotropy of these materials, which
greatly exacerbates the statistical effects of spatial variations in the
potential experienced by the quasiparticles. Using a statistical model, we are
able to account for the experimental observations. A parameter , which
characterises the spatial potential variations, may be derived from
Shubnikov-de Haas oscillation experiments. Using this value, we are able to
predict a transition width which is in good agreement with that observed in MHz
penetration-depth measurements on the same sample.Comment: 8 pages, 2 figures, submitted to J. Phys. Condens. Matte
Flow equations for QED in the light front dynamics
The method of flow equations is applied to QED on the light front. Requiring
that the partical number conserving terms in the Hamiltonian are considered to
be diagonal and the other terms off-diagonal an effective Hamiltonian is
obtained which reduces the positronium problem to a two-particle problem, since
the particle number violating contributions are eliminated. No infrared
divergencies appear. The ultraviolet renormalization can be performed
simultaneously.Comment: 15 pages, Latex, 3 pictures, Submitted to Phys.Rev.
Towards complete integrability of two dimensional Poincar\'e gauge gravity
It is shown that gravity on the line can be described by the two dimensional
(2D) Hilbert-Einstein Lagrangian supplemented by a kinetic term for the coframe
and a translational {\it boundary} term. The resulting model is equivalent to a
Yang-Mills theory of local {\it translations} and frozen Lorentz gauge degrees.
We will show that this restricted Poincar\'e gauge model in 2 dimensions is
completely integrable. {\it Exact} wave, charged black hole, and `dilaton'
solutions are then readily found. In vacuum, the integrability of the {\it
general} 2D Poincar\'e gauge theory is formally proved along the same line of
reasoning.Comment: 35 pages, report Cologne-thp-1993-H
de Sitter group and Einstein-Hilbert Lagrangian
Axial vector torsion in the Einstein-Cartan space is considered here.
By picking a particular term from the SO(4,1) Pontryagin density and then
modifying it in a SO(3,1) invariant way, we get a Lagrangian density with
Lagrange multipliers. Then considering torsion and torsion-less connection as
independent fields, it has been found that and of
Einstein-Hilbert Lagrangian, appear as integration constants in such a way that
has been found to be linked with the topological Nieh-Yan density of
space.Comment: 14 page
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