1,924,902 research outputs found
Newtonian Limit of Conformal Gravity
We study the weak-field limit of the static spherically symmetric solution of
the locally conformally invariant theory advocated in the recent past by
Mannheim and Kazanas as an alternative to Einstein's General Relativity. In
contrast with the previous works, we consider the physically relevant case
where the scalar field that breaks conformal symmetry and generates fermion
masses is nonzero. In the physical gauge, in which this scalar field is
constant in space-time, the solution reproduces the weak-field limit of the
Schwarzschild--(anti)DeSitter solution modified by an additional term that,
depending on the sign of the Weyl term in the action, is either oscillatory or
exponential as a function of the radial distance. Such behavior reflects the
presence of, correspondingly, either a tachion or a massive ghost in the
spectrum, which is a serious drawback of the theory under discussion.Comment: 9 pages, comments and references added; the version to be published
in Phys. Rev.
The TTF finite-energy spectral features in photoemission of TTF-TCNQ: The Hubbard-chain description
A dynamical theory which accounts for all microscopic one-electron processes
is used to study the spectral function of the 1D Hubbard model for the whole
-plane, beyond previous studies which focused on the weight
distribution in the vicinity of the singular branch lines only. While our
predictions agree with those of the latter studies concerning the
tetracyanoquinodimethane (TCNQ) related singular features in photoemission of
the organic compound tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)
metallic phase, the generalized theory also leads to quantitative agreement
concerning the tetrathiafulvalene (TTF) related finite-energy spectral
features, which are found to correspond to a value of the on-site repulsion
larger than for TCNQ. Our study reveals the microscopic mechanisms behind the
unusual spectral features of TTF-TCNQ and provides a good overall description
of those features for the whole -plane.Comment: To appear in Journal of Physics: Condensed Matte
A Comparison of Numerical Methods used for\ud Finite Element Modelling of Soft Tissue\ud Deformation
Soft tissue deformation is often modelled using incompressible nonlinear elasticity, with solutions computed using the finite element method. There are a range of options available when using the finite element method, in particular, the polynomial degree of the basis functions used for interpolating position and pressure, and the type of element making up the mesh. We investigate the effect of these choices on the accuracy of the computed solution, using a selection of model problems motivated by typical deformations seen in soft tissue modelling. We set up model problems with discontinuous material properties (as is the case for the breast), steeply changing gradients in the body force (as found in contracting cardiac tissue), and discontinuous first derivatives in the solution at the boundary, caused by a discontinuous applied force (as in the breast during mammography). We find that the choice of pressure basis functions are vital in the presence of a material interface, higher-order schemes do not perform as well as may be expected when there are sharp gradients, and in general that it is important to take the expected regularity of the solution into account when choosing a numerical scheme
Quantization of the Bianchi type-IX model in supergravity with a cosmological constant
Diagonal Bianchi type-IX models are studied in the quantum theory of supergravity with a cosmological constant. It is shown, by imposing the
supersymmetry and Lorentz quantum constraints, that there are no physical
quantum states in this model. The Friedmann model in supergravity
with cosmological constant does admit quantum states. However, the Bianchi
type-IX model provides a better guide to the behaviour of a generic state,
since more gravitino modes are available to be excited. These results indicate
that there may be no physical quantum states in the full theory of
supergravity with a non-zero cosmological constant. are available to be
excited. These results indicate that there may be no physical quantum states in
the full theory of supergravity with a non-zero cosmological
constant.Comment: 17 pages report DAMTP R93/3
Computer program for determination of natural frequencies of closed spherical sandwich shells
Solutions for the axially symmetric motion of an elastic spherical sandwich shell have been obtained from a theory of shells which includes the effects of transverse shear deformation and rotary inertia. Frequency equations and mode shapes are derived for the full vibrations of a closed spherical shell
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