2,960 research outputs found
Symmetry of high-piezoelectric Pb-based complex perovskites at the morphotropic phase boundary II. Theoretical treatment
The structural characteristics of the perovskite- based ferroelectric
Pb(Zn1/3Nb2/3)O3-9%PbTiO3 at the morphotropic phase boundary (MPB) region
(x≃0.09) have been analyzed. The analysis is based on the symmetry
adapted free energy functions under the assumption that the total polarization
and the unit cell volume are conserved during the transformations between
various morphotropic phases. Overall features of the relationships between the
observed lattice constants at various conditions have been consistently
explained. The origin of the anomalous physical properties at MPB is discussed
Comment on "On the -Anomaly in Betaine Calcium Chloride Dihydrate"
Recently, Hlinka and Ishibashi [J. Phys. Soc. Jpn. 67, 495 (1998)] discussed
the -anomaly in betaine calcium chloride dihydrate (BCCD) in a Landau-type
approach. We comment on the shortcomings of this approach and discuss the
-anomaly in the framework of a microscopical pseudo spin model based on a
realistic description of BCCD in terms of symmetry-adapted local modes.Comment: 2 pages, RevTex, submitted to J. Phys. Soc. Jp
Universality of Nonperturbative Effects in c<1 Noncritical String Theory
Nonperturbative effects in c<1 noncritical string theory are studied using
the two-matrix model. Such effects are known to have the form fixed by the
string equations but the numerical coefficients have not been known so far.
Using the method proposed recently, we show that it is possible to determine
the coefficients for (p,q) string theory. We find that they are indeed finite
in the double scaling limit and universal in the sense that they do not depend
on the detailed structure of the potential of the two-matrix model.Comment: 17 page
2D Yang-Mills Theory as a Matrix String Theory
Quantization of two-dimensional Yang-Mills theory on a torus in the gauge
where the field strength is diagonal leads to twisted sectors that are
completely analogous to the ones that originate long string states in Matrix
String Theory. If these sectors are taken into account the partition function
is different from the standard one found in the literature and the invariance
of the theory under modular transformations of the torus appears to hold in a
stronger sense. The twisted sectors are in one-to-one correspondence with the
coverings of the torus without branch points, so they define by themselves a
string theory. A possible duality between this string theory and the
Gross-Taylor string is discussed, and the problems that one encounters in
generalizing this approach to interacting strings are pointed out. This talk is
based on a previous paper by the same authors, but it contains some new results
and a better interpretation of the results already obtained.Comment: 11 pages, LaTeX, 2 figures included with epsf. Talk presented at the
2nd Conference on Quantum aspects of Gauge Theories, Supersymmetry and
Unification, Corfu, Greece, 21-26 September 199
Naked Singularity and Thunderbolt
We consider quantum theoretical effects of the sudden change of the boundary
conditions which mimics the occurrence of naked singularities. For a simple
demonstration, we study a massless scalar field in -dimensional
Minkowski spacetime with finite spatial interval. We calculate the vacuum
expectation value of the energy-momentum tensor and explicitly show that
singular wave or {\em thunderbolt} appears along the Cauchy horizon. The
thunderbolt possibly destroys the Cauchy horizon if its backreaction on the
geometry is taken into account, leading to quantum restoration of the global
hyperbolicity. The result of the present work may also apply to the situation
that a closed string freely oscillating is traveling to a brane and changes
itself to an open string pinned-down by the ends satisfying the Dirichlet
boundary conditions on the brane.Comment: 12 pages, 1 figure, references added, to appear in Phys. Rev.
Black-Hole Bombs and Photon-Mass Bounds
Generic extensions of the standard model predict the existence of ultralight
bosonic degrees of freedom. Several ongoing experiments are aimed at detecting
these particles or constraining their mass range. Here we show that massive
vector fields around rotating black holes can give rise to a strong
superradiant instability which extracts angular momentum from the hole. The
observation of supermassive spinning black holes imposes limits on this
mechanism. We show that current supermassive black hole spin estimates provide
the tightest upper limits on the mass of the photon (mv<4x10^{-20} eV according
to our most conservative estimate), and that spin measurements for the largest
known supermassive black holes could further lower this bound to mv<10^{-22}
eV. Our analysis relies on a novel framework to study perturbations of rotating
Kerr black holes in the slow-rotation regime, that we developed up to second
order in rotation, and that can be extended to other spacetime metrics and
other theories.Comment: 5 pages, 2 figures. References added. Matches published versio
Dynamics in Stationary, Non-Globally Hyperbolic Spacetimes
Classically, the dynamics in a non-globally hyperbolic spacetime is ill
posed. Previously, a prescription was given for defining dynamics in static
spacetimes in terms of a second order operator acting on a Hilbert space
defined on static slices. The present work extends this result by giving a
similar prescription for defining dynamics in stationary spacetimes obeying
certain mild assumptions. The prescription is defined in terms of a first order
operator acting on a different Hilbert space from the one used in the static
prescription. It preserves the important properties of the earlier one: the
formal solution agrees with the Cauchy evolution within the domain of
dependence, and smooth data of compact support always give rise to smooth
solutions. In the static case, the first order formalism agrees with second
order formalism (using specifically the Friedrichs extension). Applications to
field quantization are also discussed.Comment: 18 pages, 1 figure, AMSLaTeX; v2: expanded discussion of field
quantization, new Proposition 3.1, revised Theorem 4.2, corrected typos, and
updated reference
Quantum mechanics without spacetime II : noncommutative geometry and the free point particle
In a recent paper we have suggested that a formulation of quantum mechanics
should exist, which does not require the concept of time, and that the
appropriate mathematical language for such a formulation is noncommutative
differential geometry. In the present paper we discuss this formulation for the
free point particle, by introducing a commutation relation for a set of
noncommuting coordinates. The sought for background independent quantum
mechanics is derived from this commutation relation for the coordinates. We
propose that the basic equations are invariant under automorphisms which map
one set of coordinates to another- this is a natural generalization of
diffeomorphism invariance when one makes a transition to noncommutative
geometry. The background independent description becomes equivalent to standard
quantum mechanics if a spacetime manifold exists, because of the proposed
automorphism invariance. The suggested basic equations also give a quantum
gravitational description of the free particle.Comment: 8 page
- …