3,860 research outputs found
Pseudo Jahn-Teller Effect In The Origin Of Enhanced Flexoelectricity
The controversy between the theory and experiment in explaining the origin of enhanced flexoelectricity is removed by taking into account the pseudo Jahn-Teller effect (PJTE) which, under certain conditions, creates local dipolar distortions of dynamic nature, resonating between two or more equivalent orientations. The latter become nonequivalent under a strain gradient thus producing enhanced flexoelectricity: it is much easier to orient ready-made dipoles than to polarize an ionic solid. For BaTiO3, the obtained earlier numerical data for the adiabatic potential energy surface in the space of dipolar displacements in the Ti centers were used to estimate the flexoelectric coefficient integral in the paraelectric phase in a one-dimensional model with the strain gradient along the [111] direction: integral = -0.43 X 10(-6) Cm-1. This eliminates the huge contradiction between the experimental data of integral similar to mu Cm-1 for this case and the theoretical predictions (without the PJTE) of 3-4 orders-of-magnitude smaller values. Enhanced flexoelectricity is thus expected in solids with a sufficient density of centers that have PJTE induced dipolar instabilities. It explains also the origin of enhanced flexoelectricity observed in other solids, noticeable containing Nb perovskite centers which are known to have a PJTE instability, similar to that of Ti centers. The SrTiO3 crystal as a virtual ferroelectric in which the strain gradient eases the condition of PJTE polar instability is also discussed. (C) 2015 AIP Publishing LLC.Institute for Theoretical Chemistr
Effective Lagrangian from Higher Curvature Terms: Absence of vDVZ Discontinuity in AdS Space
We argue that the van Dam-Veltman-Zakharov discontinuity arising in the limit of the massive graviton through an explicit Pauli-Fierz mass term
could be absent in anti de Sitter space. This is possible if the graviton can
acquire mass spontaneously from the higher curvature terms or/and the massless
limit is attained faster than the cosmological constant . We discuss the effects of higher-curvature couplings and of an explicit
cosmological term () on stability of such continuity and of massive
excitations.Comment: 23 pages, Latex, the version to appear in Class. Quant. Gra
On chaotic behavior of gravitating stellar shells
Motion of two gravitating spherical stellar shells around a massive central
body is considered. Each shell consists of point particles with the same
specific angular momenta and energies. In the case when one can neglect the
influence of gravitation of one ("light") shell onto another ("heavy") shell
("restricted problem") the structure of the phase space is described. The
scaling laws for the measure of the domain of chaotic motion and for the
minimal energy of the light shell sufficient for its escape to infinity are
obtained.Comment: e.g.: 12 pages, 8 figures, CHAOS 2005 Marc
Generalized parity transformations in the regularized Chern-Simons theory
We study renormalization effects in the Abelian Chern-Simons (CS) action.
These effects can be non-trivial when the gauge field is coupled to dynamical
matter, since the regularization of the UV divergences in the model forces the
introduction of a parity even piece in the gauge field action. This changes the
classical (odd) transformation properties of the pure CS action. This effect,
already discussed for the case of a lattice regularization by F. Berruto, M.C.
Diamantini and P. Sodano in hep-th/0004203, is also present when the theory is
defined in the continuum and, indeed, it is a manifestation of a more general
`anomalous' effect, since it happens for every regularization scheme. We
explore the physical consequences of this anomaly. We also show that
generalized, nonlocal parity transformations can be defined in such a way that
the regularized theory is odd, and that those transformations tend to the usual
ones when the cutoff is removed. These generalized transformations play a role
that is tantamount to the deformed symmetry corresponding to Ginsparg-Wilson
fermions [2] (in an even number of spacetime dimensions).Comment: 16 pages, LaTeX, references added and typos correcte
Spherically symmetric spacetimes in massive gravity
We explore spherically symmetric stationary solutions, generated by ``stars''
with regular interiors, in purely massive gravity. We reexamine the claim that
the resummation of non-linear effects can cure, in a domain near the source,
the discontinuity exhibited by the linearized theory as the mass m of the
graviton tends to zero. First, we find analytical difficulties with this claim,
which appears not to be robust under slight changes in the form of the mass
term. Second, by numerically exploring the inward continuation of the class of
asymptotically flat solutions, we find that, when m is ``small'', they all end
up in a singularity at a finite radius, well outside the source, instead of
joining some conjectured ``continuous'' solution near the source. We reopen,
however, the possibility of reconciling massive gravity with phenomenology by
exhibiting a special class of solutions, with ``spontaneous symmetry breaking''
features, which are close, near the source, to general relativistic solutions
and asymptote, for large radii, a de Sitter solution of curvature ~m^2.Comment: 57 pages, references addde
Magnetic Z(N) symmetry in 2+1 dimensions
This review describes the role of magnetic symmetry in 2+1 dimensional gauge
theories. In confining theories without matter fields in fundamental
representation the magnetic symmetry is spontaneously broken. Under some mild
assumptions, the low-energy dynamics is determined universally by this
spontaneous breaking phenomenon. The degrees of freedom in the effective theory
are magnetic vortices. Their role in confining dynamics is similar to that
played by pions and sigma in the chiral symmetry breaking dynamics.
I give an explicit derivation of the effective theory in (2+1)-dimensional
weakly coupled confining models and argue that it remains qualitatively the
same in strongly coupled (2+1)-dimensional gluodynamics. Confinement in this
effective theory is a very simple classical statement about the long range
interaction between topological solitons, which follows (as a result of a
simple direct classical calculation) from the structure of the effective
Lagrangian. I show that if fundamentally charged dynamical fields are present
the magnetic symmetry becomes local rather than global. The modifications to
the effective low energy description in the case of heavy dynamical fundamental
matter are discussed. This effective lagrangian naturally yields a bag like
description of baryonic excitations. I also discuss the fate of the magnetic
symmetry in gauge theories with the Chern-Simons term
Compact QED3 with theta term and axionic confining strings
We discuss three dimensional compact QED with a theta term due to an axionic
field. The variational gauge invariant functional is considered and it is shown
that the ground state energy is independent of theta in a leading
approximation. The mass gap of the axionic field is found to be dependent upon
theta, the mass gap of the photon field and the scalar potential. The vacuum
expectation of the Wilson loop is shown to be independent of theta in a leading
approximation, to obey the area law and to lead to confinement. We also briefly
discuss the properties of axionic confining strings.Comment: 35 pages, LaTex, typing error correcte
Difficulties in Inducing a Gauge Theory at Large N
It is argued that the recently proposed Kazakov-Migdal model of induced gauge
theory, at large , involves only the zero area Wilson loops that are
effectively trees in the gauge action induced by the scalars. This retains only
a constant part of the gauge action excluding plaquettes or anything like them
and the gauge variables drop out.Comment: 6 pages, Latex, AZPH-TH/93-01, COLO-HEP/30
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