747 research outputs found
Fully Covariant Van Dam-Veltman-Zakharov Discontinuity, and Absence Thereof
In both old and recent literature, it has been argued that the celebrated van
Dam-Veltman-Zakharov (vDVZ) discontinuity of massive gravity is an artifact due
to linearization of the true equations of motion. In this letter, we
investigate that claim. First, we exhibit an explicit -albeit somewhat
arbitrary- fully covariant set of equations of motion that, upon linearization,
reduce to the standard Pauli-Fierz equations. We show that the vDVZ
discontinuity still persists in that non-linear, covariant theory. Then, we
restrict our attention to a particular system that consistently incorporates
massive gravity: the Dvali-Gabadadze-Porrati (DGP) model. DGP is fully
covariant and does not share the arbitrariness and imperfections of our
previous covariantization, and its linearization exhibits a vDVZ discontinuity.
Nevertheless, we explicitly show that the discontinuity does disappear in the
fully covariant theory, and we explain the reason for this phenomenon.Comment: Inconsequential mistakes in Eqs. (12,13,20) corrected. 10 pages,
Latex. To appear in Physics Letters
Criterion of multi-switching stability for magnetic nanoparticles
We present a procedure to study the switching and the stability of an array
of magnetic nanoparticles in the dynamical regime. The procedure leads to the
criterion of multi-switching stability to be satisfied in order to have stable
switching. The criterion is used to compare various magnetic-field-induced
switching schemes, either present in the literature or suggested in the present
work. In particular, we perform micromagnetic simulations to study the
magnetization trajectories and the stability of the magnetization after
switching for nanoparticles of elliptical shape. We evaluate the stability of
the switching as a function of the thickness of the particles and the rise and
fall times of the magnetic pulses, both at zero and room temperature.
Furthermore, we investigate the role of the dipolar interaction and its
influence on the various switching schemes. We find that the criterion of
multi-switching stability can be satisfied at room temperature and in the
presence of dipolar interactions for pulses shaped according to CMOS
specifications, for switching rates in the GHz regime
Aspects of Space-Time Dualities
Duality groups of Abelian gauge theories on four manifolds and their
reduction to two dimensions are considered. The duality groups include elements
that relate different space-times in addition to relating different
gauge-coupling matrices. We interpret (some of) such dualities as the
geometrical symmetries of compactified theories in higher dimensions. In
particular, we consider compactifications of a (self-dual) 2-form in 6-D, and
compactifications of a self-dual 4-form in 10-D. Relations with a self-dual
superstring in 6-D and with the type IIB superstring are discussed.Comment: 10 pages, plain Latex. Minor misprint correcte
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