4,736 research outputs found
Nonlinear viscoelasticity of freestanding and polymer-anchored vertically aligned carbon nanotube foams
Vertical arrays of carbon nanotubes (VACNTs) show unique mechanical behavior in compression, with a highly nonlinear response similar to that of open cell foams and the ability to recover large deformations. Here, we study the viscoelastic response of both freestanding VACNT arrays and sandwich structures composed of a VACNT array partially embedded between two layers of poly(dimethylsiloxane) (PDMS) and bucky paper. The VACNTs tested are ∼2 mm thick foams grown via an injection chemical vapor deposition method. Both freestanding and sandwich structures exhibit a time-dependent behavior under compression. A power-law function of time is used to describe the main features observed in creep and stress-relaxation tests. The power-law exponents show nonlinear viscoelastic behavior in which the rate of creep is dependent upon the stress level and the rate of stress relaxation is dependent upon the strain level. The results show a marginal effect of the thin PDMS/bucky paper layers on the viscoelastic responses. At high strain levels (ɛ = 0.8), the peak stress for the anchored CNTs reaches ∼45 MPa, whereas it is only ∼15 MPa for freestanding CNTs, suggesting a large effect of PDMS on the structural response of the sandwich structures
Dynamic scaling of fronts in the quantum XX chain
The dynamics of the transverse magnetization in the zero-temperature XX chain
is studied with emphasis on fronts emerging from steplike initial magnetization
profiles. The fronts move with fixed velocity and display a staircase like
internal structure whose dynamic scaling is explored both analytically and
numerically. The front region is found to spread with time sub-diffusively with
the height and the width of the staircase steps scaling as t^(-1/3) and t^1/3,
respectively. The areas under the steps are independent of time, thus the
magnetization relaxes in quantized "steps" of spin-flips.Comment: 4 pages, 3 eps figures, RevTe
Quaternionic and Octonionic Spinors. A Classification
Quaternionic and octonionic realizations of Clifford algebras and spinors are
classified and explicitly constructed in terms of recursive formulas. The most
general free dynamics in arbitrary signature space-times for both quaternionic
and octonionic spinors is presented. In the octonionic case we further provide
a systematic list of results and tables expressing, e.g., the relations of the
octonionic Clifford algebras with the cosets over the Lorentz algebras,
the identities satisfied by the higher-rank antisymmetric octonionic tensors
and so on. Applications of these results range from the classification of
octonionic generalized supersymmetries, the construction of octonionic
superstrings, as well as the investigations concerning the recently discovered
octonionic -superalgebra and its superconformal extension.Comment: 24 pages, LaTe
Simple algebras of Weyl type
Over a field of any characteristic, for a commutative associative algebra
with an identity element and for the polynomial algebra of a
commutative derivation subalgebra of , the associative and the Lie
algebras of Weyl type on the same vector space are
defined. It is proved that , as a Lie algebra (modular its center) or as
an associative algebra, is simple if and only if is -simple and
acts faithfully on . Thus a lot of simple algebras are obtained.Comment: 9 pages, Late
Marginal Extended Perturbations in Two Dimensions and Gap-Exponent Relations
The most general form of a marginal extended perturbation in a
two-dimensional system is deduced from scaling considerations. It includes as
particular cases extended perturbations decaying either from a surface, a line
or a point for which exact results have been previously obtained. The
first-order corrections to the local exponents, which are functions of the
amplitude of the defect, are deduced from a perturbation expansion of the
two-point correlation functions. Assuming covariance under conformal
transformation, the perturbed system is mapped onto a cylinder. Working in the
Hamiltonian limit, the first-order corrections to the lowest gaps are
calculated for the Ising model. The results confirm the validity of the
gap-exponent relations for the perturbed system.Comment: 11 pages, Plain TeX, eps
Cosmic strings in axionic-dilatonic gravity
We first consider local cosmic strings in dilaton-axion gravity and show that
they are singular solutions. Then we take a supermassive Higgs limit and
present expressions for the fields at far distances from the core by applying a
Pecci-Quinn and a duality transformation to the dilatonic Melvin's magnetic
universe.Comment: Latex file. 16 page
Fermionization and Hubbard Models
We introduce a transformation which allows the fermionization of operators of
any one-dimensional spin-chain. This fermionization procedure is independent of
any eventual integrable structure and is compatible with it. We illustrate this
method on various integrable and non-integrable chains, and deduce some general
results. In particular, we fermionize XXC spin-chains and study their
symmetries. Fermionic realizations of certain Lie algebras and superalgebras
appear naturally as symmetries of some models. We also fermionize recently
obtained Hubbard models, and obtain for the first time multispecies analogues
of the Hubbard model, in their fermionic form. We comment on the conflict
between symmetry enhancement and integrability of these models. Finally, the
fermionic versions of the non integrable spin-1 and spin-3/2 Heisenberg chains
are obtained.Comment: 24 pages, Latex. Minor typos corrected, one equation adde
Gauge theories as a geometrical issue of a Kaluza-Klein framework
We present a geometrical unification theory in a Kaluza-Klein approach that
achieve the geometrization of a generic gauge theory bosonic component.
We show how it is possible to derive the gauge charge conservation from the
invariance of the model under extra-dimensional translations and to geometrize
gauge connections for spinors, thus we can introduce the matter just by free
spinorial fields. Then, we present the applications to i)a pentadimensional
manifold , so reproducing the original Kaluza-Klein theory,
unless some extensions related to the rule of the scalar field contained in the
metric and the introduction of matter by spinors with a phase dependence from
the fifth coordinate, ii)a seven-dimensional manifold , in which we geometrize the electro-weak model by
introducing two spinors for any leptonic family and quark generation and a
scalar field with two components with opposite hypercharge, responsible of
spontaneous symmetry breaking.Comment: 37 pages, no figure
Plane Light-Like Shells and Impulsive Gravitational Waves in Scalar-Tensor Theories of Gravity
We study gravitational plane impulsive waves and electromagnetic shock waves
in a scalar-tensor theory of gravity of the Brans-Dicke type. In vacuum, we
present an exact solution of Brans-Dicke's field equations and give an example
in which a plane impulsive gravitational wave and a null shell of matter
coexist on the same hypersurface. In the homogenous case, we characterize them
by their surface energy density and wave amplitude and discuss the inhomogenous
case. We also give an exact solution of the Brans-Dicke's field equations in
the electrovacuum case which admits a true curvature singularity and use it to
built an example where a plane impulsive gravitational wave and an
electromagnetic shock wave have the same null hypersurface as history of their
wave fronts and propagate independently and decoupled from a null shell of
matter. This last solution is shown to correspond to the space-time describing
the interaction region resulting from the collision of two electromagnetic
shock waves leading to the formation of two gravitational impulsive waves. The
properties of this solution are discussed and compared to those of the
Bell-Szekeres solution of general relativity.Comment: 19 pages, latex, 1 figure, accepted for publication in Class. Quant.
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