796 research outputs found
Carboranylmethylene-substituted phosphazenes and polymers thereof
Carboranylmethylene-substituted cyclophosphazenes are described which can be thermally polymerized into carboranylmethylene-substituted phosphazene polymers. The polymers are useful as thermally stable coatings. Also, due to the characteristics of these polymers in acting as a ligand for transition metals, metalocarboranylmethylene phosphazene polymers are described which can act as immobilized catalyst systems, and are electrically conductive and superconductive
Composite absorbing potentials
The multiple scattering interferences due to the addition of several
contiguous potential units are used to construct composite absorbing potentials
that absorb at an arbitrary set of incident momenta or for a broad momentum
interval.Comment: 9 pages, Revtex, 2 postscript figures. Accepted in Phys. Rev. Let
Quantum Time and Spatial Localization: An Analysis of the Hegerfeldt Paradox
Two related problems in relativistic quantum mechanics, the apparent
superluminal propagation of initially localized particles and dependence of
spatial localization on the motion of the observer, are analyzed in the context
of Dirac's theory of constraints. A parametrization invariant formulation is
obtained by introducing time and energy operators for the relativistic particle
and then treating the Klein-Gordon equation as a constraint. The standard,
physical Hilbert space is recovered, via integration over proper time, from an
augmented Hilbert space wherein time and energy are dynamical variables. It is
shown that the Newton-Wigner position operator, being in this description a
constant of motion, acts on states in the augmented space. States with strictly
positive energy are non-local in time; consequently, position measurements
receive contributions from states representing the particle's position at many
times. Apparent superluminal propagation is explained by noting that, as the
particle is potentially in the past (or future) of the assumed initial place
and time of localization, it has time to propagate to distant regions without
exceeding the speed of light. An inequality is proven showing the Hegerfeldt
paradox to be completely accounted for by the hypotheses of subluminal
propagation from a set of initial space-time points determined by the quantum
time distribution arising from the positivity of the system's energy. Spatial
localization can nevertheless occur through quantum interference between states
representing the particle at different times. The non-locality of the same
system to a moving observer is due to Lorentz rotation of spatial axes out of
the interference minimum.Comment: This paper is identical to the version appearing in J. Math. Phys.
41; 6093 (Sept. 2000). The published version will be found at
http://ojps.aip.org/jmp/. The paper (40 page PDF file) has been completely
revised since the last posting to this archiv
Process for the preparation of polycarboranylphosphazenes
A process for the preparation of polycarboranylphosphazenes is described. Polydihalophosphazenes are allowed to react at ambient temperatures for at least one hour with a lithium carborane in a suitable inert solvent. The remaining chlorine substituents of the carboranyl polyphosphazene are then replaced with aryloxy or alkoxy groups to enhance moisture resistance. The polymers give a high char yield when exposed to extreme heat and flame and can be used as insulation
A measurement-based approach to quantum arrival times
For a quantum-mechanically spread-out particle we investigate a method for
determining its arrival time at a specific location. The procedure is based on
the emission of a first photon from a two-level system moving into a
laser-illuminated region. The resulting temporal distribution is explicitly
calculated for the one-dimensional case and compared with axiomatically
proposed expressions. As a main result we show that by means of a deconvolution
one obtains the well known quantum mechanical probability flux of the particle
at the location as a limiting distribution.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
Carboranylcyclotriphosphazenes and their polymers
Carboranyl-substituted polyphosphazenes are prepared by heat polymerizing a carboranyl halocyclophosphazene at 250 C for about 120 hours in the absence of oxygen and moisture. The cyclophosphazene is obtained by allowing a lithium carborane, e.g., the reaction product of methyl-o-carborane with n-butyllithium in ethyl ether, to react with e.g., hexachlorocyclotriphosphazene at ambient temperatures and in anhydrous conditions. For greater stability in the presence of moisture, the chlorine substituents of the polymer are then replaced by aryloxy or alkoxy groups, such as CF3CH2O. The new substantially inorganic polymers are thermally stable materials which produce a high char yield when exposed to extreme temperatures, and can thus serve to insulate less heat and fire resistant substances
Ambiguities of arrival-time distributions in quantum theory
We consider the definition that might be given to the time at which a
particle arrives at a given place, both in standard quantum theory and also in
Bohmian mechanics. We discuss an ambiguity that arises in the standard theory
in three, but not in one, spatial dimension.Comment: LaTex, 12 pages, no figure
Quantum times of arrival for multiparticle states
Using the concept of crossing state and the formalism of second quantization,
we propose a prescription for computing the density of arrivals of particles
for multiparticle states, both in the free and the interacting case. The
densities thus computed are positive, covariant in time for time independent
hamiltonians, normalized to the total number of arrivals, and related to the
flux. We investigate the behaviour of this prescriptions for bosons and
fermions, finding boson enhancement and fermion depletion of arrivals.Comment: 10 a4 pages, 5 inlined figure
Space-time properties of free motion time-of-arrival eigenstates
The properties of the time-of-arrival operator for free motion introduced by
Aharonov and Bohm and of its self-adjoint variants are studied. The domains of
applicability of the different approaches are clarified. It is shown that the
arrival time of the eigenstates is not sharply defined. However, strongly
peaked real-space (normalized) wave packets constructed with narrow Gaussian
envelopes centred on one of the eigenstates provide an arbitrarily sharp
arrival time.Comment: REVTEX, 12 pages, 4 postscript figure
Probability distribution of arrival times in quantum mechanics
In a previous paper [V. Delgado and J. G. Muga, Phys. Rev. A 56, 3425 (1997)]
we introduced a self-adjoint operator whose eigenstates
can be used to define consistently a probability distribution of the time of
arrival at a given spatial point. In the present work we show that the
probability distribution previously proposed can be well understood on
classical grounds in the sense that it is given by the expectation value of a
certain positive definite operator which is nothing but a
straightforward quantum version of the modulus of the classical current. For
quantum states highly localized in momentum space about a certain momentum , the expectation value of becomes indistinguishable
from the quantum probability current. This fact may provide a justification for
the common practice of using the latter quantity as a probability distribution
of arrival times.Comment: 21 pages, LaTeX, no figures; A Note added; To be published in Phys.
Rev.
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