1,066 research outputs found
Exponentially Large Probabilities in Quantum Gravity
The problem of topology change transitions in quantum gravity is investigated
from the Wheeler-de Witt wave function point of view. It is argued that for all
theories allowing wormhole effects the wave function of the universe is
exponentially large. If the wormhole action is positive, one can try to
overcome this difficulty by redefinition of the inner product, while for the
case of negative wormhole action the more serious problems arise.Comment: 9 pages in LaTeX, 4 figures in PostScript, the brief version of this
paper is to appear in Proceedings of the XXIV ITEP Winter School of Physic
Initial Conditions for Semiclassical Field Theory
Semiclassical approximation based on extracting a c-number classical
component from quantum field is widely used in the quantum field theory.
Semiclassical states are considered then as Gaussian wave packets in the
functional Schrodinger representation and as Gaussian vectors in the Fock
representation. We consider the problem of divergences and renormalization in
the semiclassical field theory in the Hamiltonian formulation. Although
divergences in quantum field theory are usually associated with loop Feynman
graphs, divergences in the Hamiltonian approach may arise even at the tree
level. For example, formally calculated probability of pair creation in the
leading order of the semiclassical expansion may be divergent. This observation
was interpretted as an argumentation for considering non-unitary evolution
transformations, as well as non-equivalent representations of canonical
commutation relations at different time moments. However, we show that this
difficulty can be overcomed without the assumption about non-unitary evolution.
We consider first the Schrodinger equation for the regularized field theory
with ultraviolet and infrared cutoffs. We study the problem of making a limit
to the local theory. To consider such a limit, one should impose not only the
requirement on the counterterms entering to the quantum Hamiltonian but also
the requirement on the initial state in the theory with cutoffs. We find such a
requirement in the leading order of the semiclassical expansion and show that
it is invariant under time evolution. This requirement is also presented as a
condition on the quadratic form entering to the Gaussian state.Comment: 20 pages, Plain TeX, one postscript figur
Renormalization of Poincare Transformations in Hamiltonian Semiclassical Field Theory
Semiclassical Hamiltonian field theory is investigated from the axiomatic
point of view. A notion of a semiclassical state is introduced. An "elementary"
semiclassical state is specified by a set of classical field configuration and
quantum state in this external field. "Composed" semiclassical states viewed as
formal superpositions of "elementary" states are nontrivial only if the Maslov
isotropic condition is satisfied; the inner product of "composed" semiclassical
states is degenerate. The mathematical proof of Poincare invariance of
semiclassical field theory is obtained for "elementary" and "composed"
semiclassical states. The notion of semiclassical field is introduced; its
Poincare invariance is also mathematically proved.Comment: LaTeX, 40 pages; short version of hep-th/010307
Fluorescence energy transfer in quantum dot/azo dye complexes in polymer track membranes
Fluorescence resonance energy transfer in complexes of semiconductor CdSe/ZnS quantum dots with molecules of heterocyclic azo dyes, 1-(2-pyridylazo)-2-naphthol and 4-(2-pyridylazo) resorcinol, formed at high quantum dot concentration in the polymer pore track membranes were studied by steady-state and transient PL spectroscopy. The effect of interaction between the complexes and free quantum dots on the efficiency of the fluorescence energy transfer and quantum dot luminescence quenching was found and discussed
Semiclassical interferences and catastrophes in the ionization of Rydberg atoms by half-cycle pulses
A multi-dimensional semiclassical description of excitation of a Rydberg
electron by half-cycle pulses is developed and applied to the study of energy-
and angle-resolved ionization spectra. Characteristic novel phenomena
observable in these spectra such as interference oscillations and semiclassical
glory and rainbow scattering are discussed and related to the underlying
classical dynamics of the Rydberg electron. Modifications to the predictions of
the impulse approximation are examined that arise due to finite pulse
durations
Spin-1 Antiferromagnetic Heisenberg Chains in an External Staggered Field
We present in this paper a nonlinear sigma-model analysis of a spin-1
antiferromagnetic Heisenberg chain in an external commensurate staggered
magnetic field. After rediscussing briefly and extending previous results for
the staggered magnetization curve, the core of the paper is a novel
calculation, at the tree level, of the Green functions of the model. We obtain
precise results for the elementary excitation spectrum and in particular for
the spin gaps in the transverse and longitudinal channels. It is shown that,
while the spectral weight in the transverse channel is exhausted by a single
magnon pole, in the longitudinal one, besides a magnon pole a two-magnon
continuum appears as well whose weight is a stedily increasing function of the
applied field, while the weight of the magnon decreases correspondingly. The
balance between the two is governed by a sum rule that is derived and
discussed. A detailed comparison with the present experimental and numerical
(DMRG) status of the art as well as with previous analytical approaches is also
made.Comment: 23 pages, 3 figures, LaTe
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