962 research outputs found
Quantum limitations on superluminal propagation
Unstable systems such as media with inverted atomic population have been
shown to allow the propagation of analytic wavepackets with group velocity
faster than that of light, without violating causality. We illuminate the
important role played by unstable modes in this propagation, and show that the
quantum fluctuations of these modes, and their unitary time evolution, impose
severe restrictions on the observation of superluminal phenomena.Comment: RevTeX 4 page
Dynamics of Vortex Pair in Radial Flow
The problem of vortex pair motion in two-dimensional plane radial flow is
solved. Under certain conditions for flow parameters, the vortex pair can
reverse its motion within a bounded region. The vortex-pair translational
velocity decreases or increases after passing through the source/sink region,
depending on whether the flow is diverging or converging, respectively. The
rotational motion of two corotating vortexes in a quiescent environment
transforms into motion along a logarithmic spiral in the presence of radial
flow. The problem may have applications in astrophysics and geophysics.Comment: 13 pages, 9 figure
Anomalous bond stretching phonons as a probe of charge fluctuations in perovskites
Important information on momentum resolved low energy charge response can be
extracted from anomalous properties of bond stretching in plane phonons
observed in inelastic neutron and X-ray scattering in cuprates and some other
perovskites. We discuss a semiphenomenological model based on coupling of
phonons to a single charge mode. The phonon dispersion and linewidth allow to
locate the energy of the charge excitation in the mid infrared part of the
spectrum and to determine some of its characteristics. New experiments on
oxygen isotope substitution could allow to achieve a more detailed description.
Corresponding relations following from the model can be used for the
interpretation of experiments and as test of the model.Comment: presented at the M2S-HTSC-VIII conference in Dresde
Quantum coherence of discrete kink solitons in ion traps
We propose to realize quantized discrete kinks with cold trapped ions. We
show that long-lived solitonlike configurations are manifested as deformations
of the zigzag structure in the linear Paul trap, and are topologically
protected in a circular trap with an odd number of ions. We study the
quantum-mechanical time evolution of a high-frequency, gap separated internal
mode of a static kink and find long coherence times when the system is cooled
to the Doppler limit. The spectral properties of the internal modes make them
ideally suited for manipulation using current technology. This suggests that
ion traps can be used to test quantum-mechanical effects with solitons and
explore ideas for the utilization of the solitonic internal-modes as carriers
of quantum information.Comment: 5 pages, 4 figures ; minor correction
Entropy generation in 2+1-dimensional Gravity
The tunneling approach, for entropy generation in quantum gravity, is shown
to be valid when applied to 3-D general relativity. The entropy of de Sitter
and Reissner-Nordstr\"om external event horizons and of the 3-D black hole
obtained by Ba\~nados et. al. is rederived from tunneling of the metric to
these spacetimes. The analysis for spacetimes with an external horizon is
carried out in a complete analogy with the 4-D case. However, we find
significant differences for the black hole. In particular the initial
configuration that tunnels to a 3-D black hole may not to yield an infinitely
degenerate object, as in 4-D Schwarzschild black hole. We discuss the possible
relation to the evaporation of the 3-D black hole.Comment: 22 pages, Tex, TAUP-2102-9
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