2,541 research outputs found
Collapse of Kaluza-Klein Bubbles
Kaluza-Klein theory admits ``bubble" configurations, in which the
circumference of the fifth dimension shrinks to zero on some compact surface. A
three parameter family of such bubble initial data at a moment of time-symmetry
(some including a magnetic field) has been found by Brill and Horowitz,
generalizing the (zero-energy) ``Witten bubble" solution. Some of these data
have negative total energy. We show here that all the negative energy bubble
solutions start out expanding away from the moment of time symmetry, while the
positive energy bubbles can start out either expanding or contracting. Thus it
is unlikely that the negative energy bubbles would collapse and produce a naked
singularity.Comment: 6 pages, plain LaTeX, UMDGR-94-08
Viscoelastic response of sonic band-gap materials
A brief report is presented on the effect of viscoelastic losses in a high
density contrast sonic band-gap material of close-packed rubber spheres in air.
The scattering properties of such a material are computed with an on-shell
multiple scattering method, properties which are compared with the lossless
case. The existence of an appreciable omnidirectional gap in the transmission
spectrum, when losses are present, is also reported.Comment: 5 pages, 4 figures, submitted to PR
Black Holes and Wormholes in 2+1 Dimensions
A large variety of spacetimes---including the BTZ black holes---can be
obtained by identifying points in 2+1 dimensional anti-de Sitter space by means
of a discrete group of isometries. We consider all such spacetimes that can be
obtained under a restriction to time symmetric initial data and one asymptotic
region only. The resulting spacetimes are non-eternal black holes with
collapsing wormhole topologies. Our approach is geometrical, and we discuss in
detail: The allowed topologies, the shape of the event horizons, topological
censorship and trapped curves.Comment: 23 pages, LaTeX, 11 figure
Fate of Kaluza-Klein Bubble
We numerically study classical time evolutions of Kaluza-Klein bubble
space-time which has negative energy after a decay of vacuum. As the zero
energy Witten's bubble space-time, where the bubble expands infinitely, the
subsequent evolutions of Brill and Horowitz's momentarily static initial data
show that the bubble will expand in terms of the area. At first glance, this
result may support Corley and Jacobson's conjecture that the bubble will expand
forever as well as the Witten's bubble. The irregular signatures, however, can
be seen in the behavior of the lapse function in the maximal slicing gauge and
the divergence of the Kretchman invariant. Since there is no appearance of the
apparent horizon, we suspect an appearance of a naked singularity as the final
fate of this space-time.Comment: 13 pages including 10 figures, RevTeX, epsf.sty. CGPG-99/12-8,
RESCEU-6/00 and DAMTP-2000-30. To appear in Phys. Rev.
Thermal Diffusivities of Functionalized Pentacene Semiconductors
We have measured the interlayer and in-plane (needle axis) thermal
diffusivities of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). The
needle axis value is comparable to the phonon thermal conductivities of
quasi-one dimensional organic metals with excellent pi-orbital overlap, and its
value suggests that a significant fraction of heat is carried by optical
phonons. Furthermore, the interlayer (c-axis) thermal diffusivity is at least
an order of magnitude larger, and this unusual anisotropy implies very strong
dispersion of optical modes in the interlayer direction, presumably due to
interactions between the silyl-containing side groups. Similar values for both
in-plane and interlayer diffusivities have been observed for several other
functionalized pentacene semiconductors with related structures.Comment: 9 pages, including 4 figures; submitted to Applied Physics Letter
The effect of stimulation frequency on the transmural ventricular monophasic action potential in yellowfin tuna Thunnus albacares
Monophasic action potentials (MAPs) were recorded from the spongy and compact layers of the yellowfin tuna Thunnus albacares ventricle as stimulation frequency was increased. MAP duration decreased with increase in stimulation frequency in both the spongy and compact myocardial layers, but no significant difference in MAP duration was observed between the layers
Holographic fermions in external magnetic fields
We study the Fermi level structure of 2+1-dimensional strongly interacting
electron systems in external magnetic field using the AdS/CFT correspondence.
The gravity dual of a finite density fermion system is a Dirac field in the
background of the dyonic AdS-Reissner-Nordstrom black hole. In the probe limit
the magnetic system can be reduced to the non-magnetic one, with
Landau-quantized momenta and rescaled thermodynamical variables. We find that
at strong enough magnetic fields, the Fermi surface vanishes and the
quasiparticle is lost either through a crossover to conformal regime or through
a phase transition to an unstable Fermi surface. In the latter case, the
vanishing Fermi velocity at the critical magnetic field triggers the non-Fermi
liquid regime with unstable quasiparticles and a change in transport properties
of the system. We associate it with a metal-"strange metal" phase transition.
Next we compute the DC Hall and longitudinal conductivities using the
gravity-dressed fermion propagators. For dual fermions with a large charge,
many different Fermi surfaces contribute and the Hall conductivity is quantized
as expected for integer Quantum Hall Effect (QHE). At strong magnetic fields,
as additional Fermi surfaces open up, new plateaus typical for the fractional
QHE appear. The somewhat irregular pattern in the length of fractional QHE
plateaus resemble the outcomes of experiments on thin graphite in a strong
magnetic field. Finally, motivated by the absence of the sign problem in
holography, we suggest a lattice approach to the AdS calculations of finite
density systems.Comment: 34 pages, 14 figure
Quantum Fermion Hair
It is shown that the Dirac operator in the background of a magnetic
%Reissner-Nordstr\"om black hole and a Euclidean vortex possesses normalizable
zero modes in theories containing superconducting cosmic strings. One
consequence of these zero modes is the presence of a fermion condensate around
magnetically charged black holes which violates global quantum numbers.Comment: 16pp (harvmac (l)) and 2 figs.(not included
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