4,374 research outputs found
Comments on Black Holes in String Theory
A very brief review is given of some of the developments leading to our
current understanding of black holes in string theory. This is followed by a
discussion of two possible misconceptions in this subject - one involving the
stability of small black holes and the other involving scale radius duality.
Finally, I describe some recent results concerning quasinormal modes of black
holes in anti de Sitter spacetime, and their implications for strongly coupled
conformal field theories (in various dimensions).Comment: 13 pages. Talk given at Strings '99, Potsdam, German
Probing the Continuum Limit in Non-Compact QED: New Results on Large Lattices
We present new Monte Carlo results in non-compact lattice QED with staggered
fermions down to m_0 = 0.005. This extends our previous investigations on the
nature of the continuum limit of QED.Comment: 7 pages, uuencoded compressed PostScript, also available at
ftp://ftp.zib-berlin.de/pub/zib-publications/Reports/SC-94-31.ps.
Generating Spin Currents in Semiconductors with the Spin Hall Effect
We investigate electrically-induced spin currents generated by the spin Hall
effect in GaAs structures that distinguish edge effects from spin transport.
Using Kerr rotation microscopy to image the spin polarization, we demonstrate
that the observed spin accumulation is due to a transverse bulk electron spin
current, which can drive spin polarization nearly 40 microns into a region in
which there is minimal electric field. Using a model that incorporates the
effects of spin drift, we determine the transverse spin drift velocity from the
magnetic field dependence of the spin polarization.Comment: 4 pages, 4 figure
Black Holes with Multiple Charges and the Correspondence Principle
We consider the entropy of near extremal black holes with multiple charges in
the context of the recently proposed correspondence principle of Horowitz and
Polchinski, including black holes with two, three and four Ramond-Ramond
charges. We find that at the matching point the black hole entropy can be
accounted for by massless open strings ending on the D-branes for all cases
except a black hole with four Ramond-Ramond charges, in which case a possible
resolution in terms of brane-antibrane excitations is considered.Comment: 26 pages, harvmac, minor correction
Mechanical control of spin-orbit splitting in GaAs and InGaAs epilayers
Time-resolved Kerr rotation spectroscopy as a function of pump-probe
distance, voltage and magnetic field is used to measure the momentum-dependent
spin splitting energies in GaAs and InGaAs epilayers. The strain of the samples
can be reproducibly controlled in the cryostat using three- and four-point
bending applied with a mechanical vise. We find that the magnitude of the spin
splitting increases linearly with applied tension and voltage. A strain-drift
diffusion model is used to relate the magnitude of the measured spin-orbit
splitting to the amount of strain in the sample.Comment: 4 pages, 5 figure
Tachyon Condensation and Black Strings
We show that under certain conditions, closed string tachyon condensation
produces a topology changing transition from black strings to Kaluza-Klein
"bubbles of nothing." This can occur when the curvature at the horizon is much
smaller than the string scale, so the black string is far from the
correspondence point when it would make a transition to an excited fundamental
string. This provides a dramatic new endpoint to Hawking evaporation. A similar
transition occurs for black p-branes, and can be viewed as a nonextremal
version of a geometric transition. Applications to AdS black holes and the AdS
soliton are also discussed.Comment: 23 pages, 1 figure, v2: references adde
Beyond the veil: Inner horizon instability and holography
We show that scalar perturbations of the eternal, rotating BTZ black hole
should lead to an instability of the inner (Cauchy) horizon, preserving strong
cosmic censorship. Because of backscattering from the geometry, plane wave
modes have a divergent stress tensor at the event horizon, but suitable
wavepackets avoid this difficulty, and are dominated at late times by
quasinormal behavior. The wavepackets have cuts in the complexified coordinate
plane that are controlled by requirements of continuity, single-valuedness and
positive energy. Due to a focusing effect, regular wavepackets nevertheless
have a divergent stress-energy at the inner horizon, signaling an instability.
This instability, which is localized behind the event horizon, is detected
holographically as a breakdown in the semiclassical computation of dual CFT
expectation values in which the analytic behavior of wavepackets in the
complexified coordinate plane plays an integral role. In the dual field theory,
this is interpreted as an encoding of physics behind the horizon in the
entanglement between otherwise independent CFTs.Comment: 40 pages, LaTeX, 3 eps figures, v2: references adde
Microscopic sub-barrier fusion calculations for the neutron star crust
Fusion of very neutron rich nuclei may be important to determine the
composition and heating of the crust of accreting neutron stars. Fusion cross
sections are calculated using time-dependent Hartree-Fock theory coupled with
density-constrained Hartree-Fock calculations to deduce an effective potential.
Systems studied include 16O+16O, 16O+24O, 24O+24O, 12C+16O, and 12C+24O. We
find remarkable agreement with experimental cross sections for the fusion of
stable nuclei. Our simulations use the SLy4 Skyrme force that has been
previously fit to the properties of stable nuclei, and no parameters have been
fit to fusion data. We compare our results to the simple S\~{a}o Paulo static
barrier penetration model. For the asymmetric systems 12C+24O or 16O+24O we
predict an order of magnitude larger cross section than those predicted by the
S\~{a}o Paulo model. This is likely due to the transfer of neutrons from the
very neutron rich nucleus to the stable nucleus and dynamical rearrangements of
the nuclear densities during the collision process. These effects are not
included in potential models. This enhancement of fusion cross sections, for
very neutron rich nuclei, can be tested in the laboratory with radioactive
beams.Comment: 9 pages, 11 figures, corrected small errors in Figs 10, 11, Phys.
Rev. C in pres
On the A-dependence of nuclear generalized parton distributions
We perform a microscopic evaluation of nuclear GPDs for spin-0 nuclei in the
framework of the Walecka model. We demonstrate that the meson (non-nucleon)
degrees of freedom dramatically influence nuclear GPDs, which is revealed in
the non-trivial and unexpected A-dependence of DVCS observables. In particular,
we find that the first moment of the nuclear D-term, d_A(0) ~ A^2.26, which
confirms the earlier prediction of M.Polyakov. We find that in the HERMES
kinematics, contrary to the free proton case, the nuclear meson degrees of
freedom in large nuclei enhance the nuclear DVCS amplitude which becomes
comparable to the Bethe-Heitler amplitude, and, thus, give the non-trivial
A-dependence to the DVCS asymmetries: as a function of the atomic number the
beam-charge asymmetry increases whereas the beam-spin asymmetry decreases
slowly.Comment: Final version published in J. Phys. G. 17 pages, 9 figure
An Equivalence Between Momentum and Charge in String Theory
It is shown that for a translationally invariant solution to string theory,
spacetime duality interchanges the momentum in the symmetry direction and the
axion charge per unit length. As one application, we show explicitly that
charged black strings are equivalent to boosted (uncharged) black strings. The
extremal black strings (which correspond to the field outside of a fundamental
macroscopic string) are equivalent to plane fronted waves describing strings
moving at the speed of light.Comment: 10 page
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