310 research outputs found
Trapped surfaces in spherical expanding open universes
Consider spherically symmetric initial data for a cosmology which, in the
large, approximates an open Friedmann-Lema{\^\i}tre
universe. Further assume that the data is chosen so that the trace of the
extrinsic curvature is a constant and that the matter field is at rest at this
instant of time. One expects that no trapped surfaces appear in the data if no
significant clump of excess matter is to be found. This letter confirms this
belief by displaying a necessary condition for the existence of trapped
surfaces.This necessary condition, simply stated, says that a relatively large
amount of excess matter must be concentrated in a small volume for trapped
surfaces to appear.Comment: 8 pages, Late
Schwarzschild horizon and the gravitational redshift formula
The gravitational redshift formula is usually derived in the geometric optics
approximation. In this note we consider an exact formulation of the problem in
the Schwarzschild space-time, with the intention to clarify under what
conditions this redshift law is valid. It is shown that in the case of shocks
the radial component of the Poynting vector can scale according to the redshift
formula, under a suitable condition. If that condition is not satisfied, then
the effect of the backscattering can lead to significant modifications. The
obtained results imply that the energy flux of the short wavelength radiation
obeys the standard gravitational redshift formula while the energy flux of long
waves can scale differently, with redshifts being dependent on the frequency.Comment: Revtex, 5 p. Rewritten Sec. II, minor changes in Secs III - VII. To
appear in the Classical and Quantum Gravit
Saturation of Spin-Polarized Current in Nanometer Scale Aluminum Grains
We describe measurements of spin-polarized tunnelling via discrete energy
levels of single Aluminum grains. In high resistance samples (),
the spin-polarized tunnelling current rapidly saturates as a function of the
bias voltage. This indicates that spin-polarized current is carried only via
the ground state and the few lowest in energy excited states of the grain. At
the saturation voltage, the spin-relaxation rate of the highest
excited states is comparable to the electron tunnelling rate: and in two samples. The ratio of
to the electron-phonon relaxation rate is in agreement with the Elliot-Yafet
scaling, an evidence that spin-relaxation in Al grains is governed by the
spin-orbit interaction.Comment: 5 pages, 4 figure
Transport in Graphene Tunnel Junctions
We present a technique to fabricate tunnel junctions between graphene and Al
and Cu, with a Si back gate, as well as a simple theory of tunneling between a
metal and graphene. We map the differential conductance of our junctions versus
probe and back gate voltage, and observe fluctuations in the conductance that
are directly related to the graphene density of states. The conventional
strong-suppression of the conductance at the graphene Dirac point can not be
clearly demonstrated, but a more robust signature of the Dirac point is found:
the inflection in the conductance map caused by the electrostatic gating of
graphene by the tunnel probe. We present numerical simulations of our
conductance maps, confirming the measurement results. In addition, Al causes
strong n-doping of graphene, Cu causes a moderate p-doping, and in high
resistance junctions, phonon resonances are observed, as in STM studies.Comment: 22 pages, 5 figure
Electronic Properties of Clean Au-Graphene Contacts
The effects of Au grains on graphene conduction and doping are investigated
in this report. To obtain a clean Au-graphene contact, Au grains are deposited
over graphene at elevated temperature and in high vacuum, before any chemical
processing. The bulk and the effective contact resistance versus gate voltage
demonstrate that Au grains cause p-doping in graphene. The Fermi level shift is
in agreement with first principles calculations, but the equilibrium separation
we find between the graphene and the top-most Au layer is larger than
predicted. Nonequilibrium electron transport displays giant-phonon thresholds
observed in graphene tunnel junctions, demonstrating the tunneling nature of
the contact, even though there are no dielectrics involved.Comment: 11 pages, 4 figure
Modelling Electron Spin Accumulation in a Metallic Nanoparticle
A model describing spin-polarized current via discrete energy levels of a
metallic nanoparticle, which has strongly asymmetric tunnel contacts to two
ferromagnetic leads, is presented.
In absence of spin-relaxation, the model leads to a spin-accumulation in the
nanoparticle, a difference () between the chemical potentials of
spin-up and spin-down electrons, proportional to the current and the Julliere's
tunnel magnetoresistance. Taking into account an energy dependent
spin-relaxation rate , as a function of bias
voltage () exhibits a crossover from linear to a much weaker dependence,
when equals the spin-polarized current through the
nanoparticle. Assuming that the spin-relaxation takes place via electron-phonon
emission and Elliot-Yafet mechanism, the model leads to a crossover from linear
to dependence. The crossover explains recent measurements of the
saturation of the spin-polarized current with in Aluminum nanoparticles,
and leads to the spin-relaxation rate of in an Aluminum
nanoparticle of diameter , for a transition with an energy difference of
one level spacing.Comment: 37 pages, 7 figure
Can Schwarzschildean gravitational fields suppress gravitational waves?
Gravitational waves in the linear approximation propagate in the
Schwarzschild spacetime similarly as electromagnetic waves. A fraction of the
radiation scatters off the curvature of the geometry. The energy of the
backscattered part of an initially outgoing pulse of the quadrupole
gravitational radiation is estimated by compact formulas depending on the
initial energy, the Schwarzschild radius, and the location and width of the
pulse. The backscatter becomes negligible in the short wavelength regime.Comment: 18 pages, Revtex. Added three references; a new comment in Sec. 7;
several misprints corrected. To appear in the Phys. Rev.
Collapsing Shells and the Isoperimetric Inequality for Black Holes
Recent results of Trudinger on Isoperimetric Inequalities for non-convex
bodies are applied to the gravitational collapse of a lightlike shell of matter
to form a black hole. Using some integral identities for co-dimension two
surfaces in Minkowski spacetime, the area of the apparent horizon is shown
to be bounded above in terms of the mass by the , which is
consistent with the Cosmic Censorship Hypothesis. The results hold in four
spacetime dimensions and above.Comment: 16 pages plain TE
Waves in Schwarzschild spacetimes: How strong can imprints of the spacetime curvature be
Misprints corrected, two references added. To appear in the Phys. Rev. D
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