132,398 research outputs found
Large Scale Fluctuations in the X-Ray Background
We present an attempt to measure the large angular scale fluctuations in the
X-Ray Background (XRB) from the HEAO1-A2 data, expressed in terms of spherical
harmonics. We model the harmonic coefficients assuming a power spectrum and an
epoch-dependent bias parameter, and using a phenomenological scenario
describing the evolution of the X-ray sources. From the few low-order
multipoles detected above shot noise, we estimate the power-spectrum
normalization on scales intermediate between those explored by local galaxy
redshift surveys (~ 100 Mpc) and by the COBE Microwave Background measurements
(~ 1000 Mpc). We find that the HEAO1 harmonics are consistent with present
epoch rms fluctuations of the X-ray sources bx(0)sigma8 ~ 1-2 in 8 Mpc spheres.
Therefore the observed fluctuations in the XRB are roughly as expected from
interpolating between the local galaxy surveys and the COBE CMB experiment. We
predict that an X-ray all-sky surface brightness survey resolving sources a
factor of 10 fainter than HEAO1, may reveal fluctuations to significantly
larger angular scales and therefore more strongly constrain the large scale
structure of the Universe on scales of hundreds of Mpcs.Comment: 14 pages, 3 Postscript figures, uses aaspp4.sty and psfig. Revised
following referee's report. Accepted for publication in Ap
Probing entanglement via Rashba-induced shot noise oscillations
We have recently calculated shot noise for entangled and spin-polarized
electrons in novel beam-splitter geometries with a local Rashba s-o interaction
in the incoming leads. This interaction allows for a gate-controlled rotation
of the incoming electron spins. Here we present an alternate simpler route to
the shot noise calculation in the above work and focus on only electron pairs.
Shot noise for these shows continuous bunching and antibunching behaviors. In
addition, entangled and unentangled triplets yield distinctive shot noise
oscillations. Besides allowing for a direct way to identify triplet and singlet
states, these oscillations can be used to extract s-o coupling constants
through noise measurements. Incoming leads with spin-orbit interband mixing
give rise an additional modulation of the current noise. This extra rotation
allows the design of a spin transistor with enhanced spin control.Comment: 7 pages, 3 figures; to appear in the special issue of the Journal of
Superconductivity in honor of E. I. Rashb
Quantum shot-noise at local tunneling contacts on mesoscopic multiprobe conductors
New experiments that measure the low-frequency shot-noise spectrum at local
tunneling contacts on mesoscopic structures are proposed. The current
fluctuation spectrum at a single tunneling tip is determined by local partial
densities of states. The current-correlation spectrum between two tunneling
tips is sensitive to non-diagonal density of states elements which are
expressed in terms of products of scattering states of the conductor. Thus such
an experiment permits to investigate correlations of electronic wave functions.
We present specific results for a clean wire with a single barrier and for
metallic diffusive conductors.Comment: 4 pages REVTeX, 2 figure
Shot noise in normal metal-d-wave superconducting junctions
We present theoretical calculations and predictions for the shot noise in
voltage biased junctions of superconductors and normal metal
counter-electrodes. In the clean limit for the d-wave superconductor the shot
noise vanishes at zero voltage because of resonant Andreev reflection by
zero-energy surface bound states. We examine the sensitivity of this resonance
to impurity scattering. We report theoretical results for the magnetic field
dependence of the shot noise, as well the fingerprints of subdominant - and
pairing channels.Comment: 15 pages, 8 figures and 3 tables embedde
Shot Noise Induced by Nonequilibrium Spin Accumulation
When an electric current passes across a potential barrier, the partition
process of electrons at the barrier gives rise to the shot noise, reflecting
the discrete nature of the electric charge. Here we report the observation of
excess shot noise connected with a spin current which is induced by a
nonequilibrium spin accumulation in an all-semiconductor lateral spin-valve
device. We find that this excess shot noise is proportional to the spin
current. Additionally, we determine quantitatively the spin-injection-induced
electron temperature by measuring the current noise. Our experiments show that
spin accumulation driven shot noise provides a novel means of investigating
nonequilibrium spin transport.Comment: 5 pages and Supplemental Materia
Searching for Large Scale Structure in Deep Radio Surveys
(Abridged Abstract) We calculate the expected amplitude of the dipole and
higher spherical harmonics in the angular distribution of radio galaxies. The
median redshift of radio sources in existing catalogues is z=1, which allows us
to study large scale structure on scales between those accessible to present
optical and infrared surveys, and that of the Cosmic Microwave Background
(CMB). The dipole is due to 2 effects which turn out to be of comparable
magnitude: (i) our motion with respect to the CMB, and (ii) large scale
structure, parameterised here by a family of Cold Dark Matter power-spectra. We
make specific predictions for the Green Bank (87GB) and Parkes-MIT-NRAO (PMN)
catalogues. For these relatively sparse catalogues both the motion and large
scale structure dipole effects are expected to be smaller than the Poisson
shot-noise. However, we detect dipole and higher harmonics in the combined
87GB-PMN catalogue which are far larger than expected. We attribute this to a 2
% flux mismatch between the two catalogues. We also investigate the existence
and extent of the Supergalactic Plane in the above catalogues. In a strip of +-
10 deg of the standard Supergalactic equator, we find a 3-sigma detection in
PMN, but only 1-sigma in 87GB.Comment: 15 pages, 5 ps figures, Latex, Submitted to MNRA
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