2,022 research outputs found
Inversion of magnetoresistance in magnetic tunnel junctions : effect of pinhole nanocontacts
Inverse magnetoresistance has been observed in magnetic tunnel junctions with
pinhole nanocontacts over a broad temperature range. The tunnel
magnetoresistance undergoes a change of sign at higher bias and temperature.
This phenomenon is attributed to the competition between the spin conserved
ballistic transport through the pinhole contact where the transmission
probability is close to unity and spin polarized tunneling across the
insulating spacer with weak transmittivity.Comment: Replaced with revised version and new figure, 6 figures, RevTex
Reference manual for the Langley Research Center flight simulation computing system
The researchers at the Langley Research Center Flight Simulation Computing System are provided with an advanced real-time digital simulation capability. This capability is controlled at the user interface level by the Real Time Simulation Supervisor. The Supervisor is a group of subprograms loaded with a simulation application program. The Supervisor provides the interface between the application program and the operating system, and coordinates input and output to and from the simulation hardware. The Supervisor also performs various utility functions as required by a simulation application program
The Milky Way Heart: Investigating molecular gas and gamma-ray morphologies in the Central Molecular Zone
Since the discovery of a broad distribution of very high energy (VHE; >0.1
TeV) gamma-rays in the Central Molecular Zone (CMZ) of the Galaxy in 2006 by
the HESS collaboration, the correlation of this emission with the integrated
intensity of the CS(1-0) molecular line emission has inferred a hadronic origin
for the gamma-rays. Here we describe the beginning of our investigation into
the strength of this correlation utilising new multi-line millimeter data from
the Mopra CMZ and HOP surveys and multi-wavelength GBT radio continuum
observations towards the CMZ and compare these in detail with the diffuse TeV
gamma-ray emission from HESS. The benefit of these new data is that they allow
us to simultaneously observe and analyse correlations using a large number
(>10) of molecular species, some of which contain their isotopologue pairs. The
use of isotopologue pairs is especially powerful, since it allows one to
analyse the optical depth of a number of different molecular species, thus
investigating the nature of the correlation over a range of different physical
conditions. Here we begin by comparing the integrated line emission and
continuum radio emission with the diffuse gamma-ray emission, and, by using
isotopologue pairs such as HCN/HCN, obtain optical depths throughout the
CMZ corresponding to regions of both strong and weak gamma-ray emission. We
find that the radio continuum better matches the peak of the gamma-ray
emission, which corresponds to the more compact -- compared to the relatively
coarse resolution of the gamma-ray images -- sources in the CMZ. Using the
isotopologue pairs, we find that the optical depth at all positions and
velocities within the CMZ are about 2--4. This is similar to that found for the
CS(1--0) line and would underestimate the mass of the CMZ, potentially
explaining why molecular line emission peaks appear offset from the gamma-ray
peaks.Comment: 8 pages, 5 figures. Accepted to the Proceedings of the 25th Texas
Symposium on Relativistic Astrophysics (Heidelberg, 2010
The re-emission spectrum of digital hardware subjected to EMI
The emission spectrum of digital hardware under the influence of external electromagnetic interference is shown to contain information about the interaction of the incident energy with the digital circuits in the system. The generation mechanism of the re-emission spectrum is reviewed, describing how nonlinear effects may be a precursor to the failure of the equipment under test. Measurements on a simple circuit are used to demonstrate how the characteristics of the re-emission spectrum may be correlated with changes to the digital waveform within the circuit. The technique is also applied to a piece of complex digital hardware where Similar, though more subtle, effects can be measured. It is shown that the re-emission spectrum can be used to detect the interaction of the interference with the digital devices at a level well below that which is able to cause static failures in the circuits. The utility of the technique as a diagnostic tool for immunity testing of digital hardware, by identifying which subsystems are being affected by external interference, is also demonstrated
Spontaneous Fluxon Production in Annular Josephson Tunnel Junctions in the Presence of a Magnetic Field
We report on the spontaneous production of fluxons in the presence of a
symmetry-breaking magnetic field for annular Josephson tunnel junctions during
a thermal quench. The dependence on field intensity of the probability
to trap a single defect during the N-S phase transition drastically
depends on the sample circumferences. We show that the data can be understood
in the framework of the Kibble-Zurek picture of spontaneous defect formation
controlled by causal bounds.Comment: Submitted to Phys. Rev. B with 5 figures on Nov. 15, 200
Modeling the gamma-ray emission produced by runaway cosmic rays in the environment of RX J1713.7-3946
Diffusive shock acceleration in supernova remnants is the most widely invoked
paradigm to explain the Galactic cosmic ray spectrum. Cosmic rays escaping
supernova remnants diffuse in the interstellar medium and collide with the
ambient atomic and molecular gas. From such collisions gamma-rays are created,
which can possibly provide the first evidence of a parent population of runaway
cosmic rays. We present model predictions for the GeV to TeV gamma-ray emission
produced by the collisions of runaway cosmic rays with the gas in the
environment surrounding the shell-type supernova remnant RX J1713.7-3946. The
spectral and spatial distributions of the emission, which depend upon the
source age, the source injection history, the diffusion regime and the
distribution of the ambient gas, as mapped by the LAB and NANTEN surveys, are
studied in detail. In particular, we find for the region surrounding RX
J1713-3946, that depending on the energy one is observing at, one may observe
startlingly different spectra or may not detect any enhanced emission with
respect to the diffuse emission contributed by background cosmic rays. This
result has important implications for current and future gamma-ray experiments.Comment: version published on PAS
Molecular Clouds as Cosmic-Ray Barometers
The advent of high sensitivity, high resolution gamma-ray detectors, together
with a knowledge of the distribution of the atomic hydrogen and especially of
the molecular hydrogen in the Galaxy on sub-degree scales creates a unique
opportunity to explore the flux of cosmic rays in the Galaxy. We here present
the new data on the distribution of the molecular hydrogen from a large region
of the inner Galaxy obtained by the NANTEN Collaboration. We then introduce a
methodology which aims to provide a test bed for current and future gamma-ray
observatories to explore the cosmic ray flux at various positions in our
Galaxy. In particular, for a distribution of molecular clouds, as provided by
the NANTEN survey, and local cosmic ray density as measured at the Earth, we
estimate the expected GeV to TeV gamma-ray signal, which can then be compared
with observations and use to test the cosmic ray flux.Comment: PASJ (in press
Molecular Clouds as Cosmic Ray Laboratories
We will here discuss how the gamma-ray emission from molecular clouds can be
used to probe the cosmic ray flux in distant regions of the Galaxy and to
constrain the highly unknown cosmic ray diffusion coefficient. In particular we
will discuss the GeV to TeV emission from runaway cosmic rays penetrating
molecular clouds close to young and old supernova remnants and in molecular
clouds illuminated by the background cosmic ray flux.Comment: to appear on Proceedings of 25th Texas Symposium on Relativistic
Astrophysic
Role of Quantum Confinement in Luminescence Efficiency of Group IV Nanostructures
Experimental results obtained previously for the photoluminescence efficiency
(PL) of Ge quantum dots (QDs) are theoretically studied. A
- plot of PL versus QD diameter () resulted in an
identical slope for each Ge QD sample only when . We
identified that above 6.2 nm: due to a changing
effective mass (EM), while below 4.6 nm: due to
electron/ hole confinement. We propose that as the QD size is initially
reduced, the EM is reduced, which increases the Bohr radius and interface
scattering until eventually pure quantum confinement effects dominate at small
New Experiments for Spontaneous Vortex Formation in Josephson Tunnel Junctions
It has been argued by Zurek and Kibble that the likelihood of producing
defects in a continuous phase transition depends in a characteristic way on the
quench rate. In this paper we discuss an improved experiment for measuring the
Zurek-Kibble scaling exponent for the production of fluxons in
annular symmetric Josephson Tunnel Junctions. We find .
Further, we report accurate measurements of the junction gap voltage
temperature dependence which allow for precise monitoring of the fast
temperature variations during the quench.Comment: 12 pages, 5 figures, submitted to Phys. Rev.
- …