321 research outputs found
Magnetic Field Geometry of the Broad Line Radio Galaxy 3C111
Very Long Baseline Polarimetric observations of the Broad Line Radio galaxy
3C111 performed in July and September of 1996 at 8 and 43 GHz reveal rapidly
evolving parsec-scale radio structure after a large millimetre outburst. The
B-field geometry is not simple. We present a first analysis of possible Faraday
and optical depth effects based on a comparison of the polarization images for
the two frequencies.Comment: 4 pages, 4 figures, Proceedings of conference "The Physics of
Relativistic Jets in the Chandra and XMM Era", eds. G. Brunetti, D.E. Harris,
R.M. Sambruna, and G. Setti, submitted to New Astronomy Revie
A Proposal and Simulation for Phase-locked THz-QCLs Array by Mutual Injection of the Optical Fields
Tunneling Between Two-Dimensional Electron Gases in a Strong Magnetic Field
We have measured the tunneling between two two-dimensional electron gases at
high magnetic fields , when the carrier densities of the two electron layers
are matched. For filling factors , there is a gap in the current-voltage
characteristics centered about , followed by a tunneling peak at ~mV. Both features have been observed before and have been attributed to
electron-electron interactions within a layer. We have measured high field
tunneling peak positions and fitted gap parameters that are proportional to
, and independent of the carrier densities of the two layers. This suggests
a different origin for the gap to that proposed by current theories, which
predict a dependence.Comment: 9 pages, cond-mat/yymmnn
Re-entrant resonant tunneling
We study the effect of electron-electron interactions on the
resonant-tunneling spectroscopy of the localized states in a barrier. Using a
simple model of three localized states, we show that, due to the Coulomb
interactions, a single state can give rise to two resonant peaks in the
conductance as a function of gate voltage, G(Vg). We also demonstrate that an
additional higher-order resonance with Vg-position in between these two peaks
becomes possibile when interactions are taken into account. The corresponding
resonant-tunneling process involves two-electron transitions. We have observed
both these effects in GaAs transistor microstructures by studying the time
evolution of three adjacent G(Vg) peaks caused by fluctuating occupation of an
isolated impurity (modulator). The heights of the two stronger peaks exibit
in-phase fluctuations. The phase of fluctuations of the smaller middle peak is
opposite. The two stronger peaks have their origin in the same localized state,
and the third one corresponds to a co-tunneling process.Comment: 9 pages, REVTeX, 4 figure
A relativistic model of the radio jets in NGC 315
We apply our intrinsically symmetrical, decelerating relativistic jet model
to deep VLA imaging of the inner 140 arcsec of the giant low-luminosity radio
galaxy NGC 315. An optimized model accurately fits the data in both total
intensity and linear polarization. We infer that the velocity, emissivity and
field structure in NGC 315 are very similar to those of the other
low-luminosity sources we have modelled, but that all of the physical scales
are larger by a factor of about 5. We derive an inclination to the line of
sight of 38 degrees for the jets. Where they first brighten, their on-axis
velocity is approximately v/c = 0.9. They decelerate to v/c = 0.4 between 8 and
18 kpc from the nucleus and the velocity thereafter remains constant. The speed
at the edge of the jet is roughly 0.6 of the on-axis value where it is best
constrained, but the transverse velocity profile may deviate systematically
from the Gaussian form we assume. The proper emissivity profile is split into
three power-law regions separated by shorter transition zones. In the first of
these, at 3 kpc (the flaring point) the jets expand rapidly at constant
emissivity, leading to a large increase in the observed brightness on the
approaching side. At 10 kpc, the emissivity drops abruptly by a factor of 2.
Where the jets are well resolved their rest-frame emission is
centre-brightened. The magnetic field is modelled as random on small scales but
anisotropic and we rule out a globally ordered helical configuration. To a
first approximation, the field evolves from a mixture of longitudinal and
toroidal components to predominantly toroidal, but it also shows variations in
structure along and across the jets, with a significant radial component in
places. Simple adiabatic models fail to fit the emissivity variations.Comment: 20 pages, 17 figures, MNRAS (in press
An observation of spin-valve effects in a semiconductor field effect transistor: a novel spintronic device
We present the first spintronic semiconductor field effect transistor.
The injector and collector contacts of this device were made from magnetic
permalloy thin films with different coercive fields so that they could be
magnetized either parallel or antiparallel to each other in different applied
magnetic fields. The conducting medium was a two dimensional electron gas
(2DEG) formed in an AlSb/InAs quantum well.
Data from this device suggest that its resistance is controlled by two
different types of spin-valve effect: the first occurring at the
ferromagnet-2DEG interfaces; and the second occuring in direct propagation
between contacts.Comment: 4 pages, 2 figure
Modeling and improving the output power of terahertz master-oscillator power-amplifier quantum cascade lasers
A model based on carrier rate equations is proposed to evaluate the gain saturation and predict the dependence of the output power of a terahertz master-oscillator power-amplifier quantum cascade laser (THz-MOPA-QCL) on the material and structure parameters. The model reveals the design rules of the preamplifier and the power extractor to maximize the output power and the wall-plug efficiency. The correction of the model is verified by its agreement with the experiment results. The optimized MOPA devices exhibit single-mode emission at ā¼ 2.6 THz with a side mode suppression ratio of 23 dB, a pulsed output power of 153 mW, a wall-plug efficiency of 0.22%, and a low divergence angle of ā¼6Ā°Ć16Ā°, all measured at an operation temperature of 77 K. The model developed here is helpful for the design of MOPA devices and semiconductor optical amplifiers, in which the active region is based on intersubband transitions
The FourierāKelvin Stellar Interferometer: A Concept for a Practical Interferometric Mission for Discovering and Investigating Extrasolar Giant Planets
The FourierāKelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the nearātoāmidāinfrared spectral region (3 ā 8 Ī¼m). FKSI is conceived as a scientific and technological precursor to TPF. The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will answer key questions about extrasolar planets:ā¢ What are the characteristics of the known extrasolar giant planets?ā¢ What are the characteristics of the extrasolar zodiacal clouds around nearby stars?ā¢ Are there giant planets around classes of stars other than those already studied?We present preliminary results of a detailed design study of the FKSI. Using a nulling interferometer configuration, the optical system consists of two 0.5 m telescopes on a 12.5 m boom feeding a MachāZender beam combiner with a fiber wavefront error reducer to produce a 0.01% null of the central starlight. With this system, planets around nearby stars can be detected and characterized using a combination of spectral and spatial resolution. Ā© 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87917/2/297_1.pd
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