4,684 research outputs found
Spin Current and Shot Noise in Single-Molecule Quantum Dots with a Phonon Mode
In this paper we investigate the spin-current and its shot-noise spectrum in
a single-molecule quantum dot coupled with a local phonon mode. We pay special
attention on the effect of phonon on the quantum transport property. The
spin-polarization dependent current is generated by a rotating magnetic filed
applied in the quantum dot. Our results show the remarkable influence of phonon
mode on the zero-frequency shot noise. The electron-phonon interaction leads to
sideband peaks which are located exactly on the integer number of the phonon
frequency and moreover the peak-height is sensitive to the electron-phonon
coupling.Comment: 17 pages,5 figure
T2 and T2⁎ mapping and weighted imaging in cardiac MRI
Cardiac imaging is progressing from simple imaging of heart structure and function to techniques visualizing and measuring underlying tissue biological changes that can potentially define disease and therapeutic options. These techniques exploit underlying tissue magnetic relaxation times: T1, T2 and T2*. Initial weighting methods showed myocardial heterogeneity, detecting regional disease. Current methods are now fully quantitative generating intuitive color maps that do not only expose regionality, but also diffuse changes – meaning that between-scan comparisons can be made to define disease (compared to normal) and to monitor interval change (compared to old scans). T1 is now familiar and used clinically in multiple scenarios, yet some technical challenges remain. T2 is elevated with increased tissue water – edema. Should there also be blood troponin elevation, this edema likely reflects inflammation, a key biological process. T2* falls in the presence of magnetic/paramagnetic materials – practically, this means it measures tissue iron, either after myocardial hemorrhage or in myocardial iron overload. This review discusses how T2 and T2⁎ imaging work (underlying physics, innovations, dependencies, performance), current and emerging use cases, quality assurance processes for global delivery and future research directions
A new powerful method for probing the atmospheres of transiting exoplanets
Although atmospheric transmission spectroscopy of HD209458b with the Hubble
Space Telescope has been very successful, attempts to detect its atmospheric
absorption features using ground-based telescopes have so far been fruitless.
Here we present a new method for probing the atmospheres of transiting
exoplanets which may be more suitable for ground-based observations, making use
of the Rossiter effect. During a transit, an exoplanet sequentially blocks off
light from the approaching and receding parts of the rotating star, causing an
artificial radial velocity wobble. The amplitude of this signal is directly
proportional to the effective size of the transiting object, and the wavelength
dependence of this effect can reveal atmospheric absorption features, in a
similar way as with transmission spectroscopy. The advantage of this method
over conventional atmospheric transmission spectroscopy is that it does not
rely on accurate photometric comparisons of observations on and off transit,
but instead depends on the relative velocity shifts of individual stellar
absorption lines within the same on-transit spectra. We used an archival
VLT/UVES data set to apply this method to HD209458. The amplitude of the
Rossiter effect is shown to be 1.7+-1.2 m/sec higher in the Sodium D lines than
in the weighted average of all other absorption lines in the observed
wavelength range, corresponding to an increment of 4.3+-3% (1.4 sigma). The
uncertainty in this measurement compares to a photometric accuracy of 5e-4 for
conventional atmospheric transmission spectroscopy, more than an order of
magnitude higher than previous attempts using ground-based telescopes.
Observations specifically designed for this method could increase the accuracy
further by a factor 2-3.Comment: LaTex, 5 pages, 4 figs; submitted to MNRAS Letter
Adapting robot paths for automated NDT of complex structures using ultrasonic alignment
Automated inspection systems using industrial robots have been available for several years. The IntACom robot inspection system was developed at TWI Wales and utilizes phased array ultrasonic probes to inspect complex geometries, in particular aerospace composite components. To increase inspection speed and accuracy, off-line path planning is employed to define a series of robotic movements following the surface of a component. To minimize influences of refraction at the component interface and effects of anisotropy, the ultrasonic probe must be kept perpendicular to the surface throughout the inspection. Deviations between the actual component and computer model used for path-planning result in suboptimal alignment and a subsequent reduction in the quality of the ultrasonic echo signal. In this work we demonstrate methods for using the ultrasonic echo signals to adapt a robotic path to achieve a minimal variation in the reflected surface echo. The component surface is imaged using phased array probes to calculate a sparse 3D point cloud with estimated normal directions. This is done through a preliminary alignment path covering approximately 25% of the total surface to minimize the impact on overall inspection time. The data is then compared to the expected geometry and deviations are minimized using least-squares optimization. Compared to manual alignment techniques, this method shows a reduction in surface amplitude variation of up to 32%, indicating that the robot is following the surface of the component more accurately
Yukawa Unification, b --> s gamma and Bino-Stau Coannihilation
The minimal supersymmetric standard model with universal boundary conditions
and "asymptotic" Yukawa unification is considered. The full one-loop effective
potential for radiative electroweak symmetry breaking as well as the one-loop
corrections to the charged Higgs boson, b-quark and tau lepton masses are
included. The CP-even Higgs boson masses are corrected to two-loops. The relic
abundance of the lightest supersymmetric particle (bino) is calculated by
including its coannihilations with the next-to-lightest supersymmetric particle
(lightest stau) consistently with Yukawa unification. The branching ratio of b
--> s gamma is evaluated by incorporating all the applicable next-to-leading
order QCD corrections. The bino-stau coannihilations reduce the bino relic
abundance below the upper bound from cold dark matter considerations in a
sizable fraction of the parameter space allowed by b --> s gamma for mu>0.
Thus, the mu>0 case, which also predicts an acceptable b-quark mass, is
perfectly compatible with data.Comment: 16 pages including 3 figures, Revtex, major revisions are mad
Implications of Halo Inside-out Growth on the X-Ray Properties of Nearby Galaxy Systems within the Preheating Scenario
We present an entirely analytic model for a preheated, polytropic
intergalactic medium in hydrostatic equilibrium within a NFW dark halo
potential in which the evolution of the halo structure between major merger
events proceeds inside-out by accretion. This model is used to explain, within
a standard CDM cosmogony, the observed X-ray properties of nearby
relaxed, non-cooling flow groups and clusters of galaxies. We find that our
preferred solution to the equilibrium equations produces scaling relations in
excellent agreement with observations, while simultaneously accounting for the
typical structural characteristics of the distribution of the diffuse baryons.
In the class of preheating models, ours stands out because it offers a unified
description of the intrahalo medium for galaxy systems with total masses above
\sm 2\times 10^{13}\msun, does not produce baryonic configurations with large
isentropic cores, and reproduces faithfully the observed behavior of the gas
entropy at large radii. All this is achieved with a moderate level of energy
injection of about half a keV, which can be easily accommodated within the
limits of the total energy released by the most commonly invoked feedback
mechanisms, as well as with a polytropic index of 1.2, consistent with both
many observational determinations and predictions from high-resolution
gas-dynamical simulations of non-cooling flow clusters. More interestingly, our
scheme offers a physical motivation for the adoption of this specific value of
the polytropic index, as it is the one that best ensures the conservation after
halo virialization of the balance between the total specific energies of the
gas and dark matter components for the full range of masses investigated.Comment: 18 pages, 11 figures, accepted for publication in the Astrophysical
Journa
Multi-transmission-line-beam interactive system
We construct here a Lagrangian field formulation for a system consisting of
an electron beam interacting with a slow-wave structure modeled by a possibly
non-uniform multiple transmission line (MTL). In the case of a single line we
recover the linear model of a traveling wave tube (TWT) due to J.R. Pierce.
Since a properly chosen MTL can approximate a real waveguide structure with any
desired accuracy, the proposed model can be used in particular for design
optimization. Furthermore, the Lagrangian formulation provides for: (i) a clear
identification of the mathematical source of amplification, (ii) exact
expressions for the conserved energy and its flux distributions obtained from
the Noether theorem. In the case of uniform MTLs we carry out an exhaustive
analysis of eigenmodes and find sharp conditions on the parameters of the
system to provide for amplifying regimes
Spin diffusion and injection in semiconductor structures: Electric field effects
In semiconductor spintronic devices, the semiconductor is usually lightly
doped and nondegenerate, and moderate electric fields can dominate the carrier
motion. We recently derived a drift-diffusion equation for spin polarization in
the semiconductors by consistently taking into account electric-field effects
and nondegenerate electron statistics and identified a high-field diffusive
regime which has no analogue in metals. Here spin injection from a ferromagnet
(FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying
this spin drift-diffusion equation to several typical injection structures such
as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field
regime spin injection from a ferromagnet into a semiconductor is enhanced by
several orders of magnitude. For injection structures with interfacial
barriers, the electric field further enhances spin injection considerably. In
FM/NS/FM structures high electric fields destroy the symmetry between the two
magnets at low fields, where both magnets are equally important for spin
injection, and spin injection becomes locally determined by the magnet from
which carriers flow into the semiconductor. The field-induced spin injection
enhancement should also be insensitive to the presence of a highly doped
nonmagnetic semiconductor (NS) at the FM interface, thus FM/NS/NS
structures should also manifest efficient spin injection at high fields.
Furthermore, high fields substantially reduce the magnetoresistance observable
in a recent experiment on spin injection from magnetic semiconductors
Wireless recording of the calls of Rousettus aegyptiacus and their reproduction using electrostatic transducers
Bats are capable of imaging their surroundings in great detail using echolocation. To apply similar methods to human engineering systems requires the capability to measure and recreate the signals used, and to understand the processing applied to returning echoes. In this work, the emitted and reflected echolocation signals of Rousettus aegyptiacus are recorded while the bat is in flight, using a wireless sensor mounted on the bat. The sensor is designed to replicate the acoustic gain control which bats are known to use, applying a gain to returning echoes that is dependent on the incurred time delay. Employing this technique allows emitted and reflected echolocation calls, which have a wide dynamic range, to be recorded. The recorded echoes demonstrate the complexity of environment reconstruction using echolocation. The sensor is also used to make accurate recordings of the emitted calls, and these calls are recreated in the laboratory using custom-built wideband electrostatic transducers, allied with a spectral equalization technique. This technique is further demonstrated by recreating multi-harmonic bioinspired FM chirps. The ability to record and accurately synthesize echolocation calls enables the exploitation of biological signals in human engineering systems for sonar, materials characterization and imaging
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