8,560 research outputs found
Lateral spin-orbit interaction and spin polarization in quantum point contacts
We study ballistic transport through semiconductor quantum point contact
systems under different confinement geometries and applied fields. In
particular, we investigate how the {\em lateral} spin-orbit coupling,
introduced by asymmetric lateral confinement potentials, affects the spin
polarization of the current. We find that even in the absence of external
magnetic fields, a variable {\em non-zero spin polarization} can be obtained by
controlling the asymmetric shape of the confinement potential. These results
suggest a new approach to produce spin polarized electron sources and we study
the dependence of this phenomenon on structural parameters and applied magnetic
fields. This asymmetry-induced polarization provides also a plausible
explanation of our recent observations of a 0.5 conductance plateau (in units
of ) in quantum point contacts made on InAs quantum-well structures.
Although our estimates of the required spin-orbit interaction strength in these
systems do not support this explanation, they likely play a role in the effects
enhanced by electron-electron interactions.Comment: Summited to PRB (2009
Radiation environment along the INTEGRAL orbit measured with the IREM monitor
The INTEGRAL Radiation Environment Monitor (IREM) is a payload supporting
instrument on board the INTEGRAL satellite. The monitor continually measures
electron and proton fluxes along the orbit and provides this information to the
spacecraft on board data handler. The mission alert system broadcasts it to the
payload instruments enabling them to react accordingly to the current radiation
level. Additionally, the IREM conducts its autonomous research mapping the
Earth radiation environment for the space weather program. Its scientific data
are available for further analysis almost without delay.Comment: 5 pages, 7 figures, accepted for publication in A+A letter
Anomalous magneto-oscillations and spin precession
A semiclassical analysis based on concepts developed in quantum chaos reveals
that anomalous magneto-oscillations in quasi two-dimensional systems with
spin-orbit interaction reflect the non-adiabatic spin precession of a classical
spin vector along the cyclotron orbits.Comment: 4 pages, 2 figure
Shuttle time and frequency transfer experiment
A proposed space shuttle experiment to demonstrate techniques for global high precision comparison of clocks and primary frequency standards is described. The experiment, using transmitted microwave and pulsed laser signals, compared a hydrogen maser clock onboard the space shuttle with a clock in a ground station in order to demonstrate time transfer with accuracies of 1 nsec or better and frequency comparison at the 10 to the -14th power accuracy level
Wavefunction considerations for the central spin decoherence problem in a nuclear spin bath
Decoherence of a localized electron spin in a solid state material (the
``central spin'' problem) at low temperature is believed to be dominated by
interactions with nuclear spins in the lattice. This decoherence is partially
suppressed through the application of a large magnetic field that splits the
energy levels of the electron spin and prevents depolarization. However,
dephasing decoherence resulting from a dynamical nuclear spin bath cannot be
removed in this way. Fluctuations of the nuclear field lead to uncertainty of
the electron's precessional frequency in a process known as spectral diffusion.
This article considers the effect of the electron's wavefunction shape upon
spectral diffusion and provides wavefunction dependent decoherence time
formulas for free induction decay as well as spin echoes and concatenated
dynamical decoupling schemes for enhancing coherence. We also discuss dephasing
of a qubit encoded in singlet-triplet states of a double quantum dot. A central
theoretical result of this work is the development of a continuum approximation
for the spectral diffusion problem which we have applied to GaAs and InAs
materials specifically
X-ray observations of RX J0822-4300 and Puppis-A
Based on observations with the X-ray observatories Chandra and XMM-Newton we
present results from a detailed spectro-imaging and timing analysis of the
central compact X-ray source RX J0822-4300 in the supernova remnant Puppis-A.
The superior angular resolution of Chandra allows for the first time to
pinpoint the point source nature of this object down to arcsec
(FWHM) and to determine its position:{RA=,
Dec= (J2000)} with sub-arcsecond accuracy.
Spectral fits based on Chandra and XMM-Newton data provide a tight constraint
on the emission properties of RX J0822-4300. Most of its X-ray emission seems
to be of thermal origin. A model spectrum consisting of two blackbody
components with K,
K and km, km for the blackbody temperatures
and the size of the projected emitting regions, respectively, provides the best
model description of its spectrum. A search for X-ray pulsations from RX
J0822-4300, revealed an interesting periodicity candidate which, if confirmed,
does not support a scenario of steady spin-down.Comment: Accepted by A&A, 20 pages, 10 figures, 3 table
High-accuracy global time and frequency transfer with a space-borne hydrogen maser clock
A proposed system for high-accuracy global time and frequency transfer using a hydrogen maser clock in a space vehicle is discussed. Direct frequency transfer with a accuracy of 10 to the minus 14th power and time transfer with an estimated accuracy of 1 nsec are provided by a 3-link microwave system. A short pulse laser system is included for subnanosecond time transfer and system calibration. The results of studies including operational aspects, error sources, data flow, system configuration, and implementation requirements for an initial demonstration experiment using the Space Shuttle are discussed
Real space first-principles derived semiempirical pseudopotentials applied to tunneling magnetoresistance
In this letter we present a real space density functional theory (DFT)
localized basis set semi-empirical pseudopotential (SEP) approach. The method
is applied to iron and magnesium oxide, where bulk SEP and local spin density
approximation (LSDA) band structure calculations are shown to agree within
approximately 0.1 eV. Subsequently we investigate the qualitative
transferability of bulk derived SEPs to Fe/MgO/Fe tunnel junctions. We find
that the SEP method is particularly well suited to address the tight binding
transferability problem because the transferability error at the interface can
be characterized not only in orbital space (via the interface local density of
states) but also in real space (via the system potential). To achieve a
quantitative parameterization, we introduce the notion of ghost semi-empirical
pseudopotentials extracted from the first-principles calculated Fe/MgO bonding
interface. Such interface corrections are shown to be particularly necessary
for barrier widths in the range of 1 nm, where interface states on opposite
sides of the barrier couple effectively and play a important role in the
transmission characteristics. In general the results underscore the need for
separate tight binding interface and bulk parameter sets when modeling
conduction through thin heterojunctions on the nanoscale.Comment: Submitted to Journal of Applied Physic
MeV measurements of γ-ray bursts by CGRO-COMPTEL: Revised catalog
The imaging COMPTEL telescope has accumulated 0.1–30 MeV spectra, time-histories, and positions of more than forty γ-ray bursts within its ∼3 sr field of view in the eight years since its launch. CGRO-COMPTEL measures in both imaging “telescope” and single detector “burst spectroscopy” mode. In an ongoing collaboration with BACODINE/GCN, bursts are imaged automatically, with localizations relayed to a global network of multiwavelength observers in near real time (∼10 minutes). We have updated our burst search procedure in two ways: 1) using more sensitive search algorithms; and 2) using data from more detectors. The first are double change-point algorithms. With these we can find regions of significant excess flux with no assumptions on the wide range of burst time-scales (e.g., rise-times or decay-times) or intensities, and only one adjustable parameter (the time-averaged count-rate of the detectors). This makes it simpler to combine information on burst time-histories from the larger effective area (but cruder time bins) burst spectroscopy detectors, and hence better pinpoint the best times for imaging each burst. We report the eight bursts detected during 1998–1999
MeV measurements of gamma-ray bursts by CGRO-COMPTEL
Since the launch of the Compton Gamma-Ray Observatory in April 1991, the imaging COMPTEL telescope has accumulated positions and 0.75–30 MeV spectra of more than thirty gamma-ray bursts within its ∼π sr field of view. In an ongoing collaboration with BACODINE/GCN, COMPTEL positions are relayed to a global network of multiwavelength observers in near real time (∼10 minutes). Here we summarize the MeV properties, and present spatial, spectral, and temporal data for the latest of these events, GRB 970807. In concurrence with earlier SMM and current BATSE, OSSE, and EGRET measurements, COMPTEL data add to the accumulating evidence that GRB spectra do seem to have a characteristic shape: a peak (inE2F(E) ) around several hundred keV; and a power law above (spectral index 1.5–3.5) extending beyond the COMPTEL energy range
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