4,781 research outputs found
How to make semiconductors ferromagnetic: A first course on spintronics
The rapidly developing field of ferromagnetism in diluted magnetic
semiconductors, where a semiconductor host is magnetically doped by transition
metal impurities to produce a ferromagnetic semiconductor (e.g. Ga_{1-x}Mn_xAs
with x ~ 1-10 %), is discussed with the emphasis on elucidating the physical
mechanisms underlying the magnetic properties. Recent key developments are
summarized with critical discussions of the roles of disorder, localization,
band structure, defects, and the choice of materials in producing good magnetic
quality and high Curie temperature. The correlation between magnetic and
transport properties is argued to be a crucial ingredient in developing a full
understanding of the properties of ferromagnetic semiconductors.Comment: 8 pages; to appear in the special issue 'Quantum Phases at Nanoscale'
of Solid State Communication
TBC costing
Procedures to be used in determining the cost of producing and installing a parabolic dish collector in annual production volumes of 10,000, 50,000, 100,000, and 1,000,000 units include (1) evaluating each individual part for material cost and for the type and number of operations required to work the raw material into the finished part; (2) costing labor, burden, tooling, gaging, machinery, and equipment; (3) estimating facilities requirements for each production volume; and (4) considering suggestions for design and material alterations that could result in cost reduction
Study of 'ratio' automatically assembled structures final report, 15 jun. 1963 - 15 jun. 1964
Structural analysis of RATIO automatically assembled panel sections for application to spaceborne paraboloidal antenn
Towards new background independent representations for Loop Quantum Gravity
Recently, uniqueness theorems were constructed for the representation used in
Loop Quantum Gravity. We explore the existence of alternate representations by
weakening the assumptions of the so called LOST uniqueness theorem. The
weakened assumptions seem physically reasonable and retain the key requirement
of explicit background independence. For simplicity, we restrict attention to
the case of gauge group U(1).Comment: 22 pages, minor change
In-Situ Dual-Port Polarization Contrast Imaging of Faraday Rotation in a High Optical Depth Ultracold 87Rb Atomic Ensemble
We study the effects of high optical depth and density on the performance of
a light-atom quantum interface. An in-situ imaging method, a dual-port
polarization contrast technique, is presented. This technique is able to
compensate for image distortions due to refraction. We propose our imaging
method as a tool to characterize atomic ensembles for high capacity spatial
multimode quantum memories. Ultracold dense inhomogeneous Rubidium samples are
imaged and we find a resonant optical depth as high as 680 on the D1 line. The
measurements are compared with light-atom interaction models based on
Maxwell-Bloch equations. We find that an independent atom assumption is
insufficient to explain our data and present corrections due to resonant
dipole-dipole interactions
On Pebble Automata for Data Languages with Decidable Emptiness Problem
In this paper we study a subclass of pebble automata (PA) for data languages
for which the emptiness problem is decidable. Namely, we introduce the
so-called top view weak PA. Roughly speaking, top view weak PA are weak PA
where the equality test is performed only between the data values seen by the
two most recently placed pebbles. The emptiness problem for this model is
decidable. We also show that it is robust: alternating, nondeterministic and
deterministic top view weak PA have the same recognition power. Moreover, this
model is strong enough to accept all data languages expressible in Linear
Temporal Logic with the future-time operators, augmented with one register
freeze quantifier.Comment: An extended abstract of this work has been published in the
proceedings of the 34th International Symposium on Mathematical Foundations
of Computer Science (MFCS) 2009}, Springer, Lecture Notes in Computer Science
5734, pages 712-72
Submillimeter-wave emission of three Galactic red novae: cool molecular outflows produced by stellar mergers
Red novae are optical transients erupting at luminosities typically higher
than those of classical novae. Their outbursts are believed to be caused by
stellar mergers. We present millimeter/submillimeter-wave observations with
ALMA and SMA of the three best known Galactic red novae, V4332 Sgr, V1309 Sco,
and V838 Mon. The observations were taken 22, 8, and 14 yr after their
respective eruptions and reveal the presence of molecular gas at excitation
temperatures of 35-200 K. The gas displays molecular emission in rotational
transitions with very broad lines (full width 400 km\s). We found
emission of CO, SiO, SO, SO (in all three red novae), HS (covered only
in V838 Mon) and AlO (present in V4332 Sgr and V1309 Sco). No anomalies were
found in the isotopic composition of the molecular material and the chemical
(molecular) compositions of the three red novae appear similar to those of
oxygen-rich envelopes of classical evolved stars (RSGs, AGBs, post-AGBs). The
minimum masses of the molecular material that most likely was dispersed in the
red-nova eruptions are 0.1, 0.01, and 10 M for V838 Mon, V4332
Sgr, and V1309 Sco, respectively. The molecular outflows in V4332 Sgr and V1309
Sco are spatially resolved and appear bipolar. The kinematic distances to V1309
Sco and V4332 Sgr are 2.1 and 4.2 kpc, respectively. The kinetic energy stored
in the ejecta of the two older red-nova remnants of V838 Mon and V4332 Sgr is
of order erg, similar to values found for some post-AGB (pre-PN)
objects whose bipolar ejecta were also formed in a short-duration eruption. Our
observations strengthen the link between these post-AGB objects and red novae
and support the hypothesis that some of the post-AGB objects were formed in a
common-envelope ejection event or its most catastrophic outcome, a merger.Comment: 19 pages, 13 figures, accepted to A&
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