2,339 research outputs found
Image selection system
An image selection (ISS) was developed for the NASA-Ames Research Center Earth Resources Aircraft Project. The ISS is an interactive, graphics oriented, computer retrieval system for aerial imagery. An analysis of user coverage requests and retrieval strategies is presented, followed by a complete system description. Data base structure, retrieval processors, command language, interactive display options, file structures, and the system's capability to manage sets of selected imagery are described. A detailed example of an area coverage request is graphically presented
Towards a better understanding of the dynamic role of the distance language learner: learner perceptions of personality, motivation, roles, and approaches
This study investigated the experience of learners enrolled on an Open University (UK) French course, and included personality factors, motivation, and tutor and student roles. The data gathered via multiple elicitation methods gave useful insights into issues of special relevance to distance language education, in particular the lack of fit between an inherently social discipline such as language learning and the distance context, whose main characterizing feature is remoteness from others. Motivation was seen to play a crucial role in success, along with tutor feedback, and personal responsibility for learning. Increased confidence and self?regulation were beneficial outcomes of the process of learning at a distance, and numerous suggestions for learning approaches based on personal experience were offered for language learners new to distance learning. The study concluded that the task for distance practitioners is to build on the insights shown by learners themselves, in order to target support where it is most needed
Anisotropic Hall Effect in Single Crystal Heavy Fermion YbAgGe
Temperature- and field-dependent Hall effect measurements are reported for
YbAgGe, a heavy fermion compound exhibiting a field-induced quantum phase
transition, and for two other closely related members of the RAgGe series: a
non-magnetic analogue, LuAgGe and a representative, ''good local moment'',
magnetic material, TmAgGe. Whereas the temperature dependent Hall coefficient
of YbAgGe shows behavior similar to what has been observed in a number of heavy
fermion compounds, the low temperature, field-dependent measurements reveal
well defined, sudden changes with applied field; in specific for a
clear local maximum that sharpens as temperature is reduced below 2 K and that
approaches a value of 45 kOe - a value that has been proposed as the
quantum critical point. Similar behavior was observed for where a
clear minimum in the field-dependent Hall resistivity was observed at low
temperatures. Although at our base temperatures it is difficult to distinguish
between the field-dependent behavior predicted for (i) diffraction off a
critical spin density wave or (ii) breakdown in the composite nature of the
heavy electron, for both field directions there is a distinct temperature
dependence of a feature that can clearly be associated with a field-induced
quantum critical point at persisting up to at least 2 K.Comment: revised versio
Minimal Work Principle and its Limits for Classical Systems
The minimal work principle asserts that work done on a thermally isolated
equilibrium system, is minimal for the slowest (adiabatic) realization of a
given process. This principle, one of the formulations of the second law, is
operationally well-defined for any finite (few particle) Hamiltonian system.
Within classical Hamiltonian mechanics, we show that the principle is valid for
a system of which the observable of work is an ergodic function. For
non-ergodic systems the principle may or may not hold, depending on additional
conditions. Examples displaying the limits of the principle are presented and
their direct experimental realizations are discussed.Comment: 4 + epsilon pages, 1 figure, revte
System for the measurement of ultra-low stray light levels
An apparatus is described for measuring the effectiveness of stray light suppression light shields and baffle arrangements used in optical space experiments and large space telescopes. The light shield and baffle arrangement and a telescope model are contained in a vacuum chamber. A source of short, high-powered light energy illuminates portions of the light shield and baffle arrangement and reflects a portion of same to a photomultiplier tube by virtue of multipath scattering. The resulting signal is transferred to time-channel electronics timed by the firing of the high energy light source allowing time discrimination of the signal thereby enabling the light scattered and suppressed by the model to be distinguished from the walls and holders around the apparatus
Anomalous Hall effect in Rashba two-dimensional electron systems based on narrow-band semiconductors: side-jump and skew scattering mechanisms
We employ a helicity-basis kinetic equation approach to investigate the
anomalous Hall effect in two-dimensional narrow-band semiconductors considering
both Rashba and extrinsic spin-orbit (SO) couplings, as well as a SO coupling
directly induced by an external driving electric field. Taking account of
long-range electron-impurity scattering up to the second Born approximation, we
find that the various components of the anomalous Hall current fit into two
classes: (a) side-jump and (b) skew scattering anomalous Hall currents. The
side-jump anomalous Hall current involves contributions not only from the
extrinsic SO coupling but also from the SO coupling due to the driving electric
field. It also contains a component which arises from the Rashba SO coupling
and relates to the off-diagonal elements of the helicity-basis distribution
function. The skew scattering anomalous Hall effect arises from the anisotropy
of the diagonal elements of the distribution function and it is a result of
both the Rashba and extrinsic SO interactions. Further, we perform a numerical
calculation to study the anomalous Hall effect in a typical InSb/AlInSb quantum
well. The dependencies of the side-jump and skew scattering anomalous Hall
conductivities on magnetization and on the Rashba SO coupling constant are
examined.Comment: 16 pages, 4 figures, accepted for publication in PR
Universal Magnetic-Field-Driven Metal-Insulator-Metal Transformations in Graphite and Bismuth
Applied magnetic field induces metal - insulator and re-entrant
insulator-metal transitions in both graphite and rhombohedral bismuth. The
corresponding transition boundaries plotted on the magnetic field - temperature
(B - T) plane nearly coincide for these semimetals and can be best described by
power laws T ~ (B - B_c)^k, where B_c is a critical field at T = 0 and k = 0.45
+/- 0.05. We show that insulator-metal-insulator (I-M-I) transformations take
place in the Landau level quantization regime and illustrate how the IMT in
quasi-3D graphite transforms into a cascade of I-M-I transitions, related to
the quantum Hall effect in quasi-2D graphite samples. We discuss the possible
coupling of superconducting and excitonic correlations with the observed
phenomena, as well as the signatures of quantum phase transitions associated
with the M-I and I-M transformations.Comment: 23 pages including 14 figure
The chiral Anomalous Hall effect in re-entrant AuFe alloys
The Hall effect has been studied in a series of AuFe samples in the
re-entrant concentration range, as well as in part of the spin glass range. An
anomalous Hall contribution linked to the tilting of the local spins can be
identified, confirming theoretical predictions of a novel topological Hall term
induced when chirality is present. This effect can be understood in terms of
Aharonov-Bohm-like intrinsic current loops arising from successive scatterings
by canted local spins. The experimental measurements indicate that the chiral
signal persists, meaning scattering within the nanoscopic loops remains
coherent, up to temperatures of the order of 150 K.Comment: 7 pages, 11 eps figures Published version. Minor change
c-axis magnetotransport in CeCoIn
We present the results of out-of-plane electrical transport measurements on
the heavy fermion superconductor CeCoIn at temperatures from 40 mK to 400
K and in magnetic field up to 9 T. For 10 K transport measurements show
that the zero-field resistivity changes linearly with temperature
and extrapolates nearly to zero at 0 K, indicative of non-Fermi-liquid (nFL)
behavior associated with a quantum critical point (QCP). The longitudinal
magnetoresistance (LMR) of CeCoIn for fields applied parallel to the
c-axis is negative and scales as between 50 and 100 K, revealing
the presence of a single-impurity Kondo energy scale K.
Beginning at 16 K a small positive LMR feature is evident for fields less than
3 tesla that grows in magnitude with decreasing temperature. For higher fields
the LMR is negative and increases in magnitude with decreasing temperature.
This sizable negative magnetoresistance scales as from 2.6 K to
roughly 8 K, and it arises from an extrapolated residual resistivity that
becomes negative and grows quadratically with field in the nFL temperature
regime. Applying a magnetic field along the c-axis with B B restores
Fermi-liquid behavior in at less than 130 mK. Analysis of the
resistivity coefficient's field-dependence suggests that the QCP in
CeCoIn is located \emph{below} the upper critical field, inside the
superconducting phase. These data indicate that while high- c-axis transport
of CeCoIn exhibits features typical for a heavy fermion system, low-
transport is governed both by spin fluctuations associated with the QCP and
Kondo interactions that are influenced by the underlying complex electronic
structure intrinsic to the anisotropic CeCoIn crystal structure
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