3,327 research outputs found
SETI science working group report
This report covers the initial activities and deliberations of a continuing working group asked to assist the SETI Program Office at NASA. Seven chapters present the group's consensus on objectives, strategies, and plans for instrumental R&D and for a microwave search for extraterrestrial in intelligence (SETI) projected for the end of this decade. Thirteen appendixes reflect the views of their individual authors. Included are discussions of the 8-million-channel spectrum analyzer architecture and the proof-of-concept device under development; signal detection, recognition, and identification on-line in the presence of noise and radio interference; the 1-10 GHz sky survey and the 1-3 GHz targeted search envisaged; and the mutual interests of SETI and radio astronomy. The report ends with a selective, annotated SETI reading list of pro and contra SETI publications
Polaron Transport in the Paramagnetic Phase of Electron-Doped Manganites
The electrical resistivity, Hall coefficient, and thermopower as functions of
temperature are reported for lightly electron-doped Ca(1-x)La(x)MnO(3)(0 <= x
<= 0.10). Unlike the case of hole-doped ferromagnetic manganites, the magnitude
and temperature dependence of the Hall mobility for these compounds is found to
be inconsistent with small-polaron theory. The transport data are better
described by the Feynman polaron theory and imply intermediate coupling (alpha
\~ 5.4) with a band effective mass, m*~4.3 m_0, and a polaron mass, m_p ~ 10
m_0.Comment: 7 pp., 7 Fig.s, to be published, PR
Towards Safe Machine Learning for CPS: Infer Uncertainty from Training Data
Machine learning (ML) techniques are increasingly applied to decision-making
and control problems in Cyber-Physical Systems among which many are
safety-critical, e.g., chemical plants, robotics, autonomous vehicles. Despite
the significant benefits brought by ML techniques, they also raise additional
safety issues because 1) most expressive and powerful ML models are not
transparent and behave as a black box and 2) the training data which plays a
crucial role in ML safety is usually incomplete. An important technique to
achieve safety for ML models is "Safe Fail", i.e., a model selects a reject
option and applies the backup solution, a traditional controller or a human
operator for example, when it has low confidence in a prediction.
Data-driven models produced by ML algorithms learn from training data, and
hence they are only as good as the examples they have learnt. As pointed in
[17], ML models work well in the "training space" (i.e., feature space with
sufficient training data), but they could not extrapolate beyond the training
space. As observed in many previous studies, a feature space that lacks
training data generally has a much higher error rate than the one that contains
sufficient training samples [31]. Therefore, it is essential to identify the
training space and avoid extrapolating beyond the training space. In this
paper, we propose an efficient Feature Space Partitioning Tree (FSPT) to
address this problem. Using experiments, we also show that, a strong
relationship exists between model performance and FSPT score.Comment: Publication rights licensed to AC
Theory for nucleation at an interface and magnetization reversal of a two-layer nanowire
Nucleation at the interface between two adjoining regions with dissimilar physical properties is investigated using a model for magnetization reversal of a two-layer ferromagnetic nanowire. Each layer of the nanowire is considered to have a different degree of magnetic anisotropy, representing a hard magnetic layer exchange-coupled to a softer layer. A magnetic field applied along the easy axis causes the softer layer to reverse, forming a domain wall close to the interface. For small applied fields this state is metastable and complete reversal of the nanowire takes place via activation over a barrier. A reversal mechanism involving nucleation at an interface is proposed, whereby a domain wall changes in width as it passes from the soft layer to the hard layer during activation. Langer’s statistical theory for the decay of a metastable state is used to derive rates of magnetization reversal, and simple formulas are found in limiting cases for the activation energy, rate of reversal, and critical field at which the metastable state becomes unstable. These formulas depend on the anisotropy difference between each layer, and the behavior of the reversal rate prefactor is interpreted in terms of activation entropy and domain-wall dynamics
Anisotropic weakly localized transport in nitrogen-doped ultrananocrystalline diamond films
We establish the dominant effect of anisotropic weak localization (WL) in
three dimensions associated with a propagative Fermi surface, on the
conductivity correction in heavily nitrogen doped ultrananocrystalline diamond
(UNCD) films based on magneto-resistance studies at low temperatures. Also, low
temperature electrical conductivity can show weakly localized transport in 3D
combined with the effect of electron-electron interactions in these materials,
which is remarkably different from the conductivity in 2DWL or strong
localization regime. The corresponding dephasing time of electronic
wavefunctions in these systems described as ~ T^-p with p < 1, follows a
relatively weak temperature dependence compared to the generally expected
nature for bulk dirty metals having . The temperature dependence of
Hall (electron) mobility together with an enhanced electron density has been
used to interpret the unusual magneto-transport features and show delocalized
electronic transport in these n-type UNCD films, which can be described as
low-dimensional superlattice structures.Comment: 27 pages, 6 figures, To be published in Physical Review
Effect of nonequilibrium phonons on hot-electron spin relaxation in n-type GaAs quantum wells
We have studied the effect of nonequilibrium longitudinal optical phonons on
hot-electron spin relaxation in -type GaAs quantum wells. The longitudinal
optical phonons, due to the finite relaxation rate, are driven to
nonequilibrium states by electrons under an in-plane electric field. The
nonequilibrium phonons then in turn influence the electron spin relaxation
properties via modifying the electron heating and drifting. The spin relaxation
time is elongated due to the enhanced electron heating and thus the
electron-phonon scattering in the presence of nonequilibrium phonons. The
frequency of spin precession, which is roughly proportional to the electron
drift velocity, can be either increased (at low electric field and/or high
lattice temperature) or decreased (at high electric field and/or low lattice
temperature). The nonequilibrium phonon effect is more pronounced when the
electron density is high and the impurity density is low.Comment: 6 pages, 3 figure
Vafa-Witten Estimates for Compact Symmetric Spaces
We give an optimal upper bound for the first eigenvalue of the untwisted
Dirac operator on a compact symmetric space G/H with rk G-rk H\le 1 with
respect to arbitrary Riemannian metrics. We also prove a rigidity statement.Comment: LaTeX, 11 pages. V2: Rigidity statement added, minor changes. To
appea
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