370,916 research outputs found
Cached Sufficient Statistics for Efficient Machine Learning with Large Datasets
This paper introduces new algorithms and data structures for quick counting
for machine learning datasets. We focus on the counting task of constructing
contingency tables, but our approach is also applicable to counting the number
of records in a dataset that match conjunctive queries. Subject to certain
assumptions, the costs of these operations can be shown to be independent of
the number of records in the dataset and loglinear in the number of non-zero
entries in the contingency table. We provide a very sparse data structure, the
ADtree, to minimize memory use. We provide analytical worst-case bounds for
this structure for several models of data distribution. We empirically
demonstrate that tractably-sized data structures can be produced for large
real-world datasets by (a) using a sparse tree structure that never allocates
memory for counts of zero, (b) never allocating memory for counts that can be
deduced from other counts, and (c) not bothering to expand the tree fully near
its leaves. We show how the ADtree can be used to accelerate Bayes net
structure finding algorithms, rule learning algorithms, and feature selection
algorithms, and we provide a number of empirical results comparing ADtree
methods against traditional direct counting approaches. We also discuss the
possible uses of ADtrees in other machine learning methods, and discuss the
merits of ADtrees in comparison with alternative representations such as
kd-trees, R-trees and Frequent Sets.Comment: See http://www.jair.org/ for any accompanying file
Theoretical analysis of spectral gain in a THz quantum cascade laser: prospects for gain at 1 THz
In a recent Letter [Appl. Phys. Lett. 82, 1015 (2003)], Williams et al.
reported the development of a terahertz quantum cascade laser operating at 3.4
THz or 14.2 meV. We have calculated and analyzed the gain spectra of the
quantum cascade structure described in their work, and in addition to gain at
the reported lasing energy of ~= 14 meV, we have discovered substantial gain at
a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue
for the development of a terahertz laser at this lower energy, or of a
two-color terahertz laser.Comment: in press APL, tentative publication date 29 Sep 200
Method and apparatus for producing an image from a transparent object
The contrast produced from a photographic transparency is controlled by placing the transparency between a pair of partially reflecting mirrors forming walls of an optical cavity. Mirrors trap a collimated laser beam illuminating the transparency so that at least a portion of the beam energy is passed through the transparency plural times. The distance that the light beam travels between the mirrors is controlled as a function of the wavelength of the beam energy to control the phase of light interference in the beam passing through the transparency, thereby controlling the intensity of the beam derived from the mirror downstream of the transparency
RTS amplitudes in decanano n-MOSFETs with conventional and high-k gate stacks
Low frequency (LF) noise in MOSFETs has been a topic of interest to both academia and industry in recent years. It is becoming a major concern for analogue circuit performance, DRAM operation, and will eventually impact critically upon the reliability of digital logic especially as devices continue to scale towards nano dimensions. Random telegraph signals (RTS) caused by the capture and emission of carriers in traps at the Si/SiO/sub 2/ interface have been posited as a major component of low frequency noise in semiconductor devices. The change in the drain current associated with trapping events in defect states is usually referred to as the RTS amplitude. The magnitude of the RTS amplitude is largest in the subthreshold regime at lower gate voltages and is reduced in the strong inversion regime as mobile charge in the inversion layer increasingly screens out the electrostatic influence of the trapped charge. We study the magnitude of the RTS amplitudes in nano-CMOS devices with conventional and high- gate stacks. Traps at the front and back gate dielectric interfaces, as well as traps in the body of the dielectric are considered. The impact of poly gate depletion is also taken into account
c-axis Raman Scattering in MgB2: Observation of a Dirty-Limit Gap in the pi-bands
Raman scattering spectra from the ac-face of thick MgB2 single crystals were
measured in zz, xz and xx polarisations. In zz and xz polarisations a threshold
at around 29 cm^{-1} forms in the below Tc continuum but no pair-breaking peak
is seen, in contrast to the sharp pair-breaking peak at around 100 cm^{-1} seen
in xx polarisation. The zz and xz spectra are consistent with Raman scattering
from a dirty superconductor while the sharp peak in the xx spectra argues for a
clean system. Analysis of the spectra resolves this contradiction, placing the
larger and smaller gap magnitudes in the sigma and pi bands, and indicating
that relatively strong impurity scattering is restricted to the pi bands.Comment: Revised manuscript accepted for publication in Physical Review
Letter
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