10,412 research outputs found
Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors
We calculate the low-frequency optical conductivity sigma(w) of clean metals
and superconductors at zero temperature neglecting the effects of impurities
and phonons. In general, the frequency and temperature dependences of sigma
have very little in common. For small Fermi surfaces in three dimensions (but
not in 2D) we find for example that Re sigma(w>0)=const. for low w which
corresponds to a scattering rate Gamma proportional to w^2 even in the absence
of Umklapp scattering when there is no T^2 contribution to Gamma. In the main
part of the paper we discuss in detail the optical conductivity of d-wave
superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest
frequencies and the Umklapp processes typically set in smoothly above a finite
threshold w_0 smaller than twice the maximal gap Delta. In cases where the
nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among
them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 figure
Microwave soil moisture measurements and analysis
An effort to develop a model that simulates the distribution of water content and of temperature in bare soil is documented. The field experimental set up designed to acquire the data to test this model is described. The microwave signature acquisition system (MSAS) field measurements acquired in Colby, Kansas during the summer of 1978 are pesented
Classification and Quantification of Entangled Bipartite Qutrit Pure States
A complete analysis of entangled bipartite qutrit pure states is carried out
based on a simple entanglement measure. An analysis of all possible extremally
entangled pure bipartite qutrit states is shown to reduce, with the help of
SLOCC transformations, to three distinct types. The analysis and the results
should be helpful for finding different entanglement types in multipartite pure
state systems.Comment: 10 pages, LaTe
Resolving the Controversy Over the Core Radius of 47 Tucanae (NGC 104)
This paper investigates the discrepancy between recent measurements of the
density profile of the globular cluster 47 Tuc that have used HST data sets.
Guhathakurta et al. (1992) used pre-refurbishment WFPC1 V-band images to derive
r_c = 23" +/- 2". Calzetti et al. (1993) suggested that the density profile is
a superposition of two King profiles (r_c = 8" and r_c = 25") based on U-band
FOC images. De Marchi et al. (1996) used deep WFPC1 U-band images to derive r_c
= 12" +/- 2". Differences in the adopted cluster centers are not the cause of
the discrepancy. Our independent analysis of the data used by De Marchi et al.
reaches the following conclusions: (1) De Marchi et al.'s r_c ~ 12" value is
spuriously low, a result of radially-varying bias in the star counts in a
magnitude limited sample -- photometric errors and a steeply rising stellar
luminosity function cause more stars to scatter across the limiting magnitude
into the sample than out of it, especially near the cluster center where
crowding effects are most severe. (2) Changing the limiting magnitude to the
main sequence turnoff, away from the steep part of the luminosity function,
partially alleviates the problem and results in r_c = 18". (3) Combining such a
limiting magnitude with accurate photometry derived from PSF fitting, instead
of the less accurate aperture photometry employed by De Marchi et al., results
in a reliable measurement of the density profile which is well fit by r_c = 22"
+/- 2". Archival WFPC2 data are used to derive a star list with a higher degree
of completeness, greater photometric accuracy, and wider areal coverage than
the WFPC1 and FOC data sets; the WFPC2-based density profile supports the above
conclusions, yielding r_c = 24" +/- 1.9".Comment: 22 pages, 5 figures, 1 table; accepted for publication in PASP; see
http://www.ucolick.org/~raja/hgg.tar.gz for full-resolution figure
Early Identification of Violent Criminal Gang Members
Gang violence is a major problem in the United States accounting for a large
fraction of homicides and other violent crime. In this paper, we study the
problem of early identification of violent gang members. Our approach relies on
modified centrality measures that take into account additional data of the
individuals in the social network of co-arrestees which together with other
arrest metadata provide a rich set of features for a classification algorithm.
We show our approach obtains high precision and recall (0.89 and 0.78
respectively) in the case where the entire network is known and out-performs
current approaches used by law-enforcement to the problem in the case where the
network is discovered overtime by virtue of new arrests - mimicking real-world
law-enforcement operations. Operational issues are also discussed as we are
preparing to leverage this method in an operational environment.Comment: SIGKDD 201
Observations of Binary Stars with the Differential Speckle Survey Instrument. II. Hipparcos Stars Observed in 2010 January and June
The results of 497 speckle observations of Hipparcos stars and selected other targets are presented. Of these, 367 were resolved into components and 130 were unresolved. The data were obtained using the Differential Speckle Survey Instrument at the WIYN 3.5 m Telescope. (The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National Optical Astronomy Observatories.) Since the first paper in this series, the instrument has been upgraded so that it now uses two electron-multiplying CCD cameras. The measurement precision obtained when comparing to ephemeris positions of binaries with very well known orbits is approximately 1-2 mas in separation and better than 0°.6 in position angle. Differential photometry is found to be in very good agreement with Hipparcos measures in cases where the comparison is most relevant. We derive preliminary orbits for two systems
Force distributions in 3D granular assemblies: Effects of packing order and inter-particle friction
We present a systematic investigation of the distribution of normal forces at
the boundaries of static packings of spheres. A new method for the efficient
construction of large hexagonal-close-packed crystals is introduced and used to
study the effect of spatial ordering on the distribution of forces. Under
uniaxial compression we find that the form for the probability distribution of
normal forces between particles does not depend strongly on crystallinity or
inter-particle friction. In all cases the distribution decays exponentially at
large forces and shows a plateau or possibly a small peak near the average
force but does not tend to zero at small forces.Comment: 9 pages including 8 figure
Observations from a prototype implementation of the Common APSE Interface Set (CAIS)
This paper presents an overview of the Common Ada Programming Support Environment (APSE) Interface Set (CAIS), its purpose, and its history. The paper describes an internal research and development effort at the Mitre Corporation to implement a prototype version of the current CAIS specification and to rehost existing Ada software development tools onto the CAIS prototype. Based on this effort, observations are made on the maturity and functionality of the CAIS. These observations support the government's current policy of publicizing the CAIS specification as a baseline for public review in support of its evolution into a standard which can be mandated for use as Ada is today
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