13,390 research outputs found
Far-infrared measurements of oxygen-doped polycrystalline La2CuO4.0315 superconductor under slow-cooled and fast-cooled conditions
We have studied the far-infrared (far-IR) charge dynamics of an equilibrated
pure oxygen doped La2CuO4+0.0315 under slow-cooled and fast-cooled conditions.
The superconducting transition temperature (Tc) for the slow-cooled and that
for the fast-cooled processes were respectively found to be close to the two
intrinsic Tc's: One at 30 K and the other at 15 K. Direct comparison with our
previous results and other far-IR and Raman studies on single crystalline
La2-xSrxCuO4, we conclude that the topology of the pristine electronic phases
that are responsible for the two intrinsic Tc's is holes arranged into
two-dimensional (2D) square lattices.Comment: Submitted to PR
Effects of Kinks on DNA Elasticity
We study the elastic response of a worm-like polymer chain with reversible
kink-like structural defects. This is a generic model for (a) the
double-stranded DNA with sharp bends induced by binding of certain proteins,
and (b) effects of trans-gauche rotations in the backbone of the
single-stranded DNA. The problem is solved both analytically and numerically by
generalizing the well-known analogy to the Quantum Rotator. In the small
stretching force regime, we find that the persistence length is renormalized
due to the presence of the kinks. In the opposite regime, the response to the
strong stretching is determined solely by the bare persistence length with
exponential corrections due to the ``ideal gas of kinks''. This high-force
behavior changes significantly in the limit of high bending rigidity of the
chain. In that case, the leading corrections to the mechanical response are
likely to be due to the formation of multi-kink structures, such as kink pairs.Comment: v1: 16 pages, 7 figures, LaTeX; submitted to Physical Review E; v2: a
new subsection on soft kinks added to section Theory, sections Introduction
and Conclusions expanded, references added, other minor changes; v3: a
reference adde
Toeplitz Inverse Covariance-Based Clustering of Multivariate Time Series Data
Subsequence clustering of multivariate time series is a useful tool for
discovering repeated patterns in temporal data. Once these patterns have been
discovered, seemingly complicated datasets can be interpreted as a temporal
sequence of only a small number of states, or clusters. For example, raw sensor
data from a fitness-tracking application can be expressed as a timeline of a
select few actions (i.e., walking, sitting, running). However, discovering
these patterns is challenging because it requires simultaneous segmentation and
clustering of the time series. Furthermore, interpreting the resulting clusters
is difficult, especially when the data is high-dimensional. Here we propose a
new method of model-based clustering, which we call Toeplitz Inverse
Covariance-based Clustering (TICC). Each cluster in the TICC method is defined
by a correlation network, or Markov random field (MRF), characterizing the
interdependencies between different observations in a typical subsequence of
that cluster. Based on this graphical representation, TICC simultaneously
segments and clusters the time series data. We solve the TICC problem through
alternating minimization, using a variation of the expectation maximization
(EM) algorithm. We derive closed-form solutions to efficiently solve the two
resulting subproblems in a scalable way, through dynamic programming and the
alternating direction method of multipliers (ADMM), respectively. We validate
our approach by comparing TICC to several state-of-the-art baselines in a
series of synthetic experiments, and we then demonstrate on an automobile
sensor dataset how TICC can be used to learn interpretable clusters in
real-world scenarios.Comment: This revised version fixes two small typos in the published versio
Microsaccade directions do not predict directionality of illusory brightness changes of overlapping transparent surfaces
AbstractTse (2005) recently introduced a new class of illusory brightness changes where shifts of attention lead to shifts in perceived brightness across overlapping, transparent figures, under conditions of visual fixation. In the absence of endogenous attentional shifts, illusory brightness changes appear to shift from figure to figure spontaneously, much as occurs in other multistable phenomena. The goal of the present research is to determine whether fixational microsaccades are correlated with perceived brightness changes. It has recently been demonstrated that microsaccades can reveal the direction of covert attentional shifts either toward (Engbert, R. & Kliegl, R. (2003). Microsaccades uncover the orientation of covert attention. Vision Research, 43, 1035–1045; Hafed, Z. M. & Clark, J. J. (2002). Microsaccades as an overt measure of covert attention shifts. Vision Research, 42(22), 2533–2545) or away from (Rolfs, M., Engbert, R., & Kliegl, R. (2004). Microsaccade orientation supports attentional enhancement opposite a peripheral cue: commentary on Tse, Sheinberg, and Logothetis (2003). Psychological Science, 15(10), 705–707) a peripheral cue under certain circumstances. Others (Horwitz, G. D. & Albright, T. D. (2003). Short-latency fixational saccades induced by luminance increments. Journal of Neurophysiology, 90(2), 1333–1339; Tse, P. U., Sheinberg, D. L., & Logothetis, N. K. (2002). Fixational eye movements are not affected by abrupt onsets that capture attention. Vision Research, 42, 1663–1669; Tse, P. U., Sheinberg, D. L., & Logothetis, N. K. (2004). The distribution of microsaccade directions need not reveal the location of attention. Psychological Science, 15(10), 708–710) found no change in the distribution of microsaccade directions as a function of where attention is allocated, although changes in the rate of microsaccades were observed in all of these studies in response to the onset of attentional reallocation. It is therefore possible that the distribution of microsaccade directions will change as a function of which figure is perceived to darken, or that changes in this distribution predict which figure will subsequently darken. We find no correlation between this distribution and which figure undergoes the effect, and therefore conclude that microsaccade directionality is not influenced by and does not influence which figure undergoes the effect. Moreover, the directions of microsaccades that occur immediately prior to a perceptual switch are not correlated with the perceived position of the figure that undergoes the effect. However, we do find that the rate of microsaccades decreases upon a perceptual switch, signifying an attentional shift coincident with the perceptual shift. We conclude that microsaccade directionality does not determine, predict, or cause which figure will subsequently be perceived to undergo an illusory brightness change
Polarimetry and photometry of the peculiar main-belt object 7968 = 133P/Elst-Pizarro
133P/Elst-Pizarro is an object that has been described as either an active
asteroid or a cometary object in the main asteroid belt. Here we present a
photometric and polarimetric study of this object in an attempt to infer
additional information about its origin.
With the FORS1 instrument of the ESO VLT, we have performed during the 2007
apparition of 133P/Elst-Pizarro quasi-simultaneous photometry and polarimetry
of its nucleus at nine epochs in the phase angle range 0 - 20 deg. For each
observing epoch, we also combined all available frames to obtain a deep image
of the object, to seek signatures of weak cometary activity. Polarimetric data
were analysed by means of a novel physical interference modelling.
The object brightness was found to be highly variable over timescales <1h, a
result fully consistent with previous studies. Using the albedo-polarization
relationships for asteroids and our photometric results, we found for our
target an albedo of about 0.06-0.07 and a mean radius of about 1.6 km.
Throughout the observing epochs, our deep imaging of the comet detects a tail
and an anti-tail. Their temporal variations are consistent with an activity
profile starting around mid May 2007 of minimum duration of four months. Our
images show marginal evidence of a coma around the nucleus. The overall light
scattering behaviour (photometry and polarimetry) resembles most closely that
of F-type asteroids.Comment: Accepted by Astronomy and Astrophysic
Method and System for Scatter Correction in X-Ray Imaging
Approaches for deriving scatter information using inverse tracking of scattered X-rays is disclosed. In certain embodiments scattered rays are tracked from respective locations on a detector to a source of the X-ray radiation, as opposed to tracking schemes that proceed from the source to the detector. In one such approach, the inverse tracking is implemented using a density integrated volume that reduces the integration steps performed
The activation energy for GaAs/AlGaAs interdiffusion
Copyright 1997 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article appeared in Journal of Applied Physics 82, 4842 (1997) and may be found at
Noise effect on Grover algorithm
The decoherence effect on Grover algorithm has been studied numerically
through a noise modelled by a depolarizing channel. Two types of error are
introduced characterizing the qubit time evolution and gate application, so the
noise is directly related to the quantum network construction. The numerical
simulation concludes an exponential damping law for the successive probability
of the maxima as time increases. We have obtained an allowed-error law for the
algorithm: the error threshold for the allowed noise behaves as Eth(N) ~ 1/N1.1
(N being the size of the data set). As the power of N is almost one, we
consider the Grover algorithm as robust to a certain extent against
decoherence. This law also provides an absolute threshold: if the free
evolution error is greater than 0.043, Grover algorithm does not work for any
number of qubits affected by the present error model. The improvement in the
probability of success, in the case of two qubits has been illustrated by using
a fault-tolerant encoding of the initial state by means of the [[7,1,3]]
quantum code.Comment: Accepted to be published in Eur. Phys. J. D (2008
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