1,989 research outputs found
Sensory organ like response determines the magnetism of zigzag-edged honeycomb nanoribbons
We present an analytical theory for the magnetic phase diagram for zigzag
edge terminated honeycomb nanoribbons described by a Hubbard model with an
interaction parameter U . We show that the edge magnetic moment varies as ln U
and uncover its dependence on the width W of the ribbon. The physics of this
owes its origin to the sensory organ like response of the nanoribbons,
demonstrating that considerations beyond the usual Stoner-Landau theory are
necessary to understand the magnetism of these systems. A first order magnetic
transition from an anti-parallel orientation of the moments on opposite edges
to a parallel orientation occurs upon doping with holes or electrons. The
critical doping for this transition is shown to depend inversely on the width
of the ribbon. Using variational Monte-Carlo calculations, we show that
magnetism is robust to fluctuations. Additionally, we show that the magnetic
phase diagram is generic to zigzag edge terminated nanostructures such as
nanodots. Furthermore, we perform first principles modeling to show how such
magnetic transitions can be realized in substituted graphene nanoribbons.Comment: 5 pages, 5 figure
Forecasting Stock Time-Series using Data Approximation and Pattern Sequence Similarity
Time series analysis is the process of building a model using statistical
techniques to represent characteristics of time series data. Processing and
forecasting huge time series data is a challenging task. This paper presents
Approximation and Prediction of Stock Time-series data (APST), which is a two
step approach to predict the direction of change of stock price indices. First,
performs data approximation by using the technique called Multilevel Segment
Mean (MSM). In second phase, prediction is performed for the approximated data
using Euclidian distance and Nearest-Neighbour technique. The computational
cost of data approximation is O(n ni) and computational cost of prediction task
is O(m |NN|). Thus, the accuracy and the time required for prediction in the
proposed method is comparatively efficient than the existing Label Based
Forecasting (LBF) method [1].Comment: 11 page
Abnormal diffusion of a single vortex in the two dimensional XY model
We study thermal diffusion dynamics of a single vortex in two dimensional XY
model. By numerical simulations we find an abnormal diffusion such that the
mobility decreases with time as . In addition we construct a one
dimensional diffusion-like equation to model the dynamics and confirm that it
conserves quantitative property of the abnormal diffusion. By analyzing the
reduced model, we find that the radius of the collectively moving region with
the vortex core grows as . This suggests that the
mobility of the vortex is described by dynamical correlation length as .Comment: 13 pages, 8 figure
Diluted Josephson-junction arrays in a magnetic field: phase coherence and vortex glass thresholds
The effects of random dilution of junctions on a two-dimensional
Josephson-junction array in a magnetic field are considered. For rational
values of the average flux quantum per plaquette , the superconducting
transition temperature vanishes, for increasing dilution, at a critical value
, while the vortex ordering remains stable up to , much
below the value corresponding to the geometric percolation threshold. For
, the array behaves as a zero-temperature vortex-glass.
Numerical results for from defect energy calculations are presented
which are consistent with this scenario.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Scaled free energies, power-law potentials, strain pseudospins and quasi-universality for first-order structural transitions
We consider ferroelastic first-order phase transitions with
order-parameter strains entering Landau free energies as invariant polynomials,
that have structural-variant Landau minima. The total free energy
includes (seemingly innocuous) harmonic terms, in the {\it
non}-order-parameter strains. Four 3D transitions are considered,
tetragonal/orthorhombic, cubic/tetragonal, cubic/trigonal and
cubic/orthorhombic unit-cell distortions, with respectively, and 2; and and 6. Five 2D transitions are also considered, as
simpler examples. Following Barsch and Krumhansl, we scale the free energy to
absorb most material-dependent elastic coefficients into an overall prefactor,
by scaling in an overall elastic energy density; a dimensionless temperature
variable; and the spontaneous-strain magnitude at transition .
To leading order in the scaled Landau minima become
material-independent, in a kind of 'quasi-universality'. The scaled minima in
-dimensional order-parameter space, fall at the centre and at the
corners, of a transition-specific polyhedron inscribed in a sphere, whose
radius is unity at transition. The `polyhedra' for the four 3D transitions are
respectively, a line, a triangle, a tetrahedron, and a hexagon. We minimize the
terms harmonic in the non-order-parameter strains, by substituting
solutions of the 'no dislocation' St Venant compatibility constraints, and
explicitly obtain powerlaw anisotropic, order-parameter interactions, for all
transitions. In a reduced discrete-variable description, the competing minima
of the Landau free energies induce unit-magnitude pseudospin vectors, with values, pointing to the polyhedra corners and the (zero-value) center.Comment: submitted to PR
Dewetting of Glassy Polymer Films
Dynamics and morphology of hole growth in a film of power hardening
viscoplastic solid (yield stress ~ [strain-rate]^n) is investigated. At
short-times the growth is exponential and depends on the initial hole size. At
long-times, for n > 1/3, the growth is exponential with a different exponent.
However, for n < 1/3, the hole growth slows; the hole radius approaches an
asymptotic value as time tends to infinity. The rim shape is highly asymmetric,
the height of which has a power law dependence on the hole radius (exponent
close to unity for 0.25 < n < 0.4). The above results explain recent intriguing
experiments of Reiter, Phys. Rev. Lett, 87, 186101 (2001).Comment: 4 pages, 5 figures, RevTe
A Catalog of Background Stars Reddened by Dust in the Taurus Dark Clouds
Normal field stars located behind dense clouds are a valuable resource in
interstellar astrophysics, as they provide continua in which to study phenomena
such as gas-phase and solid-state absorption features, interstellar extinction
and polarization. This paper reports the results of a search for highly
reddened stars behind the Taurus Dark Cloud complex. We use the Two Micron All
Sky Survey (2MASS) Point Source Catalog to survey a 50 sq deg area of the cloud
to a limiting magnitude of K = 10.0. Photometry in the 1.2-2.2 micron passbands
from 2MASS is combined with photometry at longer infrared wavelengths (3.6-12
micron) from the Spitzer Space Telescope and the Infrared Astronomical
Satellite to provide effective discrimination between reddened field stars and
young stellar objects (YSOs) embedded in the cloud. Our final catalog contains
248 confirmed or probable background field stars, together with estimates of
their total visual extinctions, which span the range 2-29 mag. We also identify
the 2MASS source J04292083+2742074 (IRAS 04262+2735) as a previously
unrecognized candidate YSO, based on the presence of infrared emission greatly
in excess of that predicted for a normal reddened photosphere at wavelengths >5
microns
Current responses and voltage fluctuations in Josephson-junction systems
We consider arrays of Josephson junctions as well as single junctions in both
the classical and quantum-mechanical regimes, and examine the generalized
(frequency-dependent) resistance, which describes the dynamic responses of such
Josephson-junction systems to external currents. It is shown that the
generalized resistance and the power spectrum of voltage fluctuations are
related via the fluctuation-dissipation theorem. Implications of the obtained
relations are also discussed in various experimental situations.Comment: 4 pages, To appear in Europhys. Let
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Natural history of colonization with methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE): a systematic review
Background: No published systematic reviews have assessed the natural history of colonization with methicillin-resistant Staphylococcus aureus (MRSA) or vancomycin-resistant Enterococcus (VRE). Time to clearance of colonization has important implications for patient care and infection control policy. Methods: We performed parallel searches in OVID Medline for studies that reported the time to documented clearance of MRSA and VRE colonization in the absence of treatment, published between January 1990 and July 2012. Results: For MRSA, we screened 982 articles, identified 16 eligible studies (13 observational studies and 3 randomized controlled trials), for a total of 1,804 non-duplicated subjects. For VRE, we screened 284 articles, identified 13 eligible studies (12 observational studies and 1 randomized controlled trial), for a total of 1,936 non-duplicated subjects. Studies reported varying definitions of clearance of colonization; no study reported time of initial colonization. Studies varied in the frequency of sampling, assays used for sampling, and follow-up period. The median duration of total follow-up was 38 weeks for MRSA and 25 weeks for VRE. Based on pooled analyses, the model-estimated median time to clearance was 88 weeks after documented colonization for MRSA-colonized patients and 26 weeks for VRE-colonized patients. In a secondary analysis, clearance rates for MRSA and VRE were compared by restricting the duration of follow-up for the MRSA studies to the maximum observed time point for VRE studies (43 weeks). With this restriction, the model-fitted median time to documented clearance for MRSA would occur at 41 weeks after documented colonization, demonstrating the sensitivity of the pooled estimate to length of study follow-up. Conclusions: Few available studies report the natural history of MRSA and VRE colonization. Lack of a consistent definition of clearance, uncertainty regarding the time of initial colonization, variation in frequency of sampling for persistent colonization, assays employed and variation in duration of follow-up are limitations of the existing published literature. The heterogeneity of study characteristics limits interpretation of pooled estimates of time to clearance, however, studies included in this review suggest an increase in documented clearance over time, a result which is sensitive to duration of follow-up
Fast Algorithms For Josephson Junction Arrays : Bus--bars and Defects
We critically review the fast algorithms for the numerical study of
two--dimensional Josephson junction arrays and develop the analogy of such
systems with electrostatics. We extend these procedures to arrays with
bus--bars and defects in the form of missing bonds. The role of boundaries and
of the guage choice in determing the Green's function of the system is
clarified. The extension of the Green's function approach to other situations
is also discussed.Comment: Uuencoded 1 Revtex file (11 Pages), 3 Figures : Postscript Uuencode
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