9,786 research outputs found
Monte Carlo Simulation of the Short-time Behaviour of the Dynamic XY Model
Dynamic relaxation of the XY model quenched from a high temperature state to
the critical temperature or below is investigated with Monte Carlo methods.
When a non-zero initial magnetization is given, in the short-time regime of the
dynamic evolution the critical initial increase of the magnetization is
observed. The dynamic exponent is directly determined. The results
show that the exponent varies with respect to the temperature.
Furthermore, it is demonstrated that this initial increase of the magnetization
is universal, i.e. independent of the microscopic details of the initial
configurations and the algorithms.Comment: 14 pages with 5 figures in postscrip
Tracking of MVPA across childhood and adolescence
\ua9 2024 The AuthorsObjectives: Tracking of physical activity from childhood onwards is an important public health issue, but evidence on tracking is limited. This study quantified the tracking of Moderate-Vigorous Physical Activity (MVPA) across childhood and adolescence in a recent cohort from England. Design: Longitudinal, with a socio-economically representative sample from North-East England, over an 8-year period. Methods: Measures of time spent in MVPA, with an Actigraph GT1M accelerometer, were made at age 7–8y (n = 622, T1), age 9–10y (n = 585, T2), age 12–13y (n = 525, T3) and age 14–16y (n = 361, T4). Tracking of MVPA was assessed using rank order correlations between time spent in MVPA T1–T2, T1–T3, and T1–T4, and by using Cohen\u27s kappa to examine tracking of meeting the MVPA guideline (mean of 60 min/d). We examined whether tracking varied by sex, socio-economic status (SES), initial MVPA, or initial body fatness. Results: Rank order correlations were all statistically significant at p < 0.01 and moderate: 0.58 between T1 and T2; 0.42 between T1 and T3; 0.41 between T1 and T4. Cohen\u27s kappas for meeting the global MVPA guideline were all significant, weakening from moderate to low over the 8 years. Tracking was stronger in higher SES compared to lower SES groups, and there was some evidence that it was stronger in girls than boys, but the other explanatory variables had little influence on tracking. Conclusions: Tracking of MVPA from mid-childhood to mid-adolescence in this cohort was moderate. This study suggests there is a need to establish high MVPA by mid-childhood, and to mitigate the age-related reduction in MVPA which occurs from mid-childhood
Temperature dependence of the electronic structure of semiconductors and insulators
The renormalization of electronic eigenenergies due to electron-phonon
coupling is sizable in many materials with light atoms. This effect, often
neglected in ab-initio calculations, can be computed using the
perturbation-based Allen-Heine-Cardona theory in the adiabatic or non-adiabatic
harmonic approximation. After a short description of the numerous recent
progresses in this field, and a brief overview of the theory, we focus on the
issue of phonon wavevector sampling convergence, until now poorly understood.
Indeed, the renormalization is obtained numerically through a q-point sampling
inside the BZ. For q-points close to G, we show that a divergence due to
non-zero Born effective charge appears in the electron-phonon matrix elements,
leading to a divergence of the integral over the BZ for band extrema. Although
it should vanish for non-polar materials, unphysical residual Born effective
charges are usually present in ab-initio calculations. Here, we propose a
solution that improves the coupled q-point convergence dramatically. For polar
materials, the problem is more severe: the divergence of the integral does not
disappear in the adiabatic harmonic approximation, but only in the
non-adiabatic harmonic approximation. In all cases, we study in detail the
convergence behavior of the renormalization as the q-point sampling goes to
infinity and the imaginary broadening parameter goes to zero. This allows
extrapolation, thus enabling a systematic way to converge the renormalization
for both polar and non-polar materials. Finally, the adiabatic and
non-adiabatic theory, with corrections for the divergence problem, are applied
to the study of five semiconductors and insulators: a-AlN, b-AlN, BN, diamond
and silicon. For these five materials, we present the zero-point
renormalization, temperature dependence, phonon-induced lifetime broadening and
the renormalized electronic bandstructure.Comment: 27 pages and 26 figure
Conjugation-Length Dependence of Spin-Dependent Exciton Formation Rates in Pi-Conjugated Oligomers and Polymers
We have measured the ratio, r = of the formation cross
section, of singlet () and triplet () excitons
from oppositely charged polarons in a large variety of -conjugated
oligomer and polymer films, using the photoinduced absorption and optically
detected magnetic resonance spectroscopies. The ratio r is directly related to
the singlet exciton yield, which in turn determines the maximum
electroluminescence quantum efficiency in organic light emitting diodes (OLED).
We discovered that r increases with the conjugation length, CL; in fact a
universal dependence exists in which depends linearly on ,
irrespective of the chain backbone structure. These results indicate that
-conjugated polymers have a clear advantage over small molecules in OLED
applications.Comment: 5 pages, 4 figure
Integrated impedance bridge for absolute capacitance measurements at cryogenic temperatures and finite magnetic fields
We developed an impedance bridge that operates at cryogenic temperatures
(down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This
is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed
circuit board containing the device under test and thereby parallel to the
magnetic field. The measured amplitude and phase of the output signal allows
for the separation of the total impedance into an absolute capacitance and a
resistance. Through a detailed noise characterization, we find that the best
resolution is obtained when operating the HEMT amplifier at the highest gain.
We obtained a resolution in the absolute capacitance of
6.4~aF at 77 K on a comb-drive actuator, while maintaining
a small excitation amplitude of 15~. We show the magnetic field
functionality of our impedance bridge by measuring the quantum Hall plateaus of
a top-gated hBN/graphene/hBN heterostructure at 60~mK with a probe signal of
12.8~.Comment: 7 pages, 5 figure
Giant Gravitons in type IIA PP-wave Background
We examine giant gravitons with a worldvolume magnetic flux in type IIA
pp-wave background and find that they can move away from the origin along
direction in target space satisfying . This nontrivial relation can be
regarded as a complementary relation of the giant graviton on IIA pp-wave and
is shown to be connected to the spacetime uncertainty principle. The giant
graviton is also investigated in a system of N D0-branes as a fuzzy sphere
solution. It is observed that enters into the fuzzy algebra as a
deformation parameter. Such a background dependent Myers effect guarantees that
we again get the crucial relation of our giant graviton. In the paper, we also
find a BIon configuration on the giant graviton in this background.Comment: 10 pages, no figure, content added, typo corrected, reference adde
Optimal strategies for continuous gravitational wave detection in pulsar timing arrays
Supermassive black hole binaries (SMBHBs) are expected to emit continuous
gravitational waves in the pulsar timing array (PTA) frequency band
(-- Hz). The development of data analysis techniques aimed at
efficient detection and characterization of these signals is critical to the
gravitational wave detection effort. In this paper we leverage methods
developed for LIGO continuous wave gravitational searches, and explore the use
of the -statistic for such searches in pulsar timing data. Babak &
Sesana 2012 have already used this approach in the context of PTAs to show that
one can resolve multiple SMBHB sources in the sky. Our work improves on several
aspects of prior continuous wave search methods developed for PTA data
analysis. The algorithm is implemented fully in the time domain, which
naturally deals with the irregular sampling typical of PTA data and avoids
spectral leakage problems associated with frequency domain methods. We take
into account the fitting of the timing model, and have generalized our approach
to deal with both correlated and uncorrelated colored noise sources. We also
develop an incoherent detection statistic that maximizes over all pulsar
dependent contributions to the likelihood. To test the effectiveness and
sensitivity of our detection statistics, we perform a number of monte-carlo
simulations. We produce sensitivity curves for PTAs of various configurations,
and outline an implementation of a fully functional data analysis pipeline.
Finally, we present a derivation of the likelihood maximized over the
gravitational wave phases at the pulsar locations, which results in a vast
reduction of the search parameter space.Comment: 11 pages, 5 figure
Gravitational wave detection using pulsars: status of the Parkes Pulsar Timing Array project
The first direct detection of gravitational waves may be made through
observations of pulsars. The principal aim of pulsar timing array projects
being carried out worldwide is to detect ultra-low frequency gravitational
waves (f ~ 10^-9 to 10^-8 Hz). Such waves are expected to be caused by
coalescing supermassive binary black holes in the cores of merged galaxies. It
is also possible that a detectable signal could have been produced in the
inflationary era or by cosmic strings. In this paper we review the current
status of the Parkes Pulsar Timing Array project (the only such project in the
Southern hemisphere) and compare the pulsar timing technique with other forms
of gravitational-wave detection such as ground- and space-based interferometer
systems.Comment: Accepted for publication in PAS
Nonequilibrium phase transition in a model for the propagation of innovations among economic agents
We characterize the different morphological phases that occur in a simple
one-dimensional model of propagation of innovations among economic agents [X.\
Guardiola, {\it et. al.}, Phys. Rev E {\bf 66}, 026121 (2002)]. We show that
the model can be regarded as a nonequilibrium surface growth model. This allows
us to demonstrate the presence of a continuous roughening transition between a
flat (system size independent fluctuations) and a rough phase (system size
dependent fluctuations). Finite-size scaling studies at the transition strongly
suggest that the dynamic critical transition does not belong to directed
percolation and, in fact, critical exponents do not seem to fit in any of the
known universality classes of nonequilibrium phase transitions. Finally, we
present an explanation for the occurrence of the roughening transition and
argue that avalanche driven dynamics is responsible for the novel critical
behavior
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