8,431 research outputs found
Universal Thermoelectric Effect of Dirac Fermions in Graphene
We numerically study the thermoelectric transports of Dirac fermions in
graphene in the presence of a strong magnetic field and disorder. We find that
the thermoelectric transport coefficients demonstrate universal behavior
depending on the ratio between the temperature and the width of the
disorder-broadened Landau levels(LLs). The transverse thermoelectric
conductivity reaches a universal quantum value at the center of
each LL in the high temperature regime, and it has a linear temperature
dependence at low temperatures. The calculated Nernst signal has a peak at the
central LL with heights of the order of , and changes sign near other
LLs, while the thermopower has an opposite behavior, in good agreement with
experimental data. The validity of the generalized Mott relation between the
thermoelectric and electrical transport coefficients is verified in a wide
range of temperatures.Comment: 4 pages, 4 figures, published versio
The Mercedes flap: a modified closure for circular skin defects around the eyebrow
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113709/1/ced12201.pd
Absence of dynamical gap generation in suspended graphene
There is an interesting proposal that the long-range Coulomb interaction in
suspended graphene can generate a dynamical gap, which leads to a
semimetal-insulator phase transition. We revisit this problem by solving the
self-consistent Dyson-Schwinger equations of wave function renormalization and
fermion gap. In order to satisfy the Ward identity, a suitable vertex function
is introduced. The impacts of singular velocity renormalization and dynamical
screening on gap generation are both included in this formalism, and prove to
be very important. We obtain a critical interaction strength, , which is larger than the physical value for suspended
graphene. It therefore turns out that suspended graphene is a semimetal, rather
than insulator, at zero temperature.Comment: 14 pages, 5 figures, 1 tabl
EFFECTIVENESS OF INTERGENERATIONAL PROGRAM IN IMPROVING COMMUNITY SENIOR CITIZENS’ GENERAL HEALTH AND HAPPINESS
Intergenerational programs (IGPs) encourage senior citizens tocontinue engaging in social participation, preserving their independence, andenriching their later life. However, IGPs in Taiwan (IGPs-T) are notprevalent and generally more apply between children and senior citizensexist only, which lack in-depth exchanges the senior citizen‘s life experienceand communication the young people. Relevant empirical results regardingthe effectiveness of IGPs activities on community senior citizens‘ health andhappiness are lacking. The objective of this study was to improve the mutualunderstanding and inclusion between community senior citizens and younggenerations, thereby improving the senior citizens‘ physical and mentalhealth functions and happiness.This was a two-group pre and post-test quasi-experimental studydesign and purposive sampling was used to recruit community senior citizensin Taipei City as research participants (N = 118). The experimental group (n= 55) received 90-min IGP-T activities once a week for 8 weeks. The controlgroup (n = 63) received general health services providing by communityhealth nurses in community.The outcomes of the experimental group significant progress in thehappiness (t=2.82, p=.007), and emotional well-being (EWB) (t=4.61,p≦.001) from pre-test to post-test. And the experimental group wassignificantly superior to control group in perceived health status (PHS)(t=3.79, p≦.001), happiness (t=3.82, p≦.001), and emotional well-being(EWB) (t=4.93, p≦.001) at post-test, and achieve a statistically significantdifference
Discrete chaotic states of a Bose-Einstein condensate
We find the different spatial chaos in a one-dimensional attractive
Bose-Einstein condensate interacting with a Gaussian-like laser barrier and
perturbed by a weak optical lattice. For the low laser barrier the chaotic
regions of parameters are demonstrated and the chaotic and regular states are
illustrated numerically. In the high barrier case, the bounded perturbed
solutions which describe a set of discrete chaotic states are constructed for
the discrete barrier heights and magic numbers of condensed atoms. The chaotic
density profiles are exhibited numerically for the lowest quantum number, and
the analytically bounded but numerically unbounded Gaussian-like configurations
are confirmed. It is shown that the chaotic wave packets can be controlled
experimentally by adjusting the laser barrier potential.Comment: 7 pages, 5 figure
Thermal conduction of carbon nanotubes using molecular dynamics
The heat flux autocorrelation functions of carbon nanotubes (CNTs) with
different radius and lengths is calculated using equilibrium molecular
dynamics. The thermal conductance of CNTs is also calculated using the
Green-Kubo relation from the linear response theory. By pointing out the
ambiguity in the cross section definition of single wall CNTs, we use the
thermal conductance instead of conductivity in calculations and discussions. We
find that the thermal conductance of CNTs diverges with the CNT length. After
the analysis of vibrational density of states, it can be concluded that more
low frequency vibration modes exist in longer CNTs, and they effectively
contribute to the divergence of thermal conductance.Comment: 15 pages, 6 figures, submitted to Physical Review
Persistence, extinction and spatio-temporal synchronization of SIRS cellular automata models
Spatially explicit models have been widely used in today's mathematical
ecology and epidemiology to study persistence and extinction of populations as
well as their spatial patterns. Here we extend the earlier work--static
dispersal between neighbouring individuals to mobility of individuals as well
as multi-patches environment. As is commonly found, the basic reproductive
ratio is maximized for the evolutionary stable strategy (ESS) on diseases'
persistence in mean-field theory. This has important implications, as it
implies that for a wide range of parameters that infection rate will tend
maximum. This is opposite with present results obtained in spatial explicit
models that infection rate is limited by upper bound. We observe the emergence
of trade-offs of extinction and persistence on the parameters of the infection
period and infection rate and show the extinction time having a linear
relationship with respect to system size. We further find that the higher
mobility can pronouncedly promote the persistence of spread of epidemics, i.e.,
the phase transition occurs from extinction domain to persistence domain, and
the spirals' wavelength increases as the mobility increasing and ultimately, it
will saturate at a certain value. Furthermore, for multi-patches case, we find
that the lower coupling strength leads to anti-phase oscillation of infected
fraction, while higher coupling strength corresponds to in-phase oscillation.Comment: 12page
FLEET: Butterfly Estimation from a Bipartite Graph Stream
We consider space-efficient single-pass estimation of the number of
butterflies, a fundamental bipartite graph motif, from a massive bipartite
graph stream where each edge represents a connection between entities in two
different partitions. We present a space lower bound for any streaming
algorithm that can estimate the number of butterflies accurately, as well as
FLEET, a suite of algorithms for accurately estimating the number of
butterflies in the graph stream. Estimates returned by the algorithms come with
provable guarantees on the approximation error, and experiments show good
tradeoffs between the space used and the accuracy of approximation. We also
present space-efficient algorithms for estimating the number of butterflies
within a sliding window of the most recent elements in the stream. While there
is a significant body of work on counting subgraphs such as triangles in a
unipartite graph stream, our work seems to be one of the few to tackle the case
of bipartite graph streams.Comment: This is the author's version of the work. It is posted here by
permission of ACM for your personal use. Not for redistribution. The
definitive version was published in Seyed-Vahid Sanei-Mehri, Yu Zhang, Ahmet
Erdem Sariyuce and Srikanta Tirthapura. "FLEET: Butterfly Estimation from a
Bipartite Graph Stream". The 28th ACM International Conference on Information
and Knowledge Managemen
Impacts of Filtration on Contrast-Detail Detectability of an X-ray Imaging System
The purpose of this study is to investigate the impacts of added filtration
on the contrast-detail detectability of a digital X-ray imaging system for
small animal studies. A digital X-ray imaging system specifically
designed for small animal studies was used.
This system is equipped with a micro X-ray source with a tungsten
target and a beryllium window filtration and a CCD-based digital
detector. Molybdenum filters of 0 mm, 0.02 mm, and 0.05 mm in thickness were added. The corresponding X-ray
spectra and contrast-detail detectabilities were measured using
two phantoms of different thicknesses simulating breast tissue
under different exposures. The added Mo filters
reduced the low-energy as well as the high-energy photons, hence
providing a narrowband for imaging quality improvement. In the
experiments with a 1.15 cm phantom, the optimal image detectability was observed using 22 kVp and the 0.05 mm Mo
filter. With the 2.15 cm phantom, the best detectability
was obtained with 22 kVp and the 0.02 mm Mo filter.
Our experiments showed that appropriate
filtrations could reduce certain low- and high-energy components of
X-ray spectra which have limited contributions to image contrast.
At the same time, such filtration could improve the
contrast-detail detectability, particularly at relatively low kVp
and high filtration. Therefore, optimal image quality can be
obtained with the same absorbed radiation dose by the subjects when
appropriate filtration is used
Dielectric nonlinearity of relaxor ferroelectric ceramics at low ac drives
Dielectric nonlinear response of
(PbMgNbO)(PbTiO) (0.9PMN-0.1PT) relaxor
ceramics was investigated under different ac drive voltages. It was observed
that: (i) the dielectric permittivity is independent on ac field amplitude at
high temperatures; (ii) with increasing ac drive, the permittivity maximum
increases, and the temperature of the maximum shifts to lower temperature;
(iii) the nonlinear effect is weakened when the measurement frequency
increases. The influences of increasing ac drive were found to be similar to
that of decreasing frequency. It is believed that the dielectric nonlinearities
of relaxors at low drives can be explained by the phase transition theory of
ergodic space shrinking in succession. A Monte Carlo simulation was performed
on the flips of micro polarizations at low ac drives to verify the theory.Comment: Submitted to J. Phys.: Cond. Matte
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