7,726 research outputs found
Electric Transport Theory of Dirac Fermions in Graphene
Using the self-consistent Born approximation to the Dirac fermions under
finite-range impurity scatterings, we show that the current-current correlation
function is determined by four-coupled integral equations. This is very
different from the case for impurities with short-range potentials. As a test
of the present approach, we calculate the electric conductivity in graphene for
charged impurities with screened Coulomb potentials. The obtained conductivity
at zero temperature varies linearly with the carrier concentration, and the
minimum conductivity at zero doping is larger than the existing theoretical
predictions, but still smaller than that of the experimental measurement. The
overall behavior of the conductivity obtained by the present calculation at
room temperature is similar to that at zero temperature except the minimum
conductivity is slightly larger.Comment: 6 pages, 3 figure
Magnetoconductivity of Dirac fermions in graphene under charged impurity scatterings
On the basis of self-consistent Born approximation, we solve the
Bethe-Salpeter matrix equations for Cooperon propagator of the Dirac fermions
in graphene under the charged impurity scatterings and a weak external magnetic
field. In the absence of the magnetic field, the quantum interference effect in
the electric conductivity from the contribution of Cooperon propagator will be
studied and possible weak localization in the system is discussed in terms of
the sample length and temperature. The magnetoconductivity stemming from the
quantum interference effect is calculated, and the obtained results are in good
agreement with experimental measurements.Comment: 13 pages, 9 figure
CSNL: A cost-sensitive non-linear decision tree algorithm
This article presents a new decision tree learning algorithm called CSNL that induces Cost-Sensitive Non-Linear decision trees. The algorithm is based on the hypothesis that nonlinear decision nodes provide a better basis than axis-parallel decision nodes and utilizes discriminant analysis to construct nonlinear decision trees that take account of costs of misclassification.
The performance of the algorithm is evaluated by applying it to seventeen datasets and the results are compared with those obtained by two well known cost-sensitive algorithms, ICET and MetaCost, which generate multiple trees to obtain some of the best results to date. The results show that CSNL performs at least as well, if not better than these algorithms, in more than twelve of the datasets and is considerably faster. The use of bagging with CSNL further enhances its performance showing the significant benefits of using nonlinear decision nodes.
The performance of the algorithm is evaluated by applying it to seventeen data sets and the results are
compared with those obtained by two well known cost-sensitive algorithms, ICET and MetaCost, which generate multiple trees to obtain some of the best results to date.
The results show that CSNL performs at least as well, if not better than these algorithms, in more than twelve of the data sets and is considerably faster.
The use of bagging with CSNL further enhances its performance showing the significant benefits of using non-linear decision nodes
Long-range dynamics of magnetic impurities coupled to a two-dimensional Heisenberg antiferromagnet
We consider a two-dimensional Heisenberg antiferromagnet on a square lattice
with weakly coupled impurities, i.e. additional spins interacting with the host
magnet by a small dimensionless coupling constant g<<1. Using linear spin-wave
theory, we find that the magnetization disturbance at distance r from a single
impurity behaves as g/r for 1>1/g. Surprisingly
the disturbance is inversely proportional to the coupling constant! The
interaction between two impurities separated by a distance r is proportional to
g^2/r for 1>1/g. Hence at large distances, the
interaction is universal and independent of the coupling constant. We also find
that the frequency of Rabi oscillations between two impurities is proportional
to g^2 ln(gr) at 1<<r<<1/g, logarithmically enhanced compared to the spin-wave
width. This leads to a new mechanism for NMR, NQR and EPR line broadening. All
these astonishing results are due to the gapless spectrum of the magnetic
excitations in the quantum antiferromagnet.Comment: 6 pages, 5 figure
Gratitude: Does it have a place within media-practice education?
Gratitude may be an important yet largely untapped aspect of media-practice education. This research uses an exploratory approach with media-practice academics and students in order to examine the evidence and nature of gratitude within media-practice education. Given the exploratory nature of the study, interim findings are reported. The research finds media-practice students to be open to gratitude having a place within their educational experiences, indeed students exclusively speak about situations in which they feel grateful, as positive aspects of their student experience. However academics see gratitude in a more varied way. For some, gratitude and its cyclical nature resonate; for others, gratitude is inappropriate and loaded with notions of power. These different perspectives may be partially explained by the different ways in which students and academics perceive gratitude. Whereas initial student voices suggest that gratitude is a positive emotion associated with a desire to reciprocate, academics’ interpretation seems to emphasise obligation. This mismatch may inhibit the current impact of gratitude within the media-practice learning context. The research suggests that gratitude may be a defining aspect of a functional, productive student experience which those working within media-practice education might usefully aim to generate
Absence of residual quasiparticle conductivity in the underdoped cuprate YBa2Cu4O8
We report here measurements of the in-plane thermal conductivity K(T) of the
underdoped cuprate YBa2Cu4O8 (Y124) below 1K. K(T) is shown to follow a simple,
phononic T^3 dependence at the lowest temperatures for both current directions,
with a negligible linear, quasiparticle contribution. This observation is in
marked contrast with behavior reported in optimally doped cuprates, and implies
that extended zero-energy (or low energy) low-energy quasiparticles are absent
in YBa2Cu4O8 at low temperatures.Comment: RevTex, 4 pages, 2 figures, Revised versio
Research in Pediatric Residency: National Experience of Pediatric Chief Residents
Objective
To determine factors associated with increased research productivity, satisfaction, and perceived barriers to research within residency from the experience of pediatric chief residents.
Methods
An online cross-sectional survey was administered to academic year 2014–15 chief residents. Topics assessed included program demographic characteristics, career intentions, research productivity, satisfaction with research training and opportunities, and research barriers. Chi-square and Fisher exact tests were used for descriptive statistics. Multivariable logistic regression analysis was used to determine factors associated with productivity and research satisfaction.
Results
The response rate was 63% (165 of 261). Half (82 of 165) were productive in research. Most were satisfied with their quality of research training (55%; 90 of 165) and research opportunities (69%; 114 of 165). Chiefs reporting interest in research were 5 times more likely to be productive than those who did not (odds ratio [OR] = 5.2; 95% confidence interval [CI], 2.3–11.8). Productive chiefs were more likely to report including research time in future careers (P = .003). Most (83%; 137 of 165) thought their programs were supportive of resident research, but lack of time was frequently cited as a major barrier. Those satisfied with research opportunities were less likely to find lack of training (OR = 0.3; 95% CI, 0.1–0.7) or faculty mentorship (OR = 0.2; 95% CI, 0.0–0.9) as a major barrier.
Conclusions
Pediatric chief resident interest in research is strongly associated with research productivity during residency, and research productivity is strongly associated with career plans including research time. By cultivating research interest through faculty mentorship, research training, and dedicated time, pediatric residency programs might help foster early research success and, potentially lead to continued engagement with research in trainees' future careers
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