5,496 research outputs found
Anisotropic conductivity of doped graphene due to short-range non-symmetric scattering
The conductivity of doped graphene is considered taking into account
scattering by short-range nonsymmetric defects, when the longitudinal and
transverse components of conductivity tensor appear to be different. The
calculations of the anisotropic conductivity tensor are based on the
quasiclassical kinetic equation for the case of monopolar transport at low
temperatures. The effective longitudinal conductivity and the transverse
voltage, which are controlled by orientation of sample and by gate voltage
(i.e. doping level), are presented.Comment: 3 pages, 2 figure
Polarization of tightly focused laser beams
The polarization properties of monochromatic light beams are studied. In
contrast to the idealization of an electromagnetic plane wave, finite beams
which are everywhere linearly polarized in the same direction do not exist.
Neither do beams which are everywhere circularly polarized in a fixed plane. It
is also shown that transversely finite beams cannot be purely transverse in
both their electric and magnetic vectors, and that their electromagnetic energy
travels at less than c. The electric and magnetic fields in an electromagnetic
beam have different polarization properties in general, but there exists a
class of steady beams in which the electric and magnetic polarizations are the
same (and in which energy density and energy flux are independent of time).
Examples are given of exactly and approximately linearly polarized beams, and
of approximately circularly polarized beams.Comment: 9 pages, 6 figure
A selected history of expectation bias in physics
The beliefs of physicists can bias their results towards their expectations
in a number of ways. We survey a variety of historical cases of expectation
bias in observations, experiments, and calculations.Comment: 6 pages, 2 figure
Ab-initio elastic tensor of cubic TiAlN alloy: the dependence of the elastic constants on the size and shape of the supercell model
In this study we discuss the performance of approximate SQS supercell models
in describing the cubic elastic properties of B1 (rocksalt)
TiAlN alloy by using a symmetry based projection technique. We
show on the example of TiAlN alloy, that this projection
technique can be used to align the differently shaped and sized SQS structures
for a comparison in modeling elasticity. Moreover, we focus to accurately
determine the cubic elastic constants and Zener's type elastic anisotropy of
TiAlN. Our best supercell model, that captures accurately both
the randomness and cubic elastic symmetry, results in GPa,
GPa and GPa with 3% of error and for Zener's
elastic anisotropy with 6% of error. In addition, we establish the general
importance of selecting proper approximate SQS supercells with symmetry
arguments to reliably model elasticity of alloys. In general, we suggest the
calculation of nine elastic tensor elements - , , ,
, , , , and , to evaluate and
analyze the performance of SQS supercells in predicting elasticity of cubic
alloys via projecting out the closest cubic approximate of the elastic tensor.
The here described methodology is general enough to be applied in discussing
elasticity of substitutional alloys with any symmetry and at arbitrary
composition.Comment: Submitted to Physical Review
Evidence Inference 2.0: More Data, Better Models
How do we most effectively treat a disease or condition? Ideally, we could
consult a database of evidence gleaned from clinical trials to answer such
questions. Unfortunately, no such database exists; clinical trial results are
instead disseminated primarily via lengthy natural language articles. Perusing
all such articles would be prohibitively time-consuming for healthcare
practitioners; they instead tend to depend on manually compiled systematic
reviews of medical literature to inform care.
NLP may speed this process up, and eventually facilitate immediate consult of
published evidence. The Evidence Inference dataset was recently released to
facilitate research toward this end. This task entails inferring the
comparative performance of two treatments, with respect to a given outcome,
from a particular article (describing a clinical trial) and identifying
supporting evidence. For instance: Does this article report that chemotherapy
performed better than surgery for five-year survival rates of operable cancers?
In this paper, we collect additional annotations to expand the Evidence
Inference dataset by 25\%, provide stronger baseline models, systematically
inspect the errors that these make, and probe dataset quality. We also release
an abstract only (as opposed to full-texts) version of the task for rapid model
prototyping. The updated corpus, documentation, and code for new baselines and
evaluations are available at http://evidence-inference.ebm-nlp.com/.Comment: Accepted as workshop paper into BioNLP Updated results from SciBERT
to Biomed RoBERT
Rotator and extender ferroelectrics: Importance of the shear coefficient to the piezoelectric properties of domain-engineered crystals and ceramics
The importance of a high shear coefficient d15 (or d24) to the piezoelectric
properties of domain-engineered and polycrystalline ferroelectrics is
discussed. The extent of polarization rotation, as a mechanism of piezoelectric
response, is directly correlated to the shear coefficient. The terms "rotator"
and "extender" are introduced to distinguish the contrasting behaviors of
crystals such as 4mm BaTiO3 and PbTiO3. In "rotator" ferroelectrics, where d15
is high relative to the longitudinal coefficient d33, polarization rotation is
the dominant mechanism of piezoelectric response; the maximum longitudinal
piezoelectric response is found away from the polar axis. In "extender"
ferroelectrics, d15 is low and the collinear effect dominates; the maximum
piezoelectric response is found along the polar axis. A variety of 3m, mm2 and
4mm ferroelectrics, with various crystal structures based on oxygen octahedra,
are classified in this way. It is shown that the largest piezoelectric
anisotropies d15/d33 are always found in 3m crystals; this is a result of the
intrinsic electrostrictive anisotropy of the constituent oxygen octahedra.
Finally, for a given symmetry, the piezoelectric anisotropy increases close to
ferroelectric-ferroelectric phase transitions; this includes morphotropic phase
boundaries and temperature induced polymorphic transitions.Comment: accepted in J. Appl. Phy
Thermal noise of whispering gallery resonators
By direct application of the fluctuation-dissipation theorem, we numerically
calculate the fundamental dimensional fluctuations of crystalline CaF2
whispering gallery resonators in the case of structural damping, and the limit
that this noise imposes on the frequency stability of such resonators at both
room and cryogenic temperatures. We analyze elasto-optic noise - the effect of
Brownian dimensional fluctuation on frequency via the strain-dependence of the
refractive index - a noise term that has so far not been considered for
whispering-gallery resonators. We find that dimensional fluctuation sets a
lower limit of 1E-16 to the Allan deviation for a 10-millimeter-radius sphere
at 5 K, predominantly via induced fluctuation of the refractive index.Comment: 7 pages, 3 figure
Positive effective Q12 electrostrictive coefficient in perovskites
It is demonstrated that for classical perovskites such as BaTiO3, SrTiO3 and
PbTiO3 electrostrictive strain induced by an electric field may not obey
traditionally considered "extension along the field, contraction perpendicular
to it" behavior if a sample is cut obliquely to the cubic crystallographic
directions
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