5,806 research outputs found
The Schwinger mechanism and graphene
The Schwinger mechanism, the production of charged particle-antiparticle
pairs in a macroscopic external electric field, is derived for 2+1 dimensional
theories. The rate of pair production per unit area for four species of
massless fermions, with charge , in a constant electric field is given
by where is
the speed of light for the two-dimensional system. To the extent undoped
graphene behaves like the quantum field-theoretic vacuum for massless fermions
in 2+1 dimensions, the Schwinger mechanism should be testable experimentally. A
possible experimental configuration for this is proposed. Effects due to
deviations from this idealized picture of graphene are briefly considered. It
is argued that with present day samples of graphene, tests of the Schwinger
formula may be possible.Comment: Extensive revisions. The distinction between the vacuum decay rate
and the pair production rate in the Schwinger mechanism is now stressed. The
discussion of quality of sample needed for a viable experimental test has
been significantly expanded. References adde
Front Propagation up a Reaction Rate Gradient
We expand on a previous study of fronts in finite particle number
reaction-diffusion systems in the presence of a reaction rate gradient in the
direction of the front motion. We study the system via reaction-diffusion
equations, using the expedient of a cutoff in the reaction rate below some
critical density to capture the essential role of fl uctuations in the system.
For large density, the velocity is large, which allows for an approximate
analytic treatment. We derive an analytic approximation for the front velocity
depe ndence on bulk particle density, showing that the velocity indeed diverge
s in the infinite density limit. The form in which diffusion is impleme nted,
namely nearest-neighbor hopping on a lattice, is seen to have an essential
impact on the nature of the divergence
Twisted-light-induced optical transitions in semiconductors: Free-carrier quantum kinetics
We theoretically investigate the interband transitions and quantum kinetics
induced by light carrying orbital angular momentum, or twisted light, in bulk
semiconductors. We pose the problem in terms of the Heisenberg equations of
motion of the electron populations, and inter- and intra-band coherences. Our
theory extends the free-carrier Semiconductor Bloch Equations to the case of
photo-excitation by twisted light. The theory is formulated using cylindrical
coordinates, which are better suited to describe the interaction with twisted
light than the usual cartesian coordinates used to study regular optical
excitation. We solve the equations of motion in the low excitation regime, and
obtain analytical expressions for the coherences and populations; with these,
we calculate the orbital angular momentum transferred from the light to the
electrons and the paramagnetic and diamagnetic electric current densities.Comment: 11 pages, 3 figure
Fracture of a viscous liquid
When a viscous liquid hits a pool of liquid of same nature, the impact region
is hollowed by the shock. Its bottom becomes extremely sharp if increasing the
impact velocity, and we report that the curvature at that place increases
exponentially with the flow velocity, in agreement with a theory by Jeong and
Moffatt. Such a law defines a characteristic velocity for the collapse of the
tip, which explains both the cusp-like shape of this region, and the
instability of the cusp if increasing (slightly) the impact velocity. Then, a
film of the upper phase is entrained inside the pool. We characterize the
critical velocity of entrainment of this phase and compare our results with
recent predictions by Eggers
Relativistic Quantum Mechanics and Relativistic Entanglement in the Rest-Frame Instant Form of Dynamics
A new formulation of relativistic quantum mechanics is proposed in the
framework of the rest-frame instant form of dynamics with its instantaneous
Wigner 3-spaces and with its description of the particle world-lines by means
of derived non-canonical predictive coordinates. In it we quantize the frozen
Jacobi data of the non-local 4-center of mass and the Wigner-covariant relative
variables in an abstract (frame-independent) internal space whose existence is
implied by Wigner-covariance. The formalism takes care of the properties of
both relativistic bound states and scattering ones. There is a natural solution
to the \textit{relativistic localization problem}. The non-relativistic limit
leads to standard quantum mechanics but with a frozen Hamilton-Jacobi
description of the center of mass. Due to the \textit{non-locality} of the
Poincar\'e generators the resulting theory of relativistic entanglement is both
\textit{kinematically non-local and spatially non-separable}: these properties,
absent in the non-relativistic limit, throw a different light on the
interpretation of the non-relativistic quantum non-locality and of its impact
on foundational problems.Comment: 73 pages, includes revision
Reply to "Comment on `First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism'"
The recent preprint by Mazin et al. [cond-mat/0212417] contains many
inappropriate evaluations and/or criticisms on our published work [Phys. Rev. B
66, 020513 (2002) and Nature 418, 758 (2002)]. The preprint
[cond-mat/0212417v1] was submitted to Physical Review B as a comment on one of
our papers [Phys. Rev. B 66, 020513 (2002)]. In the reviewing process, Mazin et
al. have withdrawn many of the statements contained in cond-mat/0212417v1,
however two claims remain in their revised manuscript [cond-mat/0212417v3]: (1)
the calculated variations of the superconducting energy gap within the sigma-
or the pi-bands are not observable in real samples due to scatterings, and (2)
the Coulomb repulsion mu(k,k') is negligibly small between sigma- and pi-states
and thus should be approximated by a diagonal 2 x 2 matrix in the sigma and pi
channels. Here, we point out that the former does not affect the validity of
our theoretical work which is for the clean limit, and that the latter is not
correct
Radio astronomy
The following subject areas are covered: (1) scientific opportunities (millimeter and sub-millimeter wavelength astronomy; meter to hectometer astronomy; the Sun, stars, pulsars, interstellar masers, and extrasolar planets; the planets, asteroids, and comets; radio galaxies, quasars, and cosmology; and challenges for radio astronomy in the 1990's); (2) recommendations for new facilities (the millimeter arrays, medium scale instruments, and small-scale projects); (3) continuing activities and maintenance, upgrading of telescopes and instrumentation; (4) long range programs and technology development; and (5) social, political, and organizational considerations
First-principles investigation of 180-degree domain walls in BaTiO_3
We present a first-principles study of 180-degree ferroelectric domain walls
in tetragonal barium titanate. The theory is based on an effective Hamiltonian
that has previously been determined from first-principles
ultrasoft-pseudopotential calculations. Statistical properties are investigated
using Monte Carlo simulations. We compute the domain-wall energy, free energy,
and thickness, analyze the behavior of the ferroelectric order parameter in the
interior of the domain wall, and study its spatial fluctuations. An abrupt
reversal of the polarization is found, unlike the gradual rotation typical of
the ferromagnetic case.Comment: Revtex (preprint style, 13 pages) + 3 postscript figures. A version
in two-column article style with embedded figures is available at
http://electron.rutgers.edu/~dhv/preprints/index.html#pad_wal
Value within otolaryngology: Assessment of the cost-utility analysis literature
AbstractObjectiveTo assess the characteristics and quality of cost utility analyses (CUA) related to otolaryngology within the CEA registry and to summarize their collective results.MethodsAll cost-utility analyses published between 1976 and 2011 contained in the Cost-Effectiveness Analysis Registry (CEA Registry) were evaluated. Topics that fall within the care of an otolaryngologist were included in the review regardless of the presence of an otolaryngologist author. Potential associations between various study characteristics and CEA registry quality scores were evaluated using the Pearson product moment correlation coefficient.ResultsSixty-one of 2913 (2.1%) total CUA publications screened were related to otolaryngology. Eighteen of 61 (29.5%) publications included an otolaryngologist as an author. Fourteen studies agreed on the cost effectiveness of at least unilateral cochlear implantation and six of seven (85.7%) studies demonstrated the cost effectiveness of continuous positive airway pressure (CPAP) for obstructive sleep apnea (OSA). Forty-six percent (28 of 61) of all manuscripts were published between 2008 and 2011. A more recent publication year was associated with a higher CEA registry quality score while the presence of an otolaryngologist author and journal impact factor had no significant correlation with the quality of the CUA.ConclusionBased on current evidence in the CEA registry, unilateral cochlear implantation for hearing loss and CPAP for OSA are both cost-effective therapeutic interventions. Although CUAs in otolaryngology have increased in quantity and improved in quality in more recent years, there is a relative lack of CUAs in otolaryngology in comparison to other subspecialties
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