5,985 research outputs found
Signature of Schwinger's pair creation rate via radiation generated in graphene by strong electric current
Electron - hole pairs are copuously created by an applied electric field near
the Dirac point in graphene or similar 2D electronic systems. It was shown
recently that for sufficiently large electric fields and ballistic times the
I-V characteristics become strongly nonlinear due to Schwinger's pair creation.
Since there is no energy gap the radiation from the pairs' annihilation is
enhanced. The spectrum of radiation is calculated. The angular and polarization
dependence of the emitted photons with respect to the graphene sheet is quite
distinctive. For very large currents the recombination rate becomes so large
that it leads to the second Ohmic regime due to radiation friction.Comment: 9 pages, 7 figure
One-dimensional transport in polymer nanofibers
We report our transport studies in quasi one-dimensional (1D) conductors -
helical polyacetylene fibers doped with iodine and the data analysis for other
polymer single fibers and tubes. We found that at 30 K < T < 300 K the
conductance and the current-voltage characteristics follow the power law: G(T)
~ T^alpha with alpha ~ 2.2-7.2 and I(V) ~ V^betta with betta ~ 2-5.7. Both G(T)
and I(V) show the features characteristic of 1D systems such as Luttinger
liquid or Wigner crystal. The relationship between our results and theories for
tunneling in 1D systems is discussed.Comment: 11 pages, 3 figures, accepted for publication in Phys. Rev. Letter
RF bifurcation of a Josephson junction: microwave embedding circuit requirements
A Josephson tunnel junction which is RF-driven near a dynamical bifurcation
point can amplify quantum signals. The bifurcation point will exist robustly
only if the electrodynamic environment of the junction meets certain criteria.
In this article we develop a general formalism for dealing with the non-linear
dynamics of Josephson junction embedded in an arbitrary microwave circuit. We
find sufficient conditions for the existence of the bifurcation regime: a) the
embedding impedance of the junction need to present a resonance at a particular
frequency , with the quality factor of the resonance and the
participation ratio of the junction satisfying , b) the drive
frequency should be low frequency detuned away from by more than
.Comment: Submitted to Phys. Rev. B, 12 pages, 6 figure
A simpler and more efficient algorithm for the next-to-shortest path problem
Given an undirected graph with positive edge lengths and two
vertices and , the next-to-shortest path problem is to find an -path
which length is minimum amongst all -paths strictly longer than the
shortest path length. In this paper we show that the problem can be solved in
linear time if the distances from and to all other vertices are given.
Particularly our new algorithm runs in time for general
graphs, which improves the previous result of time for sparse
graphs, and takes only linear time for unweighted graphs, planar graphs, and
graphs with positive integer edge lengths.Comment: Partial result appeared in COCOA201
Associations between body mass index and serum levels of C-reactive protein
Background. Obesity leads to increased risk of cardiovascular disease and glucose intolerance, which are phenomena of chronic inflammation. This study was performed to determine whether a higher body mass index (BMI) and central obesity are associated with low-grade inflammation.Methods. An analysis of 8 453 adults aged .20 years was performed. Every subject completed a household interview and a questionnaire regarding personal health, and their BMI and serum C-reactive protein (CRP) level were measured. The BMI data were divided into quintiles, using multiple linear regression to estimate the relationship between CRP level and BMI quintiles. An extended-model approach was used forcovariate adjustment. The association between central obesity and CRP level was examined by this method as well.Results. After controlling for demographics, chronic diseases, health behaviours and levels of folate and vitamin B12, the ƒÀ coefficient (which represents the change of naturallog- transformed levels of CRP for each kg/m2 increase in BMI) was 0.078 (
Kaluza-Klein Higher Derivative Induced Gravity
The existence and stability analysis of an inflationary solution in a
-dimensional anisotropic induced gravity is presented in this paper.
Nontrivial conditions in the field equations are shown to be compatible with a
cosmological model in which the 4-dimension external space evolves
inflationary, while, the D-dimension internal one is static. In particular,
only two additional constraints on the coupling constants are derived from the
abundant field equations and perturbation equations. In addition, a compact
formula for the non-redundant 4+D dimensional Friedmann equation is also
derived for convenience. Possible implications are also discussed in this
paper.Comment: 13 pages, typos/errors corrected, three additional appendices adde
Inflationary Universe in Higher Derivative Induced Gravity
In an induced-gravity model, the stability condition of an inflationary
slow-rollover solution is shown to be . The presence of higher derivative terms
will, however, act against the stability of this expanding solution unless
further constraints on the field parameters are imposed. We find that these
models will acquire a non-vanishing cosmological constant at the end of
inflation. Some models are analyzed for their implication to the early
universe.Comment: 6 pages, two typos correcte
Kaluza-Klein Induced Gravity Inflation
A D-dimensional induced gravity theory is studied carefully in a
dimensional Friedmann-Robertson-Walker space-time. We try to extract
information of the symmetry breaking potential in search of an inflationary
solution with non-expanding internal-space. We find that the induced gravity
model imposes strong constraints on the form of symmetry breaking potential in
order to generate an acceptable inflationary universe. These constraints are
analyzed carefully in this paper.Comment: 10 pages, title changed, corrected some typos, two additional
comments adde
Ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model on the triangular lattice: A variational study based on entangled-plaquette states
We study, on the basis of the general entangled-plaquette variational ansatz,
the ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model
on the triangular lattice. Our numerical estimates are in good agreement with
available exact results and comparable, for large system sizes, to those
computed via the best alternative numerical approaches, or by means of
variational schemes based on specific (i.e., incorporating problem dependent
terms) trial wave functions. The extrapolation to the thermodynamic limit of
our results for lattices comprising up to N=324 spins yields an upper bound of
the ground-state energy per site (in units of the exchange coupling) of
[ for the XX model], while the estimated
infinite-lattice order parameter is (i.e., approximately 64% of the
classical value).Comment: 8 pages, 3 tables, 2 figure
Learning a Factor Model via Regularized PCA
We consider the problem of learning a linear factor model. We propose a
regularized form of principal component analysis (PCA) and demonstrate through
experiments with synthetic and real data the superiority of resulting estimates
to those produced by pre-existing factor analysis approaches. We also establish
theoretical results that explain how our algorithm corrects the biases induced
by conventional approaches. An important feature of our algorithm is that its
computational requirements are similar to those of PCA, which enjoys wide use
in large part due to its efficiency
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