9,072 research outputs found
Full-vector analysis of a realistic photonic crystal fiber
We analyze the guiding problem in a realistic photonic crystal fiber using a
novel full-vector modal technique, a biorthogonal modal method based on the
nonselfadjoint character of the electromagnetic propagation in a fiber.
Dispersion curves of guided modes for different fiber structural parameters are
calculated along with the 2D transverse intensity distribution of the
fundamental mode. Our results match those achieved in recent experiments, where
the feasibility of this type of fiber was shown.Comment: 3 figures, submitted to Optics Letter
Large isotope effect on in cuprates despite of a small electron-phonon coupling
We calculate the isotope coefficients and for the
superconducting critical temperature and the pseudogap temperature
in a mean-field treatment of the t-J model including phonons. The
pseudogap phase is identified with the -charge-density wave (-CDW) phase
in this model. Using the small electron-phonon coupling constant obtained previously in LDA calculations in YBaCuO,
is negative but negligible small whereas increases
from about 0.03 at optimal doping to values around 1 at small dopings in
agreement with the general trend observed in many cuprates. Using a simple
phase fluctuation model where the -CDW has only short-range correlations it
is shown that the large increase of at low dopings is rather universal
and does not depend on the existence of sharp peaks in the density of states in
the pseudogap state or on specific values of the phonon cutoff. It rather is
caused by the large depletion of spectral weight at low frequencies by the
-CDW and thus should also occur in other realizations of the pseudogap.Comment: 8 pages, 5 figures, to be publ. in PR
Universal geometrical scaling of the elliptic flow
The presence of scaling variables in experimental observables provide very
valuable indications of the dynamics underlying a given physical process. In
the last years, the search for geometric scaling, that is the presence of a
scaling variable which encodes all geometrical information of the collision as
well as other external quantities as the total energy, has been very active.
This is motivated, in part, for being one of the genuine predictions of the
Color Glass Condensate formalism for saturation of partonic densities. Here we
extend these previous findings to the case of experimental data on elliptic
flow. We find an excellent scaling for all centralities and energies, from RHIC
to LHC, with a simple generalization of the scaling previously found for other
observables and systems. Interestingly the case of the photons, difficult to
reconcile in most formalisms, nicely fit the scaling curve. We discuss the
possible interpretations of this finding in terms of initial or final state
effects.Comment: 6 pages, 4 figures, accepted for publication in Phys Rev
Bayesian analysis of 210Pb dating
In many studies of environmental change of the past few centuries, 210Pb
dating is used to obtain chronologies for sedimentary sequences. One of the
most commonly used approaches to estimate the ages of depths in a sequence is
to assume a constant rate of supply (CRS) or influx of `unsupported' 210Pb from
the atmosphere, together with a constant or varying amount of `supported'
210Pb. Current 210Pb dating models do not use a proper statistical framework
and thus provide poor estimates of errors. Here we develop a new model for
210Pb dating, where both ages and values of supported and unsupported 210Pb
form part of the parameters. We apply our model to a case study from Canada as
well as to some simulated examples. Our model can extend beyond the current CRS
approach, deal with asymmetric errors and mix 210Pb with other types of dating,
thus obtaining more robust, realistic and statistically better defined
estimates.Comment: 22 Pages, 4 Figure
Small but long koch fractal monopole
A small but long wire fractal antenna based on the Koch curve is presented. Experimental and numerical results show that the antenna improves the features of a common linear monopole. The radiation resistance is increased and the Q is reduced at each fractal iteration, approaching the fundamental limit on small antennas.Peer ReviewedPostprint (published version
High Curie temperatures in (Ga,Mn)N from Mn clustering
The effect of microscopic Mn cluster distribution on the Curie temperature
(Tc) is studied using density-functional calculations. We find that the
calculated Tc depends crucially on the microscopic cluster distribution, which
can explain the abnormally large variations in experimental Tc values from a
few K to well above room temperature. The partially dimerized Mn_2-Mn_1
distribution is found to give the highest Tc > 500 K, and in general, the
presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values
close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.Comment: To appear in Applied Phyiscs Letter
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