9,749 research outputs found
Electromagnetic form factors of the nucleon in effective field theory
We calculate the electromagnetic form factors of the nucleon to third chiral
order in manifestly Lorentz-invariant effective field theory. The rho and omega
mesons as well as the Delta(1232) resonance are included as explicit dynamical
degrees of freedom. To obtain a self-consistent theory with respect to
constraints we consider the proper relations among the couplings of the
effective Lagrangian. For the purpose of generating a systematic power
counting, the extended on-mass-shell renormalization scheme is applied in
combination with the small-scale expansion. The results for the electric and
magnetic Sachs form factors are analyzed in terms of experimental data and
compared to previous findings in the framework of chiral perturbation theory.
The pion-mass dependence of the form factors is briefly discussed.Comment: 26 pages, 9 figure
Cavity ringdown laser absorption spectroscopy and time-of-flight mass spectroscopy of jet-cooled silver silicides
The cavity ringdown technique has been employed for the first spectroscopic characterization of the AgSi molecule, which is generated in a pulsed laser vaporization plasma reactor. A total of 20 rovibronic bands between 365 and 385 nm have been measured and analyzed to yield molecular properties for the X, B, and C 2Sigma states of AgSi. A time-of-flight mass spectrometer simultaneously monitors species produced in the molecular beam and has provided the first direct evidence for the existence of polyatomic silver silicides. Comparison of the AgSi data to our recent results for the CuSi diatom reveals very similar chemical bonding in the two coinage metal silicides, apparently dominated by covalent interactions
Lasers incorporating 2D photonic bandgap mirrors
Semiconductor lasers incorporating a 2D photonic lattice as a one end mirror in a Fabry-Perot cavity are demonstrated. The photonic lattice is a 2D hexagonal close-packed array with a lattice constant of 220 nm. Pulsed threshold currents of 110 mA were observed from a 180 μm laser
Two-dimensional photonic band-gap mirrors at 850 and 980 nm
Summary form only given. Photonic band-gap (PBG) crystals can be fabricated in semiconductor devices through the etching of patterns of holes in the device, resulting in a periodic dielectric structure. One of the more practical uses of photonic crystals in optoelectronic devices is for thin, high-reflectivity mirrors. The use of hexagonal arrays of etched circular holes results in a 2-D photonic band-gap mirror that can be tuned to a specific wavelength by varying the hole radius and the lattice spacing. 2-D mirror characterization is performed by evaluating the light emission from an active waveguide
On the ground electronic states of copper silicide and its ions
The low-lying electronic states of SiCu, SiCu^+, and SiCu^− have been studied using a variety of high-level ab initio techniques. As expected on the basis of simple orbital occupancy and bond forming for Si(s^2p^2)+Cu(s^1) species, ^2Π_r, ^1Σ^+, and ^3Σ^− states were found to be the ground electronic states for SiCu, SiCu^+, and SiCu^−, respectively; the ^2Π_r state is not that suggested in most recent experimental studies. All of these molecules were found to be quite strongly bound although the bond lengths, bond energies, and harmonic frequencies vary slightly among them, as a result of the nonbonding character of the 2π-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the ^1Σ^+, ^2Π_r, and ^3Σ^− series. The neutral SiCu is found to have bound excited electronic states of ^4Σ^−, ^2Δ, ^2Σ^+, and ^2Π_i symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the ^2Π_r ground state. It is possible but not yet certain that the ^2Π_i state is, in fact, the “B state” observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally
30% external quantum efficiency from surface textured, thin-film light-emitting diodes
There is a significant gap between the internal efficiency of light-emitting diodes (LEDs) and their external efficiency. The reason for this shortfall is the narrow escape cone for light in high refractive index semiconductors. We have found that by separating thin-film LEDs from their substrates (by epitaxial lift-off, for example), it is much easier for light to escape from the LED structure and thereby avoid absorption. Moreover, by nanotexturing the thin-film surface using "natural lithography," the light ray dynamics becomes chaotic, and the optical phase-space distribution becomes "ergodic," allowing even more of the light to find the escape cone. We have demonstrated 30% external efficiency in GaAs LEDs employing these principles
Strong Coupling Theory of Two Level Atoms in Periodic Fields
We present a new convergent strong coupling expansion for two-level atoms in
external periodic fields, free of secular terms. As a first application, we
show that the coherent destruction of tunnelling is a third-order effect. We
also present an exact treatment of the high-frequency region, and compare it
with the theory of averaging. The qualitative frequency spectrum of the
transition probability amplitude contains an effective Rabi frequency.Comment: 4 pages with 3 figure
Quantum electrodynamics for vector mesons
Quantum electrodynamics for mesons is considered. It is shown that, at
tree level, the value of the gyromagnetic ratio of the is fixed to 2
in a self-consistent effective quantum field theory. Further, the mixing
parameter of the photon and the neutral vector meson is equal to the ratio of
electromagnetic and strong couplings, leading to the mass difference
at tree order.Comment: 4 pages, 2 figures, REVTeX 4, accepted for publication in PR
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