435 research outputs found
Ferrimagnetism of the magnetoelectric compound CuOSeO probed by Se NMR
We present a thorough Se NMR study of a single crystal of the
magnetoelectric compound CuOSeO. The temperature dependence of the
local electronic moments extracted from the NMR data is fully consistent with a
magnetic phase transition from the high-T paramagnetic phase to a low-T
ferrimagnetic state with 3/4 of the Cu ions aligned parallel and 1/4
aligned antiparallel to the applied field of 14.09 T. The transition to this
3up-1down magnetic state is not accompanied by any splitting of the NMR lines
or any abrupt modification in their broadening, hence there is no observable
reduction of the crystalline symmetry from its high-T cubic \textit{P}23
space group. These results are in agreement with high resolution x-ray
diffraction and magnetization data on powder samples reported previously by Bos
{\it et al.} [Phys. Rev. B, {\bf 78}, 094416 (2008)]. We also develop a mean
field theory description of the problem based on a microscopic spin Hamiltonian
with one antiferromagnetic ( K) and one ferromagnetic
( K) nearest-neighbor exchange interaction
Analytical theory of short-pulse free-electron laser oscillators
A simple model for the nonlinear evolution of a short-pulse free-electron laser oscillator in the small gain regime is derived. An analysis of the linearized system allows the definition and calculation of the eigenmodes characterizing the small signal regime. An arbitrary solution of the nonlinear system can then be expanded in terms of these supermodes. In the single-supermode approximation, the system reduces to a Landau-Ginzburg equation, which allows the efficiency and saturated power to be obtained as functions of cavity detuning and cavity losses. In the limit of small cavity detuning, electrons emit superradiantly, with an efficiency inversely proportional to the number of radiation wavelengths within the optical pulse, and power proportional to the square of the bunch charge. In the multisupermode regime, limit cycles and period doubling behavior are observed and interpreted as a competition between supermodes. Finally, the analytical and numerical results are compared with the experimental observations from the Free-Electron Laser for Infrared eXperiments experiment
Phase Conjugation and Negative Refraction Using Nonlinear Active Metamaterials
We present experimental demonstration of phase conjugation using nonlinear
metamaterial elements. Active split-ring resonators loaded with varactor diodes
are demonstrated theoretically to act as phase-conjugating or time-reversing
discrete elements when parametrically pumped and illuminated with appropriate
frequencies. The metamaterial elements were fabricated and shown experimentally
to produce a time reversed signal. Measurements confirm that a discrete array
of phase-conjugating elements act as a negatively-refracting time reversal RF
lens only 0.12 thick
Engineering Electromagnetic Properties of Periodic Nanostructures Using Electrostatic Resonances
Electromagnetic properties of periodic two-dimensional sub-wavelength
structures consisting of closely-packed inclusions of materials with negative
dielectric permittivity in a dielectric host with positive
can be engineered using the concept of multiple electrostatic
resonances. Fully electromagnetic solutions of Maxwell's equations reveal
multiple wave propagation bands, with the wavelengths much longer than the
nanostructure period. It is shown that some of these bands are described using
the quasi-static theory of the effective dielectric permittivity
, and are independent of the nanostructure period. Those bands
exhibit multiple cutoffs and resonances which are found to be related to each
other through a duality condition. An additional propagation band characterized
by a negative magnetic permeability develops when a magnetic moment is induced
in a given nano-particle by its neighbors. Imaging with sub-wavelength
resolution in that band is demonstrated
Simulations of stable compact proton beam acceleration from a two-ion-species ultrathin foil
We report stable laser-driven proton beam acceleration from ultrathin foils
consisting of two ion species: heavier carbon ions and lighter protons.
Multi-dimensional particle-in-cell (PIC) simulations show that the radiation
pressure leads to very fast and complete spatial separation of the species. The
laser pulse does not penetrate the carbon ion layer, avoiding the proton
Rayleigh-Taylor-like (RT) instability. Ultimately, the carbon ions are heated
and spread extensively in space. In contrast, protons always ride on the front
of the carbon ion cloud, forming a compact high quality bunch. We introduce a
simple three-interface model to interpret the instability suppression in the
proton layer. The model is backed by simulations of various compound foils such
as carbon-deuterium (C-D) and carbon-tritium (C-T) foils. The effects of the
carbon ions' charge state on proton acceleration are also investigated. It is
shown that with the decrease of the carbon ion charge state, both the RT-like
instability and the Coulomb explosion degrade the energy spectrum of the
protons. Finally, full 3D simulations are performed to demonstrate the
robustness of the stable two-ion-species regime.Comment: 14 pages, 10figures, to be published in PO
Materials of the final reports on the joint Soviet-American experiment on the Kosmos-936 biosatellite
Biological experiments onboard the Kosmos-936 investigated the effect of weightlessness on the basic components of cells, the genetic structure and energy apparatus. Genetic studies were made on the Drosophila melanogaster. Experiments were made on higher vegetation and fungi as well. The results indicate that weightlessness cannot be the principal barrier for normal development. An experiment with ectopic osteogenesis in weightlessness was carried out. Measurements were made of cosmic radiation inside and outside the biosatellite
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