Search results for "Miscellaneous"
showing 10 items of 26553 documents
Charge breeding time investigations of electron cyclotron resonance charge breeders
2018
To qualify electron cyclotron resonance charge breeders, the method that is traditionally used to evaluate the charge breeding time consists in generating a rising edge of the injected beam current and measuring the time in which the extracted multicharged ion beam reaches 90% of its final current. It is demonstrated in the present paper that charge breeding times can be more accurately measured by injecting short pulses of 1 + ions and recording the time resolved responses of N + ions. This method is used to probe the effect of the 1 + ion accumulation in the plasma known to disturb the buffer gas plasma equilibrium and is a step further in understanding the large discrepancies reported in…
Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers: Experimental an…
2020
We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow…
Electromagnetically induced switching of ferroelectric thin films
2007
We analyze the interaction of an electromagnetic spike (one cycle) with a thin layer of ferroelectric medium with two equilibrium states. The model is the set of Maxwell equations coupled to the undamped Landau-Khalatnikov equation, where we do not assume slowly varying envelopes. From linear-scattering theory, we show that low-amplitude pulses can be completely reflected by the medium. Large-amplitude pulses can switch the ferroelectric. Using numerical simulations and analysis, we study this switching for long and short pulses, estimate the switching times, and provide useful information for experiments.
Reversed polarized emission in highly strained a-plane GaN/AlN multiple quantum wells
2010
The polarization of the emission from a set of highly strained $a$-plane GaN/AlN multiple quantum wells of varying well widths has been studied. A single photoluminescence peak is observed that shifts to higher energies as the quantum well thickness decreases due to quantum confinement. The emitted light is linearly polarized. For the thinnest samples the preferential polarization direction is perpendicular to the wurtzite $c$ axis with a degree of polarization that decreases with increasing well width. However, for the thickest well the preferred polarization direction is parallel to the $c$ axis. Raman scattering, x-ray diffraction, and transmission electron microscopy studies have been p…
Enhancement of the Multipactor Threshold Inside Nonrectangular Iris
2018
Multipactor breakdown is studied inside the capacitive iris of a rectangular waveguide with a skewed slot along its longitudinal cross section. Both the iris length and height are assumed to be small compared to the electromagnetic wavelength. Therefore, the quasi-static approximation is applied so as to describe the RF field distribution inside the iris gap, whereas a 2-D model is used to analyze the electron motion. The peculiarities of RF field structure are studied using the conformal mapping approach, which shows that the electric field lines can be approximated by circular arcs when the iris length is much larger than its height. The electron motion inside the iris gap is analyzed usi…
Accumulation of positrons from a LINAC based source
2020
International audience; The GBAR experiment aims to measure the gravitational acceleration of antihydrogen H̅. It will use H̅+ ions formed by the interaction of antiprotons with a dense positronium cloud, which will require about 1010 positrons to produce one H̅+. We present the first results on the positron accumulation, reaching 3.8±0.4×108 e+ collected in 560 s.
Piezo-electrical control of gyration dynamics of magnetic vortices
2019
In this work, we first statically image the electrically controlled magnetostatic configuration of magnetic vortex states and then we dynamically image the time-resolved vortex core gyration tuned by electric fields. We demonstrate the manipulation of the vortex core gyration orbit by engineering the magnetic anisotropies. We achieve this by electric fields in a synthetic heterostructure consisting of a piezoelement coupled with magnetostrictive microstructures, where the magnetic anisotropy can be controlled by strain. We directly show the strong impact of the tailored anisotropy on the static shape of the vortex state and the dynamic vortex core orbit. The results demonstrate the possibil…
Current induced chiral domain wall motion in CuIr/CoFeB/MgO thin films with strong higher order spin–orbit torques
2020
We investigate the Dzyaloshinskii–Moriya interaction (DMI) and spin–orbit torque effects in CuIr/CoFeB/MgO heterostructures. To this end, harmonic Hall measurements and current induced domain wall motion experiments are performed. The motion of domain walls at zero applied field due to current demonstrates the presence of DMI in this system. We determine the strength of the DMI to be D = + 5 ± 3 μ J / m 2 and deduce right-handed chirality in domain walls showing a partial Neel type spin structure. To ascertain the torques, we perform a second harmonic measurement to quantify the damping- and field-like current induced effective fields as a function of the magnetization direction. From the a…
Interferences in Locally Resonant Sonic Metamaterials Formed from Helmholtz Resonators
2019
[EN] The emergence of materials artificially designed to control the transmission of waves, generally called metamaterials, has been a hot topic in the field of acoustics for several years. The design of these metamaterials is usually carried out by overlapping different wave control mechanisms. An example of this trend is the so-called Locally Resonant Sonic Materials, being one of them the Phononic Crystals with a local resonant structure. These metamaterials are formed by sets of isolated resonators in such a way that the control of the waves is carried out by resonances and by the existence of Bragg bandgaps, which appear due to the ordered distribution of the resonators. Their use is b…