1,127 research outputs found
Hierarchical Fractal Weyl Laws for Chaotic Resonance States in Open Mixed Systems
In open chaotic systems the number of long-lived resonance states obeys a
fractal Weyl law, which depends on the fractal dimension of the chaotic saddle.
We study the generic case of a mixed phase space with regular and chaotic
dynamics. We find a hierarchy of fractal Weyl laws, one for each region of the
hierarchical decomposition of the chaotic phase-space component. This is based
on our observation of hierarchical resonance states localizing on these
regions. Numerically this is verified for the standard map and a hierarchical
model system.Comment: 5 pages, 3 figure
A discontinuous Galerkin method for the Vlasov-Poisson system
A discontinuous Galerkin method for approximating the Vlasov-Poisson system
of equations describing the time evolution of a collisionless plasma is
proposed. The method is mass conservative and, in the case that piecewise
constant functions are used as a basis, the method preserves the positivity of
the electron distribution function and weakly enforces continuity of the
electric field through mesh interfaces and boundary conditions. The performance
of the method is investigated by computing several examples and error estimates
associated system's approximation are stated. In particular, computed results
are benchmarked against established theoretical results for linear advection
and the phenomenon of linear Landau damping for both the Maxwell and Lorentz
distributions. Moreover, two nonlinear problems are considered: nonlinear
Landau damping and a version of the two-stream instability are computed. For
the latter, fine scale details of the resulting long-time BGK-like state are
presented. Conservation laws are examined and various comparisons to theory are
made. The results obtained demonstrate that the discontinuous Galerkin method
is a viable option for integrating the Vlasov-Poisson system.Comment: To appear in Journal for Computational Physics, 2011. 63 pages, 86
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Mass Density of Individual Cobalt Nanowires
The mass density of nanowires is determined using in-situ resonance frequency
experiments combined with quasi-static nanotensile tests. Our results reveal a
mass density of 7.36 g/cm3 on average which is below the theoretical density of
bulk cobalt. Also the density of electrodeposited cobalt nanowires is found to
decrease with the aspect ratio. The results are discussed in terms of the
measurement accuracy and the microstructure of the nanowires.Comment: 3 Figure
InP-quantum dots in Al0.20Ga0.80InP with different barrier configurations
Systematic ensemble photoluminescence studies have been performed on type-I
InP-quantum dots in Al0.20Ga0.80InP barriers, emitting at approximately 1.85 eV
at 5 K. The influence of different barrier configurations as well as the
incorporation of additional tunnel barriers on the optical properties has been
investigated. The confinement energy between the dot barrier and the
surrounding barrier layers, which is the sum of the band discontinuities for
the valence and the conduction bands, was chosen to be approximately 190 meV by
using Al0.50Ga0.50InP. In combination with 2 nm thick AlInP tunnel barriers,
the internal quantum efficiency of these barrier configurations can be
increased by up to a factor of 20 at elevated temperatures with respect to
quantum dots without such layers.Comment: physica status solidi (c) (Proceedings of QD 2008
Quantitative Topographical Characterization of Thermally Sprayed Coatings by Optical Microscopy
Topography measurements and roughness calculations for different rough surfaces (Rugotest surface comparator and thermally sprayed coatings) are presented. The surfaces are measured with a novel quantitative topography measurement technique based on optical stereomicroscopy and a comparison is made with established scanning stylus and optical profilometers. The results show that for most cases the different methods yield similar results. Stereomicroscopy is therefore a valuable method for topographical investigations in both quality control and research. On the other hand, the method based on optical microscopy demands a careful optimization of the experimental settings like the magnification and the illumination to achieve satisfactory result
Fracture strength and Young's modulus of ZnO nanowires
The fracture strength of ZnO nanowires vertically grown on sapphire
substrates was measured in tensile and bending experiments. Nanowires with
diameters between 60 and 310 nm and a typical length of 2 um were manipulated
with an atomic force microscopy tip mounted on a nanomanipulator inside a
scanning electron microscope. The fracture strain of (7.7 +- 0.8)% measured in
the bending test was found close to the theoretical limit of 10% and revealed a
strength about twice as high as in the tensile test. From the tensile
experiments the Young's modulus could be measured to be within 30% of that of
bulk ZnO, contrary to the lower values found in literature.Comment: 5 pages, 3 figures, 1 tabl
Characterisation of micromechanical properties using advanced techniques
Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 - August 2, 201
Influence of carrier-carrier and carrier-phonon correlations on optical absorption and gain in quantum-dot systems
A microscopic theory is used to study the optical properties of semiconductor
quantum dots. The dephasing of a coherent excitation and line-shifts of the
interband transitions due to carrier-carrier Coulomb interaction and
carrier-phonon interaction are determined from a quantum kinetic treatment of
correlation processes. We investigate the density dependence of both mechanisms
and clarify the importance of various dephasing channels involving the
localized and delocalized states of the system.Comment: 12 pages, 10 figure
Strength and fracture of Si micropillars: A new scanning electron microscopy-based micro-compression test
A novel method for in situ scanning electron microscope (SEM) micro-compression tests is presented. The direct SEM observation during the instrumented compression testing allows for very efficient positioning and assessment of the failure mechanism. Compression tests on micromachined Si pillars with volumes down to 2 μm3 are performed inside the SEM, and the results demonstrate the potential of the method. In situ observation shows that small diameter pillars tend to buckle while larger ones tend to crack before failure. Compressive strength increases with decreasing pillar diameter and reaches almost 9 GPa for submicrometer diameter pillars. This result is in agreement with earlier bending experiments on Si. Difficulties associated with precise strain measurements are discusse
A comparison of microtensile and microcompression methods for studying plastic properties of nanocrystalline electrodeposited nickel at different length scales
A comparison of microcompression and microtensile methods to study mechanical properties of electrodeposited nanocrystalline (nc) nickel has been performed. Microtensile tests that probe a volume of more than 2 × 106 μm3 show reasonable agreement with results from microcompression tests that probe much smaller volumes down to a few μm3. Differences between the two uniaxial techniques are discussed in terms of measurements errors, probed volume and surface effects, strain rate, and influence of stress state. Uniaxial solicitation in compression mode revealed several advantages for studying stress-strain propertie
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