299,622 research outputs found
Mapping carbon nanotube orientation by fast fourier transform of scanning electron micrographs
A novel method of applying a two-dimensional Fourier transform (2D-FFT) to SEM was developed to map the CNT orientation in pre-formed arrays. Local 2D-FFTs were integrated azimuthally to determine an orientation distribution function and the associated Herman parameter. This approach provides data rapidly and over a wide range of lengthscales. Although likely to be applicable to a wide range of anisotropic nanoscale structures, the method was specifically developed to study CNT veils, a system in which orientation critically controls mechanical properties. Using this system as a model, key parameters for the 2D-FFT analysis were optimised, including magnification and domain size; a model set of CNT veils were pre-strained to 5%, 10% and 15%, to vary the alignment degree. The algorithm confirmed a narrower orientation distribution function and increasing Herman parameter, with increasing pre-strain. To validate the algorithm, the local orientation was compared to that derived from a common polarised Raman spectroscopy. Orientation maps of the Herman parameter, derived by both methods, showed good agreement. Quantitatively, the mean Herman parameter calculated using the polarised Raman spectroscopy was 0.42 ± 0.004 compared to 0.32 ± 0.002 for the 2D-FFT method, with a correlation coefficient of 0.73. Possible reasons for the modest and systematic discrepancy were discussed
Spin dynamics of two-dimensional electrons with Rashba spin-orbit coupling and electron-electron interactions
We study the spin dynamics of two dimensional electron gases (2DEGs) with
Rashba spin-orbit coupling by taking account of electron-electron interactions.
The diffusion equations for charge and spin densities are derived by making use
of the path-integral approach and the quasiclassical Green's function.
Analyzing the effect of the interactions, we show that the spin-relaxation time
can be enhanced by the electron-electron interaction in the ballistic regime.Comment: accepted for publication in Phys. Rev.
Quantum Langevin model for exoergic ion-molecule reactions and inelastic processes
We presents a fully quantal version of the Langevin model for the total rate
of exoergic ion-molecule reactions or inelastic processes. The model, which is
derived from a rigorous multichannel quantum-defect formulation of bimolecular
processes, agrees with the classical Langevin model at sufficiently high
temperatures. It also gives the first analytic description of ion-molecule
reactions and inelastic processes in the ultracold regime where the quantum
nature of the relative motion between the reactants becomes important.Comment: 5 pages, 3 figure
Critical-state effects on microwave losses in type-II superconductors
We discuss the microwave energy losses in superconductors in the critical
state. The field-induced variations of the surface resistance are determined,
in the framework of the Coffey and Clem model, by taking into account the
distribution of the vortex magnetic field inside the sample. It is shown that
the effects of the critical state cannot generally be disregarded to account
for the experimental data. Results obtained in bulk niobium at low temperatures
are quantitatively justified.Comment: 4 pages, 4 embedded figures, to be published on Eur. Phys. J.
Light sterile neutrinos
The theory and phenomenology of light sterile neutrinos at the eV mass scale
is reviewed. The reactor, Gallium and LSND anomalies are briefly described and
interpreted as indications of the existence of short-baseline oscillations
which require the existence of light sterile neutrinos. The global fits of
short-baseline oscillation data in 3+1 and 3+2 schemes are discussed, together
with the implications for beta-decay and neutrinoless double-beta decay. The
cosmological effects of light sterile neutrinos are briefly reviewed and the
implications of existing cosmological data are discussed. The review concludes
with a summary of future perspectives.Comment: 41 pages; final version to be published as a Topical Review in
Journal of Physics
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