25 research outputs found
Theory of Magneto--Acoustic Transport in Modulated Quantum Hall Systems Near
Motivated by the experimental results of Willett et al [Phys.Rev. Lett., {\bf
78}, 4478 (1997)] we develop a magneto-transport theory for the response of a
two dimensional electron gas (2DEG) in the Fractional Quantum Hall Regime near
Landau level filling factor to the surface acoustic wave (SAW) in
the presence of an added periodic density modulation. We assume there exists a
Composite Fermion Fermi Surface (CF-FS) at , and we show that the
deformation of the (CF-FS) due to the density modulation can be at the origin
of the observed transport anomalies for the experimental conditions. Our
analysis is carried out particularly for the non-local case which corresponds
to the SAW experiments. We introduce a new model of a deformed CF-FS. The model
permits us to explain anomalous features of the response of the modulated 2DEG
to the SAW near namely the nonlinear wave vector dependence of the
electron conductivity, the appearance of peaks in the SAW velocity shift and
attenuation and the anisotropy of the effect, all of which originate from
contributions to the conductivity tensor due to the regions of the CF-FS which
are flattened by the applied modulation.Comment: 13 pages, 4 figures, the published versio
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Surface permeability of porous media particles and capillary transport
We have established previously, in a lead-in study, that the spreading of liquids in particulate
porous media at low saturation levels, characteristically less than 10% of the void space, has very
distinctive features in comparison to that at higher saturation levels. In particular, we have found
that the dispersion process can be accurately described by a special class of partial differential
equations, the super-fast non-linear diffusion equation. The results of mathematical modelling have
demonstrated very good agreement with experimental observations. However, any enhancement of
the accuracy and predictive power of the model, keeping in mind practical applications, requires the
knowledge of the effective surface permeability of the constituent particles, which defines the global,
macroscopic permeability of the particulate media. In the paper, we demonstrate how this quantity
can be determined through the solution of the Laplace-Beltrami Dirichlet problem, we study this
using the well-developed surface finite element method
Kelley's formula as a basis for the assessment of reliable change
difference scores, reliable change index, Kelley's formula,