12,037 research outputs found
Fourier analyses of commensurability oscillations in Fibonacci lateral superlattices
Magnetotransport measurements have been performed on Fibonacci lateral
superlattices (FLSLs) -- two-dimensional electron gases subjected to a weak
potential modulation arranged in the Fibonacci sequence, LSLLSLS..., with
L/S=tau (the golden ratio). Complicated commensurability oscillation (CO) is
observed, which can be accounted for as a superposition of a series of COs each
arising from a sinusoidal modulation representing the characteristic length
scale of one of the self-similar generations in the Fibonacci sequence.
Individual CO components can be separated out from the magnetoresistance trace
by performing a numerical Fourier band-pass filter. From the analysis of the
amplitude of a single-component CO thus extracted, the magnitude of the
corresponding Fourier component in the potential modulation can be evaluated.
By examining all the Fourier contents observed in the magnetoresistance trace,
the profile of the modulated potential seen by the electrons can be
reconstructed with some remaining ambiguity about the interrelation of the
phase between different components.Comment: 11 pages, 10 figures, added references in Introduction, minor
revision
Effect of Oscillating Landau Bandwidth on the Integer Quantum Hall Effect in a Unidirectional Lateral Superlattice
We have measured activation gaps for odd-integer quantum Hall states in a
unidirectional lateral superlattice (ULSL) -- a two-dimensional electron gas
(2DEG) subjected to a unidirectional periodic modulation of the electrostatic
potential. By comparing the activation gaps with those simultaneously measured
in the adjacent section of the same 2DEG sample without modulation, we find
that the gaps are reduced in the ULSL by an amount corresponding to the width
acquired by the Landau levels through the introduction of the modulation. The
decrement of the activation gap varies with the magnetic field following the
variation of the Landau bandwidth due to the commensurability effect. Notably,
the decrement vanishes at the flat band conditions.Comment: 7 pages, 6 figures, minor revisio
Transport of polymer particles in a oil-water flow in porous media: enhancing oil recovery
We study a heuristic, core-scale model for the transport of polymer particles
in a two phase (oil and water) porous medium. We are motivated by recent
experimental observations which report increased oil recovery when polymers are
injected after the initial waterflood. The recovery mechanism is believed to be
microscopic diversion of the flow, where injected particles can accumulate in
narrow pore throats and clog it, in a process known as a log-jamming effect.
The blockage of the narrow pore channels lead to a microscopic diversion of the
water flow, causing a redistribution of the local pressure, which again can
lead to the mobilization of trapped oil, enhancing its recovery. Our objective
herein is to develop a core-scale model that is consistent with the observed
production profiles. We show that previously obtained experimental results can
be qualitatively explained by a simple two-phase flow model with an additional
transport equation for the polymer particles. A key aspect of the formulation
is that the microscopic heterogeneity of the rock and a dynamic altering of the
permeability must be taken into account in the rate equations.Comment: 20 pages, 9 Figures Submitted to Transport in Porous Medi
Modulation Induced Phase Transition from Fractional Quantum Hall to Stripe State at nu=5/3
We have investigated the effect of unidirectional periodic potential
modulation on the fractional quantum Hall (FQH) state at filling factors nu=5/3
and 4/3. For large enough modulation amplitude, we find that the resistivity
minimum at nu=5/3 gives way to a peak that grows with decreasing temperature.
Density matrix renormalization group calculation reveals that phase transition
from FQH state to unidirectional striped state having a period sim 4 l (with l
the magnetic length) takes place at nu=1/3 (equivalent to nu=5/3 by the
particle-hole symmetry) with the increase of the modulation amplitude,
suggesting that the observed peak is the manifestation of the stripe phase.Comment: 4 pages, 6 figures; minor revisio
Anisotropic transport in unidirectional lateral superlattice around half-filling of the second Landau level
We have observed marked transport anisotropy in short period (a=92 nm)
unidirectional lateral superlattices around filling factors nu=5/2 and 7/2:
magnetoresistance shows a sharp peak for current along the modulation grating
while a dip appears for current across the grating. By altering the ratio a/l
(with l=sqrt{hbar/eB_perp} the magnetic length) via changing the electron
density n_e, it is shown that the nu=5/2 anisotropic features appear in the
range 6.6 alt a/l alt 7.2 varying their intensities, becoming most conspicuous
at a/l simeq 6.7. The peak/dip broadens with temperature roughly preserving its
height/depth up to 250 mK. Tilt experiments reveal that the structures are
slightly enhanced by an in-plane magnetic field B_| perpendicular to the
grating but are almost completely destroyed by B_| parallel to the grating. The
observations suggest the stabilization of a unidirectional charge-density-wave
or stripe phase by weak external periodic modulation at the second Landau
level.Comment: REVTeX, 5 pages, 3 figures, Some minor revisions, Added notes and
reference
Origin of positive magnetoresistance in small-amplitude unidirectional lateral superlattices
We report quantitative analysis of positive magnetoresistance (PMR) for
unidirectional-lateral-superlattice samples with relatively small periods
(a=92-184 nm) and modulation amplitudes (V_0=0.015-0.25 meV). By comparing
observed PMR's with ones calculated using experimentally obtained mobilities,
quantum mobilities, and V_0's, it is shown that contribution from streaming
orbits (SO) accounts for only small fraction of the total PMR. For small V_0,
the limiting magnetic field B_e of SO can be identified as an inflection point
of the magnetoresistance trace. The major part of PMR is ascribed to drift
velocity arising from incompleted cyclotron orbits obstructed by scatterings.Comment: 12 pages, 9 figures, REVTe
Commutators, Lefschetz fibrations and the signatures of surface bundles
We construct examples of Lefschetz fibrations with prescribed singular
fibers. By taking differences of pairs of such fibrations with the same
singular fibers, we obtain new examples of surface bundles over surfaces with
non-zero signature. From these we derive new upper bounds for the minimal genus
of a surface representing a given element in the second homology of a mapping
class group.Comment: 20 pages, 7 figures, accepted for publication in Topolog
Fundamental relation between longitudinal and transverse conductivities in the quantum Hall system
We investigate the relation between the diagonal () and
off-diagonal () components of the conductivity tensor in the
quantum Hall system. We calculate the conductivity components for a short-range
impurity potential using the linear response theory, employing an approximation
that simply replaces the self-energy by a constant value
with the scattering time. The approximation is equivalent to assuming
that the broadening of a Landau level due to disorder is represented by a
Lorentzian with the width . Analytic formulas are
obtained for both and within the framework of this
simple approximation at low temperatures. By examining the leading terms in
and , we find a proportional relation between
and . The relation, after
slight modification to account for the long-range nature of the impurity
potential, is shown to be in quantitative agreement with experimental results
obtained in the GaAs/AlGaAs two-dimensional electron system at the low
magnetic-field regime where spin splitting is negligibly small.Comment: 21 pages, 8 figures, accepted for publication in J. Phys.: Condens.
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