2,895 research outputs found
Composite Fermions in Modulated Structures: Transport and Surface Acoustic Waves
Motivated by a recent experiment of Willett et al. [Phys. Rev. Lett. 78, 4478
(1997)], we employ semiclassical composite-fermion theory to study the effect
of a periodic density modulation on a quantum Hall system near Landau level
filling factor nu=1/2. We show that even a weak density modulation leads to
dramatic changes in surface-acoustic-wave (SAW) propagation, and propose an
explanation for several key features of the experimental observations. We
predict that properly arranged dc transport measurements would show a structure
similar to that seen in SAW measurements.Comment: Version published in Phys. Rev. Lett. Figures changed to show SAW
velocity shift. LaTeX, 5 pages, two included postscript figure
Experimental Demonstration of Fermi Surface Effects at Filling Factor 5/2
Using small wavelength surface acoustic waves (SAW) on ultra-high mobility
heterostructures, Fermi surface properties are detected at 5/2 filling factor
at temperatures higher than those at which the quantum Hall state forms. An
enhanced conductivity is observed at 5/2 by employing sub 0.5 micron wavelength
SAW, indicating a quasiparticle mean-free-path substantially smaller than that
in the lowest Landau level. These findings are consistent with the presence of
a filled Fermi sea of composite fermions, which may pair at lower temperatures
to form the 5/2 ground state.Comment: 11 pages, 4 figure
A Fermi Fluid Description of the Half-Filled Landau Level
We present a many-body approach to calculate the ground state properties of a
system of electrons in a half-filled Landau level. Our starting point is a
simplified version of the recently proposed trial wave function where one
includes the antisymmetrization operator to the bosonic Laughlin state. Using
the classical plasma analogy, we calculate the pair-correlation function, the
static structure function and the ground state energy in the thermodynamic
limit. These results are in good agreement with the expected behavior at
.Comment: 4 pages, REVTEX, and 4 .ps file
Composite fermions in the Fractional Quantum Hall Effect: Transport at finite wavevector
We consider the conductivity tensor for composite fermions in a close to
half-filled Landau band in the temperature regime where the scattering off the
potential and the trapped gauge field of random impurities dominates. The
Boltzmann equation approach is employed to calculate the quasiclassical
transport properties at finite effective magnetic field, wavevector and
frequency. We present an exact solution of the kinetic equation for all
parameter regimes. Our results allow a consistent description of recently
observed surface acoustic wave resonances and other findings.Comment: REVTEX, 4 pages, 1 figur
Dietary flavonoid intake and weight maintenance: three prospective cohorts of 124,086 US men and women followed for up to 24 years
Objective: To examine whether dietary intake of specific flavonoid sub-classes is associated with weight change over time, including flavonols, flavones, flavanones, flavan-3-ols, anthocyanins, and flavonoid polymers. Design: Three prospective cohort studies. Setting: Health professionals in the United States. Participants: 124,086 men and women participating in the Health Professionals Follow-up Study (HPFS), Nurses’ Health Study (NHS), and Nurses’ Health Study II (NHS II). Main outcome measure: Self-reported change in weight over multiple 4-year time intervals between 1986 and 2011. Results: Increased consumption of most flavonoid sub-classes, including flavonols, flavan-3-ols, anthocyanins, and flavonoid polymers was inversely associated with weight change over 4-year time intervals, after adjustment for simultaneous changes in other lifestyle factors including other aspects of diet, smoking status, and physical activity. In the pooled results, the greatest magnitude of association was observed for anthocyanins (-0.22 lbs, 95% CI -0.30 to -0.15 lbs per additional SD/day, 10 mg), flavonoid polymers (-0.18 lbs, 95% CI -0.28 to -0.08 lbs per additional SD/day, 138 mg), and flavonols (-0.16 lbs, 95% CI -0.26 to -0.06 lbs per additional SD/day, 7 mg). After additional adjustment for fiber intake associations remained significant for anthocyanins, proanthocyanidins, and total flavonoid polymers but were attenuated and no longer statistically significant for other sub-classes. Conclusions: Higher intake of foods rich in flavonols, flavan-3-ols, anthocyanins, and flavonoid polymers, may contribute to weight maintenance in adulthood, and may help to refine dietary recommendations for the prevention of obesity and its potential sequelae
Stability and effective masses of composite-fermions in the first and second Landau Level
We propose a measure of the stability of composite fermions (CF's) at
even-denominator Landau-level filling fractions. Assuming Landau-level mixing
effects are not strong, we show that the CF liquid at in the
Landau level cannot exist and relate this to the absence of a hierarchy of
incompressible states for filling fractions . We find that
a polarized CF liquid should exist at . We also show that, for CF
states, the variation with system size of the ground state energy of
interacting electrons follows that for non-interacting particles in zero
magnetic field. We use this to estimate the CF effective masses.Comment: 9 pages, Revtex, PSIZ-TP-940
Effective mass of composite fermion: a phenomenological fit in with anomalous propagation of surface acoustic wave
We calculate the conductivity associated with the anomalous propagation of a
surface acoustic wave above a two-dimensional electron gas at .
Murthy-Shankar's middle representation is adopted and a contribution to the
response functions beyond the random phase approximation has been taken into
account. We give a phenomenological fit for the effective mass of composite
fermion in with the experimental data of the anomalous propagation of surface
acoustic wave at and find the phenomenological value of the effective
mass is several times larger than the theoretical value
derived from the Hartree-Fock approximation. We
compare our phenomenologically fitting composite fermion effective mass with
those appeared in the measurements of the activation energy and the
Shubnikov-de Haas effect and find that our result is fairly reasonable.Comment: 8 pages, 5 figures, the longer version of cond-mat/9801131 with
crucial corrections, accepted for publication by PR
Explanation for the Resistivity Law in Quantum Hall System
We consider a 2D electron system in a strong magnetic field, where the local
Hall resistivity is a function of position and
is small compared to . Particularly if the
correlations fall off slowly with distance, or if fluctuations exist on several
length scales, one finds that the macroscopic longitudinal resistivity
is only weakly dependent on and is approximately proportional to
the magnitude of fluctuations in . This may provide an explanation
of the empirical law where is
the Hall resistance, and is the magnetic field.Comment: 11 pages (REVTeX 3.0). Revised Version. Complete postscript file for
this paper is available on the World Wide Web at
http://cmtw.harvard.edu/~simon/ ; Preprint number HU-CMT-94S0
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