11,648 research outputs found
Breakdown of Particle-Hole Symmetry in the Lowest Landau Level Revealed by Tunneling Spectroscopy
Tunneling measurements on 2D electron gases at high magnetic field reveal a
qualitative difference between the two spin sublevels of the lowest Landau
level. While the tunneling current-voltage characteristic at filling factor
is a single peak shifted from zero bias by a Coulomb pseudogap, the
spectrum at shows a well-resolved double peak structure. This
difference is present regardless of whether and occur at
the same or different magnetic fields. No analogous effect is seen at and 7/2 in the first excited Landau level.Comment: 5 pages, 4 figure
Edge State Transport of Separately Contacted Bilayer Systems in the Fractional Quantum Hall Regime
Hall and diagonal resistances of bilayer fractional quantum Hall systems are
discussed theoretically. The bilayers have electrodes attached separately to
each layer. They are assumed to be coupled weakly by interlayer tunneling,
while the interlayer Coulomb interaction is negligibly small. It is shown that
source-drain voltage dependence of the resistances reflects the Luttinger
liquid parameter of the edge state.Comment: 3 pages with 2 eps figure, Revtex, contributed paper to EP2DS-1
Quantum Lifetime of Two-Dimensional Holes
The quantum lifetime of two-dimensional holes in a GaAs/AlGaAs double quantum
well is determined via tunneling spectroscopy. At low temperatures the lifetime
is limited by impurity scattering but at higher temperatures hole-hole Coulomb
scattering dominates. Our results are consistent with Fermi liquid theory, at
least up to r_s = 11. At the highest temperatures the measured width of the
hole spectral function becomes comparable to the Fermi energy. A new,
tunneling-spectroscopic, method for determining the in-plane effective mass of
the holes is also demonstrated.Comment: 5 pages, 4 figures. Published versio
Confounds and Consequences in Geotagged Twitter Data
Twitter is often used in quantitative studies that identify
geographically-preferred topics, writing styles, and entities. These studies
rely on either GPS coordinates attached to individual messages, or on the
user-supplied location field in each profile. In this paper, we compare these
data acquisition techniques and quantify the biases that they introduce; we
also measure their effects on linguistic analysis and text-based geolocation.
GPS-tagging and self-reported locations yield measurably different corpora, and
these linguistic differences are partially attributable to differences in
dataset composition by age and gender. Using a latent variable model to induce
age and gender, we show how these demographic variables interact with geography
to affect language use. We also show that the accuracy of text-based
geolocation varies with population demographics, giving the best results for
men above the age of 40.Comment: final version for EMNLP 201
Detecting Baryon Acoustic Oscillations
Baryon Acoustic Oscillations are a feature imprinted in the galaxy
distribution by acoustic waves traveling in the plasma of the early universe.
Their detection at the expected scale in large-scale structures strongly
supports current cosmological models with a nearly linear evolution from
redshift approximately 1000, and the existence of dark energy. Besides, BAOs
provide a standard ruler for studying cosmic expansion. In this paper we focus
on methods for BAO detection using the correlation function measurement. For
each method, we want to understand the tested hypothesis (the hypothesis H0 to
be rejected) and the underlying assumptions. We first present wavelet methods
which are mildly model-dependent and mostly sensitive to the BAO feature. Then
we turn to fully model-dependent methods. We present the most often used method
based on the chi^2 statistic, but we find it has limitations. In general the
assumptions of the chi^2 method are not verified, and it only gives a rough
estimate of the significance. The estimate can become very wrong when
considering more realistic hypotheses, where the covariance matrix of the
measurement depends on cosmological parameters. Instead we propose to use a new
method based on two modifications: we modify the procedure for computing the
significance and make it rigorous, and we modify the statistic to obtain better
results in the case of varying covariance matrix. We verify with simulations
that correct significances are different from the ones obtained using the
classical chi^2 procedure. We also test a simple example of varying covariance
matrix. In this case we find that our modified statistic outperforms the
classical chi^2 statistic when both significances are correctly computed.
Finally we find that taking into account variations of the covariance matrix
can change both BAO detection levels and cosmological parameter constraints
Making "fetch" happen: The influence of social and linguistic context on nonstandard word growth and decline
In an online community, new words come and go: today's "haha" may be replaced
by tomorrow's "lol." Changes in online writing are usually studied as a social
process, with innovations diffusing through a network of individuals in a
speech community. But unlike other types of innovation, language change is
shaped and constrained by the system in which it takes part. To investigate the
links between social and structural factors in language change, we undertake a
large-scale analysis of nonstandard word growth in the online community Reddit.
We find that dissemination across many linguistic contexts is a sign of growth:
words that appear in more linguistic contexts grow faster and survive longer.
We also find that social dissemination likely plays a less important role in
explaining word growth and decline than previously hypothesized
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