13,655 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
High-precision torsional magnetometer: Application to two-dimensional electron systems
A dc torsional magnetometer for use in high magnetic fields is described. With a resolution of 10^–12 J/T at 5 T and excellent rejection of background moments, this device has been used to study the de Haas–van Alphen effect in two-dimensional electron systems. This resolution is about 100 times that obtained with a commercially available superconducting quantum interference device magnetometer. The device is useful over a wide temperature range including that below 1 K
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
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