4,883 research outputs found
Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO/SrTiO interfaces
Novel electronic systems forming at oxide interfaces comprise a class of new
materials with a wide array of potential applications. A high mobility electron
system forms at the LaAlO/SrTiO interface and, strikingly, both
superconducts and displays indications of hysteretic magnetoresistance. An
essential step for device applications is establishing the ability to vary the
electronic conductivity of the electron system by means of a gate. We have
fabricated metallic top gates above a conductive interface to vary the electron
density at the interface. By monitoring capacitance and electric field
penetration, we are able to tune the charge carrier density and establish that
we can completely deplete the metallic interface with small voltages. Moreover,
at low carrier densities, the capacitance is significantly enhanced beyond the
geometric capacitance for the structure. In the same low density region, the
metallic interface overscreens an external electric field. We attribute these
observations to a negative compressibility of the electronic system at the
interface. Similar phenomena have been observed previously in semiconducting
two-dimensional electronic systems. The observed compressibility result is
consistent with the interface containing a system of mobile electrons in two
dimensions.Comment: 4 figures in main text; 4 figures in the supplemen
Quantum Hall Phase Diagram of Second Landau-level Half-filled Bilayers: Abelian versus Non-Abelian States
The quantum Hall phase diagram of the half-filled bilayer system in the
second Landau level is studied as a function of tunneling and layer separation
using exact diagonalization. We make the striking prediction that bilayer
structures would manifest two distinct branches of incompressible fractional
quantum Hall effect (FQHE) corresponding to the Abelian 331 state (at moderate
to low tunneling and large layer separation) and the non-Abelian Pfaffian state
(at large tunneling and small layer separation). The observation of these two
FQHE branches and the quantum phase transition between them will be compelling
evidence supporting the existence of the non-Abelian Pfaffian state in the
second Landau level.Comment: 4 pages, 3 figure
The scale-dependence of relative galaxy bias: encouragement for the halo model description
We investigate the relationship between the colors, luminosities, and
environments of galaxies in the Sloan Digital Sky Survey spectroscopic sample,
using environmental measurements on scales ranging from 0.2 to 6 Mpc/h. We
find: (1) that the relationship between color and environment persists even to
the lowest luminosities we probe (absolute magnitude in the r band of about -14
for h=1); (2) at luminosities and colors for which the galaxy correlation
function has a large amplitude, it also has a steep slope; and (3) in regions
of a given overdensity on small scales (1 Mpc/h), the overdensity on large
scales (6 Mpc/h) does not appear to relate to the recent star formation history
of the galaxies. Of these results, the last has the most immediate application
to galaxy formation theory. In particular, it lends support to the notion that
a galaxy's properties are related only to the mass of its host dark matter
halo, and not to the larger scale environment.Comment: submitted to ApJ; full resolution figures and slide material
available at http://cosmo.nyu.edu/blanton/scale_density.htm
The Spectral Types of White Dwarfs in Messier 4
We present the spectra of 24 white dwarfs in the direction of the globular
cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs.
Determining the spectral types of the stars in this sample, we find 24 type DA
and 0 type DB (i.e., atmospheres dominated by hydrogen and helium
respectively). Assuming the ratio of DA/DB observed in the field with effective
temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster
environment, the chance of finding no DBs in our sample due simply to
statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100
white dwarfs previously identified in open clusters indicate that DB formation
is strongly suppressed in that environment. Furthermore, all the ~10 white
dwarfs previously identified in other globular clusters are exclusively type
DA. In the context of these two facts, this finding suggests that DB formation
is suppressed in the cluster environment in general. Though no satisfactory
explanation for this phenomenon exists, we discuss several possibilities.Comment: Accepted for Publication in Astrophys. J. 11 pages including 4
figures and 2 tables (journal format
Tunneling Between a Pair of Parallel Hall Droplets
In this paper, we examine interwell tunneling between a pair of fractional
quantum Hall liquids in a double quantum well system in a tilted magnetic
field. Using a variational Monte Carlo method, we calculate moments of the
intra-Landau level tunneling spectrum as a function of in-plane field component
and interwell spacing . This is done for variety of
incompressible states including a pair of layers ([330]), pair of
layers ([550]), and Halperin's [331] state. The results suggest a
technique to extract interwell correlations from the tunneling spectral data.Comment: 21 pages and 8 figures (included), RevTeX, preprint no. UCSDCU
Inferring the Origin Locations of Tweets with Quantitative Confidence
Social Internet content plays an increasingly critical role in many domains,
including public health, disaster management, and politics. However, its
utility is limited by missing geographic information; for example, fewer than
1.6% of Twitter messages (tweets) contain a geotag. We propose a scalable,
content-based approach to estimate the location of tweets using a novel yet
simple variant of gaussian mixture models. Further, because real-world
applications depend on quantified uncertainty for such estimates, we propose
novel metrics of accuracy, precision, and calibration, and we evaluate our
approach accordingly. Experiments on 13 million global, comprehensively
multi-lingual tweets show that our approach yields reliable, well-calibrated
results competitive with previous computationally intensive methods. We also
show that a relatively small number of training data are required for good
estimates (roughly 30,000 tweets) and models are quite time-invariant
(effective on tweets many weeks newer than the training set). Finally, we show
that toponyms and languages with small geographic footprint provide the most
useful location signals.Comment: 14 pages, 6 figures. Version 2: Move mathematics to appendix, 2 new
references, various other presentation improvements. Version 3: Various
presentation improvements, accepted at ACM CSCW 201
Can Baryonic Features Produce the Observed 100 Mpc Clustering?
We assess the possibility that baryonic acoustic oscillations in adiabatic
models may explain the observations of excess power in large-scale structure on
100h^-1 Mpc scales. The observed location restricts models to two extreme areas
of parameter space. In either case, the baryon fraction must be large
(Omega_b/Omega_0 > 0.3) to yield significant features. The first region
requires Omega_0 < 0.2h to match the location, implying large blue tilts
(n>1.4) to satisfy cluster abundance constraints. The power spectrum also
continues to rise toward larger scales in these models. The second region
requires Omega_0 near 1, implying Omega_b well out of the range of big bang
nucleosynthesis constraints; moreover, the peak is noticeably wider than the
observations suggest. Testable features of both solutions are that they require
moderate reionization and thereby generate potentially observable (about 1 uK)
large-angle polarization, as well as sub-arc-minute temperature fluctuations.
In short, baryonic features in adiabatic models may explain the observed excess
only if currently favored determinations of cosmological parameters are in
substantial error or if present surveys do not represent a fair sample of
100h^-1 Mpc structures.Comment: LaTeX, 7 pages, 5 Postscript figures, submitted to ApJ Letter
Flowing with Time: a New Approach to Nonlinear Cosmological Perturbations
Nonlinear effects are crucial in order to compute the cosmological matter
power spectrum to the accuracy required by future generation surveys. Here, a
new approach is presented, in which the power spectrum, the bispectrum and
higher order correlations, are obtained -- at any redshift and for any momentum
scale -- by integrating a system of differential equations. The method is
similar to the familiar BBGKY hierarchy. Truncating at the level of the
trispectrum, the solution of the equations corresponds to the summation of an
infinite class of perturbative corrections. Compared to other resummation
frameworks, the scheme discussed here is particularly suited to cosmologies
other than LambdaCDM, such as those based on modifications of gravity and those
containing massive neutrinos. As a first application, we compute the Baryonic
Acoustic Oscillation feature of the power spectrum, and compare the results
with perturbation theory, the halo model, and N-body simulations. The
density-velocity and velocity-velocity power spectra are also computed, showing
that they are much less contaminated by nonlinearities than the density-density
one. The approach can be seen as a particular formulation of the
renormalization group, in which time is the flow parameter.Comment: 20 pages, 7 figures. Matches version published on JCA
Evidence for a fractional quantum Hall state with anisotropic longitudinal transport
At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau
level (LL), a clean two-dimensional electron system (2DES) exhibits numerous
incompressible liquid phases which display the fractional quantized Hall effect
(FQHE) (Das Sarma and Pinczuk, 1997). These liquid phases do not break
rotational symmetry, exhibiting resistivities which are isotropic in the plane.
In contrast, at lower fields, when the Fermi level lies in the third
and several higher LLs, the 2DES displays a distinctly different class of
collective states. In particular, near half filling of these high LLs the 2DES
exhibits a strongly anisotropic longitudinal resistance at low temperatures
(Lilly et al., 1999; Du et al., 1999). These "stripe" phases, which do not
exhibit the quantized Hall effect, resemble nematic liquid crystals, possessing
broken rotational symmetry and orientational order (Koulakov et al., 1996;
Fogler et al., 1996; Moessner and Chalker, 1996; Fradkin and Kivelson, 1999;
Fradkin et al, 2010). Here we report a surprising new observation: An
electronic configuration in the N=1 second LL whose resistivity tensor
simultaneously displays a robust fractionally quantized Hall plateau and a
strongly anisotropic longitudinal resistance resembling that of the stripe
phases.Comment: Nature Physics, (2011
Fractional quantum Hall effect without energy gap
In the fractional quantum Hall effect regime we measure diagonal
() and Hall () magnetoresistivity tensor components of
two-dimensional electron system (2DES) in gated GaAs/AlGaAs
heterojunctions, together with capacitance between 2DES and the gate. We
observe 1/3- and 2/3-fractional quantum Hall effect at rather low magnetic
fields where corresponding fractional minima in the thermodynamical density of
states have already disappeared manifesting complete suppression of the
quasiparticle energy gaps.Comment: 4 pages, 4 figure
- …