5,396 research outputs found
Measurement of the electronic compressibility of bilayer graphene
We present measurements of the electronic compressibility, , of bilayer
graphene in both zero and finite magnetic fields up to 14 T, and as a function
of both the carrier density and electric field perpendicular to the graphene
sheet. The low energy hyperbolic band structure of bilayer graphene is clearly
revealed in the data, as well as a sizable asymmetry between the conduction and
valence bands. A sharp increase in near zero density is observed for
increasing electric field strength, signaling the controlled opening of a gap
between these bands. At high magnetic fields, broad Landau level (LL)
oscillations are observed, directly revealing the doubled degeneracy of the
lowest LL and allowing for a determination of the disorder broadening of the
levels.Comment: 5 pages, 3 figures; final version for publicatio
Theoretical uncertainty in baryon oscillations
We discuss the systematic uncertainties in the recovery of dark energy
properties from the use of baryon acoustic oscillations as a standard ruler. We
demonstrate that while unknown relativistic components in the universe prior to
recombination would alter the sound speed, the inferences for dark energy from
low-redshift surveys are unchanged so long as the microwave background
anisotropies can measure the redshift of matter-radiation equality, which they
can do to sufficient accuracy. The mismeasurement of the radiation and matter
densities themselves (as opposed to their ratio) would manifest as an incorrect
prediction for the Hubble constant at low redshift. In addition, these
anomalies do produce subtle but detectable features in the microwave
anisotropies.Comment: 4 pages, REVTeX, 1 figure. Submitted to PR
Quantum Hall Effect and Semimetallic Behavior of Dual-Gated ABA-Stacked Trilayer Graphene
The electronic structure of multilayer graphenes depends strongly on the
number of layers as well as the stacking order. Here we explore the electronic
transport of purely ABA-stacked trilayer graphenes in a dual-gated field-effect
device configuration. We find that both the zero-magnetic-field transport and
the quantum Hall effect at high magnetic fields are distinctly different from
the monolayer and bilayer graphenes, and that they show electron-hole
asymmetries that are strongly suggestive of a semimetallic band overlap. When
the ABA trilayers are subjected to an electric field perpendicular to the
sheet, Landau level splittings due to a lifting of the valley degeneracy are
clearly observed.Comment: 5 figure
Hot-electron thermocouple and the diffusion thermopower of two-dimensional electrons in GaAs
A simple hot-electron thermocouple is realized in a two-dimensional electron system (2DES) and used to measure the diffusion thermopower of the 2DES at zero magnetic field. This hot-electron technique, which requires no micron-scale patterning of the 2DES, is much less sensitive than conventional methods to phonon-drag effects. Our thermopower results are in good agreement with the Mott formula for diffusion thermopower for temperatures up to T~2 K
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
At total Landau level filling factor a double layer
two-dimensional electron system with small interlayer separation supports a
collective state possessing spontaneous interlayer phase coherence. This state
exhibits the quantized Hall effect when equal electrical currents flow in
parallel through the two layers. In contrast, if the currents in the two layers
are equal, but oppositely directed, both the longitudinal and Hall resistances
of each layer vanish in the low temperature limit. This finding supports the
prediction that the ground state at is an excitonic superfluid.Comment: 4 pages, 4 figure
RascalC: A Jackknife Approach to Estimating Single and Multi-Tracer Galaxy Covariance Matrices
To make use of clustering statistics from large cosmological surveys,
accurate and precise covariance matrices are needed. We present a new code to
estimate large scale galaxy two-point correlation function (2PCF) covariances
in arbitrary survey geometries that, due to new sampling techniques, runs times faster than previous codes, computing finely-binned covariance
matrices with negligible noise in less than 100 CPU-hours. As in previous
works, non-Gaussianity is approximated via a small rescaling of shot-noise in
the theoretical model, calibrated by comparing jackknife survey covariances to
an associated jackknife model. The flexible code, RascalC, has been publicly
released, and automatically takes care of all necessary pre- and
post-processing, requiring only a single input dataset (without a prior 2PCF
model). Deviations between large scale model covariances from a mock survey and
those from a large suite of mocks are found to be be indistinguishable from
noise. In addition, the choice of input mock are shown to be irrelevant for
desired noise levels below mocks. Coupled with its generalization
to multi-tracer data-sets, this shows the algorithm to be an excellent tool for
analysis, reducing the need for large numbers of mock simulations to be
computed.Comment: 29 pages, 8 figures. Accepted by MNRAS. Code is available at
http://github.com/oliverphilcox/RascalC with documentation at
http://rascalc.readthedocs.io
Duality, the Semi-Circle Law and Quantum Hall Bilayers
There is considerable experimental evidence for the existence in Quantum Hall
systems of an approximate emergent discrete symmetry, . The evidence consists of the robustness of the tests of a suite a
predictions concerning the transitions between the phases of the system as
magnetic fields and temperatures are varied, which follow from the existence of
the symmetry alone. These include the universality of and quantum numbers of
the fixed points which occur in these transitions; selection rules governing
which phases may be related by transitions; and the semi-circular trajectories
in the Ohmic-Hall conductivity plane which are followed during the transitions.
We explore the implications of this symmetry for Quantum Hall systems involving
{\it two} charge-carrying fluids, and so obtain predictions both for bilayer
systems and for single-layer systems for which the Landau levels have a spin
degeneracy. We obtain similarly striking predictions which include the novel
new phases which are seen in these systems, as well as a prediction for
semicircle trajectories which are traversed by specific combinations of the
bilayer conductivities as magnetic fields are varied at low temperatures.Comment: 12 pages, 8 figures; discussion of magnetic field dependence modified
and figures and references updated in v
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
Constraints on perfect fluid and scalar field dark energy models from future redshift surveys
We discuss the constraints that future photometric and spectroscopic redshift
surveys can put on dark energy through the baryon oscillations of the power
spectrum. We model the dark energy either with a perfect fluid or a scalar
field and take into account the information contained in the linear growth
function. We show that the growth function helps to break the degeneracy in the
dark energy parameters and reduce the errors on roughly by 30% making
more appealing multicolor surveys based on photometric redshifts. We find that
a 200 square degrees spectroscopic survey reaching can constrain
to within and to using photometric redshifts with absolute uncertainty
of 0.02. In the scalar field case we show that the slope of the inverse
power-law potential for dark energy can be constrained to
(spectroscopic redshifts) or (photometric redshifts), i.e.
better than with future ground-based supernovae surveys or CMB data.Comment: 27 pages, submitted to MNRA
Coulomb Drag in the Extreme Quantum Limit
Coulomb drag resulting from interlayer electron-electron scattering in double
layer 2D electron systems at high magnetic field has been measured. Within the
lowest Landau level the observed drag resistance exceeds its zero magnetic
value by factors of typically 1000. At half-filling of the lowest Landau level
in each layer (nu = 1/2) the data suggest that our bilayer systems are much
more strongly correlated than recent theoretical models based on perturbatively
coupled composite fermion metals.Comment: 4 pages, 4 figure
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