5,181 research outputs found
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
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
Non-LTE spectral analyses of the lately discovered DB-gap white dwarfs from the SDSS
For a long time, no hydrogen-deficient white dwarfs have been known that have
effective temperature between 30 kK and < 45 kK, i.e. exceeding those of DB
white dwarfs and having lower ones than DO white dwarfs. Therefore, this
temperature range was long known as the DB-gap. Only recently, the SDSS
provided spectra of several candidate DB-gap stars. First analyses based on
model spectra calculated under the assumption of local thermodynamic
equilibrium (LTE) confirmed that these stars had 30 kK < Teff < 45 kK
(Eisenstein et al. 2006). It has been shown for DO white dwarfs that the
relaxation of LTE is necessary to account for non local effects in the
atmosphere caused by the intense radiation field. Therefore, we calculated a
non-LTE model grid and re-analysed the aforementioned set of SDSS spectra. Our
results confirm the existence of DB-gap white dwarfs.Comment: 4 pages, 2 figures, to appear in: Proceedings of the 16th European
Workshop on White Dwarf
Tunneling Conductance Between Parallel Two Dimensional Electron Systems
We derive and evaluate expressions for the low temperature {\it dc}
equilibrium tunneling conductance between parallel two-dimensional electron
systems. Our theory is based on a linear-response formalism and on
impurity-averaged perturbation theory. The disorder broadening of features in
the dependence of tunneling conductance on sheet densities and in-plane
magnetic field strengths is influenced both by the finite lifetime of electrons
within the wells and by non-momentum-conserving tunneling events. Disorder
vertex corrections are important only for weak in-plane magnetic fields and
strong interwell impurity-potential correlations. We comment on the basis of
our results on the possibility of using tunneling measurements to determine the
lifetime of electrons in the quantum wells.Comment: 14 pages, 5 Fig. not included, revtex, IUcm92-00
A New Statistic for Analyzing Baryon Acoustic Oscillations
We introduce a new statistic omega_l for measuring and analyzing large-scale
structure and particularly the baryon acoustic oscillations. omega_l is a
band-filtered, configuration space statistic that is easily implemented and has
advantages over the traditional power spectrum and correlation function
estimators. Unlike these estimators, omega_l can localize most of the acoustic
information into a single dip at the acoustic scale while also avoiding
sensitivity to the poorly constrained large scale power (i.e., the integral
constraint) through the use of a localized and compensated filter. It is also
sensitive to anisotropic clustering through pair counting and does not require
any binning. We measure the shift in the acoustic peak due to nonlinear effects
using the monopole omega_0 derived from subsampled dark matter catalogues as
well as from mock galaxy catalogues created via halo occupation distribution
(HOD) modeling. All of these are drawn from 44 realizations of 1024^3 particle
dark matter simulations in a 1h^{-1}Gpc box at z=1. We compare these shifts
with those obtained from the power spectrum and conclude that the results
agree. This indicates that any distance measurements obtained from omega_0 and
P(k) will be consistent with each other. We also show that it is possible to
extract the same amount of acoustic information using either omega_0 or P(k)
from equal volume surveys.Comment: 12 pages, 7 figures. ApJ accepted. Edit: Now updated with final
accepted versio
Quantum Hall Exciton Condensation at Full Spin Polarization
Using Coulomb drag as a probe, we explore the excitonic phase transition in
quantum Hall bilayers at nu=1 as a function of Zeeman energy, E_Z. The critical
layer separation d/l for exciton condensation initially increases rapidly with
E_Z, but then reaches a maximum and begins a gentle decline. At high E_Z, where
both the excitonic phase at small d/l and the compressible phase at large d/l
are fully spin polarized, we find that the width of the transition, as a
function of d/l, is much larger than at small E_Z and persists in the limit of
zero temperature. We discuss these results in the context of two models in
which the system contains a mixture of the two fluids.Comment: 4 pages, 3 eps figure
Improved forecasts for the baryon acoustic oscillations and cosmological distance scale
We present the cosmological distance errors achievable using the baryon
acoustic oscillations as a standard ruler. We begin from a Fisher matrix
formalism that is upgraded from Seo & Eisenstein (2003). We isolate the
information from the baryonic peaks by excluding distance information from
other less robust sources. Meanwhile we accommodate the Lagrangian displacement
distribution into the Fisher matrix calculation to reflect the gradual loss of
information in scale and in time due to nonlinear growth, nonlinear bias, and
nonlinear redshift distortions. We then show that we can contract the
multi-dimensional Fisher matrix calculations into a 2-dimensional or even
1-dimensional formalism with physically motivated approximations. We present
the resulting fitting formula for the cosmological distance errors from galaxy
redshift surveys as a function of survey parameters and nonlinearity, which
saves us going through the 12-dimensional Fisher matrix calculations. Finally,
we show excellent agreement between the distance error estimates from the
revised Fisher matrix and the precision on the distance scale recovered from
N-body simulations.Comment: Submitted to ApJ, 21 pages, LaTe
Quantum Hall Exciton Condensation at Full Spin Polarization
Using Coulomb drag as a probe, we explore the excitonic phase transition in quantum Hall bilayers at Îœ_T = 1 as a function of Zeeman energy E_Z. The critical layer separation (d/â)_c for exciton condensation initially increases rapidly with E_Z, but then reaches a maximum and begins a gentle decline. At high E_Z, where both the excitonic phase at small d/â and the compressible phase at large d/â are fully spin polarized, we find that the width of the transition, as a function of d/â, is much larger than at small E_Z and persists in the limit of zero temperature. We discuss these results in the context of two models in which the system contains a mixture of the two fluids
Improving Cosmological Distance Measurements by Reconstruction of the Baryon Acoustic Peak
The baryon acoustic oscillations are a promising route to the precision
measure of the cosmological distance scale and hence the measurement of the
time evolution of dark energy. We show that the non-linear degradation of the
acoustic signature in the correlations of low-redshift galaxies is a
correctable process. By suitable reconstruction of the linear density field,
one can sharpen the acoustic peak in the correlation function or, equivalently,
restore the higher harmonics of the oscillations in the power spectrum. With
this, one can achieve better measurements of the acoustic scale for a given
survey volume. Reconstruction is particularly effective at low redshift, where
the non-linearities are worse but where the dark energy density is highest. At
z=0.3, we find that one can reduce the sample variance error bar on the
acoustic scale by at least a factor of 2 and in principle by nearly a factor of
4. We discuss the significant implications our results have for the design of
galaxy surveys aimed at measuring the distance scale through the acoustic peak.Comment: 5 pages, LaTeX. Submitted to the Astrophysical Journa
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