3,035 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
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
RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster
An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi
X-ray Timing Explorer proportional counter array produced a high quality
spectrum in the 2 - 30 keV range. Joint fitting with the 0.7 - 11 keV spectrum
obtained with the Advanced Satellite for Astrophysics and Cosmology gas imaging
spectrometer gives an upperlimit of 2.3x10^-7 photons/cm^2/sec/keV for
non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic
field of 0.36 micro Gauss (uG) and an upperlimit of 1.8x10^-13 ergs/cm^3 for
the cosmic-ray electron energy density. The resulting lower limit to the
central magnetic field is ~1 - 3 uG While a magnetic field of ~0.1 - 0.2 uG can
be created by galaxy wakes, a magnetic field of several uG is usually
associated with a cooling flow or, as in the case of the Coma cluster, a
subcluster merger. However, for A2256, the evidence for a merger is weak and
the main cluster shows no evidence of a cooling flow. Thus, there is presently
no satisfactory hypothesis for the origin of an average cluster magnetic field
as high as >0.36 uG in the A2256 cluster.Comment: 8 pages, Astrophysical Journal (in press
Symbolic Algorithms for Language Equivalence and Kleene Algebra with Tests
We first propose algorithms for checking language equivalence of finite
automata over a large alphabet. We use symbolic automata, where the transition
function is compactly represented using a (multi-terminal) binary decision
diagrams (BDD). The key idea consists in computing a bisimulation by exploring
reachable pairs symbolically, so as to avoid redundancies. This idea can be
combined with already existing optimisations, and we show in particular a nice
integration with the disjoint sets forest data-structure from Hopcroft and
Karp's standard algorithm. Then we consider Kleene algebra with tests (KAT), an
algebraic theory that can be used for verification in various domains ranging
from compiler optimisation to network programming analysis. This theory is
decidable by reduction to language equivalence of automata on guarded strings,
a particular kind of automata that have exponentially large alphabets. We
propose several methods allowing to construct symbolic automata out of KAT
expressions, based either on Brzozowski's derivatives or standard automata
constructions. All in all, this results in efficient algorithms for deciding
equivalence of KAT expressions
Disorder mediated splitting of the cyclotron resonance in two-dimensional electron systems
We perform a direct study of the magnitude of the anomalous splitting in the
cyclotron resonance (CR) of a two-dimensional electron system (2DES) as a
function of sample disorder. In a series of AlGaAs/GaAs quantum wells,
identical except for a range of carbon doping in the well, we find the CR
splitting to vanish at high sample mobilities but to increase dramatically with
increasing impurity density and electron scattering rates. This observation
lends strong support to the conjecture that the non-zero wavevector, roton-like
minimum in the dispersion of 2D magnetoplasmons comes into resonance with the
CR, with the two modes being coupled via disorder.Comment: accepted to PRB Rapid Com
Acoustic phonon scattering in a low density, high mobility AlGaN/GaN field effect transistor
We report on the temperature dependence of the mobility, , of the
two-dimensional electron gas in a variable density AlGaN/GaN field effect
transistor, with carrier densities ranging from 0.4 cm to
3.0 cm and a peak mobility of 80,000 cm/Vs. Between
20 K and 50 K we observe a linear dependence T
indicating that acoustic phonon scattering dominates the temperature dependence
of the mobility, with being a monotonically increasing function of
decreasing 2D electron density. This behavior is contrary to predictions of
scattering in a degenerate electron gas, but consistent with calculations which
account for thermal broadening and the temperature dependence of the electron
screening. Our data imply a deformation potential D = 12-15 eV.Comment: 3 pages, 2 figures, RevTeX. Submitted to Appl Phys Let
- …