176 research outputs found
Quantized Landau level spectrum and its density dependence
Scanning tunneling microscopy and spectroscopy in magnetic field was used to
study Landau quantization in graphene and its dependence on charge carrier
density. Measurements were carried out on exfoliated graphene samples deposited
on a chlorinated SiO2 thermal oxide which allowed observing the Landau level
sequences characteristic of single layer graphene while tuning the density
through the Si backgate. Upon changing the carrier density we find abrupt jumps
in the Fermi level after each Landau level is filled. Moreover, the Landau
level spacing shows a marked increase at low doping levels, consistent with an
interaction-induced renormalization of the Dirac cone.Comment: 11 pages, 4 figure
Dynamical polarization, screening, and plasmons in gapped graphene
The one-loop polarization function of graphene has been calculated at zero
temperature for arbitrary wavevector, frequency, chemical potential (doping),
and band gap. The result is expressed in terms of elementary functions and is
used to find the dispersion of the plasmon mode and the static screening within
the random phase approximation. At long wavelengths the usual square root
behaviour of plasmon spectra for two-dimensional (2D) systems is obtained. The
presence of a small (compared to a chemical potential) gap leads to the
appearance of a new undamped plasmon mode. At greater values of the gap this
mode merges with the long-wavelength one, and vanishes when the Fermi level
enters the gap. The screening of charged impurities at large distances differs
from that in gapless graphene by slower decay of Friedel oscillations (
instead of ), similarly to conventional 2D systems.Comment: 8 pages, 8 figures, v2: to match published versio
Scanning Tunneling Spectroscopy of Graphene on Graphite
We report low temperature high magnetic field scanning tunneling microscopy
and spectroscopy of graphene flakes on graphite that exhibit the structural and
electronic properties of graphene decoupled from the substrate. Pronounced
peaks in the tunneling spectra develop with field revealing a Landau level
sequence that provides a direct way to identify graphene and to determine the
degree of its coupling to the substrate. The Fermi velocity and quasiparticle
lifetime, obtained from the positions and width of the peaks, provide access to
the electron-phonon and electron-electron interactions
Power-Aware Memory Allocation for Embedded Data-Intensive Signal Processing Applications
Many signal processing systems, particularly in the multimedia and telecommunication domains, are synthesized to execute data-intensive applications: their cost related aspects  namely power consumption and chip area  are heavily influenced, if not dominated, by the data access and storage aspects. This chapter presents a power-aware memory allocation methodology. Starting from the high-level behavioral specification of a given application, this framework performs the assignment of of the multidimensional signals to the memory layers  the on-chip scratch-pad memory and the off-chip main memory  the goal being the reduction of the dynamic energy consumption in the memory subsystem. Based on the assignment results, the framework subsequently performs the mapping of signals into the memory layers such that the overall amount of data storage be reduced. This software system yields a complete allocation solution: the exact storage amount on each memory layer, the mapping functions that determine the exact locations for any array element (scalar signal) in the specification, and, in addition, an estimation of the dynamic energy consumption in the memory subsystem
Local, global, and nonlinear screening in twisted double-layer graphene
One-atom-thick crystalline layers and their vertical heterostructures carry the promise of designer electronic materials that are unattainable by standard growth techniques. To realize their potential it is necessary to isolate them from environmental disturbances, in particular those introduced by the substrate. However, finding and characterizing suitable substrates, and minimizing the random potential fluctuations they introduce, has been a persistent challenge in this emerging field. Here we show that Landau-level (LL) spectroscopy offers the unique capability to quantify both the reduction of the quasiparticles\u27 lifetime and the long-range inhomogeneity due to random potential fluctuations. Harnessing this technique together with direct scanning tunneling microscopy and numerical simulations we demonstrate that the insertion of a graphene buffer layer with a large twist angle is a very effective method to shield a 2D system from substrate interference that has the additional desirable property of preserving the electronic structure of the system under study. We further show that owing to its remarkable nonlinear screening capability a single graphene buffer layer provides better shielding than either increasing the distance to the substrate or doubling the carrier density and reduces the amplitude of the potential fluctuations in graphene to values even lower than the ones in AB-stacked bilayer graphene
Integer and Fractional Quantum Hall Effect in Two-Terminal Measurements on Suspended Graphene
We report the observation of the quantized Hall effect in suspended graphene
probed with a two-terminal lead geometry. The failure of earlier Hall-bar
measurements is discussed and attributed to the placement of voltage probes in
mesoscopic samples. New quantized states are found at integer Landau level
fillings outside the sequence 2,6,10.., as well as at a fractional filling
\nu=1/3. Their presence is revealed by plateaus in the two-terminal conductance
which appear in magnetic fields as low as 2 Tesla at low temperatures and
persist up to 20 Kelvin in 12 Tesla. The excitation gaps, extracted from the
data with the help of a theoretical model, are found to be significantly larger
than in GaAs based electron systems.Comment: 17 pages, 4 figure
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