151 research outputs found
Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer
We have studied temperature dependences of electron transport in graphene and
its bilayer and found extremely low electron-phonon scattering rates that set
the fundamental limit on possible charge carrier mobilities at room
temperature. Our measurements have shown that mobilities significantly higher
than 200,000 cm2/Vs are achievable, if extrinsic disorder is eliminated. A
sharp (threshold-like) increase in resistivity observed above approximately
200K is unexpected but can qualitatively be understood within a model of a
rippled graphene sheet in which scattering occurs on intra-ripple flexural
phonons
Dirac fermions in a power-law-correlated random vector potential
We study localization properties of two-dimensional Dirac fermions subject to
a power-law-correlated random vector potential describing, e.g., the effect of
"ripples" in graphene. By using a variety of techniques (low-order perturbation
theory, self-consistent Born approximation, replicas, and supersymmetry) we
make a case for a possible complete localization of all the electronic states
and compute the density of states.Comment: Latex, 4+ page
Gap opening in the zeroth Landau level of graphene
We have measured a strong increase of the low-temperature resistivity
and a zero-value plateau in the Hall conductivity at
the charge neutrality point in graphene subjected to high magnetic fields up to
30 T. We explain our results by a simple model involving a field dependent
splitting of the lowest Landau level of the order of a few Kelvin, as extracted
from activated transport measurements. The model reproduces both the increase
in and the anomalous plateau in in terms of
coexisting electrons and holes in the same spin-split zero-energy Landau level.Comment: 4 pages, 3 figure
Density of states and zero Landau level probed through capacitance of graphene
We report capacitors in which a finite electronic compressibility of graphene
dominates the electrostatics, resulting in pronounced changes in capacitance as
a function of magnetic field and carrier concentration. The capacitance
measurements have allowed us to accurately map the density of states D, and
compare it against theoretical predictions. Landau oscillations in D are robust
and zero Landau level (LL) can easily be seen at room temperature in moderate
fields. The broadening of LLs is strongly affected by charge inhomogeneity that
leads to zero LL being broader than other levels
Unconventional quantum Hall effect and Berry’s phase 2pi in bilayer graphene.
There are known two distinct types of the integer quantum Hall effect. One is the conventional quantum Hall effect, characteristic of two-dimensional semiconductor systems, and the other is its relativistic counterpart recently observed in graphene, where charge carriers mimic Dirac fermions characterized by Berry’s phase pi, which results in a shifted positions of Hall plateaus. Here we report a third type of the integer quantum Hall effect. Charge carriers in bilayer graphene have a parabolic energy spectrum but are chiral and exhibit Berry’s phase 2pi affecting their quantum dynamics. The Landau quantization of these fermions results in plateaus in Hall conductivity at standard integer positions but the last (zero-level) plateau is missing. The zero-level anomaly is accompanied by metallic conductivity in the limit of low concentrations and high magnetic fields, in stark contrast to the conventional, insulating behavior in this regime. The revealed chiral fermions have no known analogues and present an intriguing case for quantum-mechanical studies
Quark--hadron duality in lepton scattering off nuclei
A phenomenological study of quark--hadron duality in electron and neutrino
scattering on nuclei is performed. We compute the structure functions and
in the resonance region within a framework that includes the
Dortmund-group model for the production of the {f}{i}rst four lowest-lying
baryonic resonances and a relativistic mean-field model for nuclei. We consider
four-momentum transfers between 0.2 and 2.5 GeV. The results indicate that
nuclear effects play a different role in the resonance and DIS region. We find
that global but not local duality works well. In the studied range of
four-momentum transfers, the integrated strength of the computed nuclear
structure functions in the resonance region, is considerably lower than the DIS
one.Comment: 18 pages, 11 figure
The structure of suspended graphene sheets
The recent discovery of graphene has sparked significant interest, which has
so far been focused on the peculiar electronic structure of this material, in
which charge carriers mimic massless relativistic particle. However, the
structure of graphene - a single layer of carbon atoms densely packed in a
honeycomb crystal lattice - is also puzzling. On the one hand, graphene appears
to be a strictly two-dimensional (2D) material and exhibits such a high crystal
quality that electrons can travel submicron distances without scattering. On
the other hand, perfect 2D crystals cannot exist in the free state, according
to both theory and experiment. This is often reconciled by the fact that all
graphene structures studied so far were an integral part of larger 3D
structures, either supported by a bulk substrate or embedded in a 3D matrix.
Here we report individual graphene sheets freely suspended on a microfabricated
scaffold in vacuum or air. These membranes are only one atom thick and still
display a long-range crystalline order. However, our studies by transmission
electron microscopy (TEM) have revealed that suspended graphene sheets are not
perfectly flat but exhibit intrinsic microscopic roughening such that the
surface normal varies by several degrees and out-of-plane deformations reach 1
nm. The atomically-thin single-crystal membranes offer an ample scope for
fundamental research and new technologies whereas the observed corrugations in
the third dimension may shed light on subtle reasons behind the stability of 2D
crystals.Comment: 14 pages, includes supplementary informatio
Coexistence of electron and hole transport in graphene
When sweeping the carrier concentration in monolayer graphene through the
charge neutrality point, the experimentally measured Hall resistivity shows a
smooth zero crossing. Using a two- component model of coexisting electrons and
holes around the charge neutrality point, we unambiguously show that both types
of carriers are simultaneously present. For high magnetic fields up to 30 T the
electron and hole concentrations at the charge neutrality point increase with
the degeneracy of the zero-energy Landau level which implies a quantum Hall
metal state at \nu=0 made up by both electrons and holes.Comment: 5 pages, 6 figure
Splitting of critical energies in the =0 Landau level of graphene
The lifting of the degeneracy of the states from the graphene =0 Landau
level (LL) is investigated through a non-interacting tight-binding model with
random hoppings. A disorder-driven splitting of two bands and of two critical
energies is observed by means of density of states and participation ratio
calculations. The analysis of the probability densities of the states within
the =0 LL provides some insights into the interplay of lattice and disorder
effects on the splitting process. An uneven spatial distribution of the wave
function amplitudes between the two graphene sublattices is found for the
states in between the two split peaks. It is shown that as the splitting is
increased (linear increasing with disorder and square root increasing with
magnetic field), the two split levels also get increasingly broadened, in such
a way that the proportion of the overlapped states keeps approximately constant
for a wide range of disorder or magnetic field variation.Comment: 6 figure
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