900 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
Two Dimensional Electron and Hole Gases at the Surface of Graphite
We report high-quality two-dimensional (2D) electron and hole gases induced
at the surface of graphite by the electric field effect. The 2D carriers reside
within a few near-surface atomic layers and exhibit mobilities up to 15,000 and
60,000 cm2/Vs at room and liquid-helium temperatures, respectively. The
mobilities imply ballistic transport at micron scale. Pronounced Shubnikov-de
Haas oscillations reveal the existence of two types of carries in both 2D
electron and hole gases.Comment: related to cond-mat/0410631 where preliminary data for this
experimental system were reporte
On resonant scatterers as a factor limiting carrier mobility in graphene
We show that graphene deposited on a substrate has a non-negligible density
of atomic scale defects. This is evidenced by a previously unnoticed D peak in
the Raman spectra with intensity of about 1% with respect to the G peak. We
evaluated the effect of such impurities on electron transport by mimicking them
with hydrogen adsorbates and measuring the induced changes in both mobility and
Raman intensity. If the intervalley scatterers responsible for the D peak are
monovalent, their concentration is sufficient to account for the limited
mobilities achievable in graphene on a substrate.Comment: version 2: several comments are taken into account and refs adde
Strong suppression of weak (anti)localization in graphene
Low-field magnetoresistance is ubiquitous in low-dimensional metallic systems
with high resistivity and well understood as arising due to quantum
interference on self-intersecting diffusive trajectories. We have found that in
graphene this weak-localization magnetoresistance is strongly suppressed and,
in some cases, completely absent. This unexpected observation is attributed to
mesoscopic corrugations of graphene sheets which cause a dephasing effect
similar to that of a random magnetic field.Comment: improved presentation of the theory part after referees comments;
important experimental info added (see "note added in proof"
Singular-phase nanooptics: towards label-free single molecule detection
Non-trivial topology of phase is crucial for many important physics phenomena
such as, for example, the Aharonov-Bohm effect 1 and the Berry phase 2. Light
phase allows one to create "twisted" photons 3, 4 , vortex knots 5,
dislocations 6 which has led to an emerging field of singular optics relying on
abrupt phase changes 7. Here we demonstrate the feasibility of singular
visible-light nanooptics which exploits the benefits of both plasmonic field
enhancement and non-trivial topology of light phase. We show that properly
designed plasmonic nanomaterials exhibit topologically protected singular phase
behaviour which can be employed to radically improve sensitivity of detectors
based on plasmon resonances. By using reversible hydrogenation of graphene 8
and a streptavidin-biotin test 9, we demonstrate areal mass sensitivity at a
level of femto-grams per mm2 and detection of individual biomolecules,
respectively. Our proof-of-concept results offer a way towards simple and
scalable single-molecular label-free biosensing technologies.Comment: 19 pages, 4 figure
A self-consistent theory for graphene transport
We demonstrate theoretically that most of the observed transport properties
of graphene sheets at zero magnetic field can be explained by scattering from
charged impurities. We find that, contrary to common perception, these
properties are not universal but depend on the concentration of charged
impurities . For dirty samples (), the value of the minimum
conductivity at low carrier density is indeed in agreement with early
experiments, with weak dependence on impurity concentration. For cleaner
samples, we predict that the minimum conductivity depends strongly on , increasing to for . A clear strategy to improve graphene mobility is to eliminate
charged impurities or use a substrate with a larger dielectric constant.Comment: To be published in Proc. Natl. Acad. Sci. US
Two Dimensional Atomic Crystals
We report free-standing atomic crystals that are strictly 2D and can be
viewed as individual atomic planes pulled out of bulk crystals or as unrolled
single-wall nanotubes. By using micromechanical cleavage, we have prepared and
studied a variety of 2D crystals, including single layers of boron nitride,
graphite, several dichalcogenides and complex oxides. These atomically-thin
sheets (essentially gigantic 2D molecules unprotected from the immediate
environment) are stable under ambient conditions, exhibit high crystal quality
and are continuous on a macroscopic scale.Comment: 4 page
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
Modeling electrolytically top gated graphene
We investigate doping of a single-layer graphene in the presence of
electrolytic top gating. The interfacial phenomena is modeled using a modified
Poisson-Boltzmann equation for an aqueous solution of simple salt. We
demonstrate both the sensitivity of graphene's doping levels to the salt
concentration and the importance of quantum capacitance that arises due to the
smallness of the Debye screening length in the electrolyte.Comment: 7 pages, including 4 figures, submitted to Nanoscale Research Letters
for a special issue related to the NGC 2009 conference
(http://asdn.net/ngc2009/index.shtml
Impurity-assisted tunneling in graphene
The electric conductance of a strip of undoped graphene increases in the
presence of a disorder potential, which is smooth on atomic scales. The
phenomenon is attributed to impurity-assisted resonant tunneling of massless
Dirac fermions. Employing the transfer matrix approach we demonstrate the
resonant character of the conductivity enhancement in the presence of a single
impurity. We also calculate the two-terminal conductivity for the model with
one-dimensional fluctuations of disorder potential by a mapping onto a problem
of Anderson localization.Comment: 6 pages, 3 figures, final version, typos corrected, references adde
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