173 research outputs found
Electronic transport in locally gated graphene nanoconstrictions
We have developed the combination of an etching and deposition technique that
enables the fabrication of locally gated graphene nanostructures of arbitrary
design. Employing this method, we have fabricated graphene nanoconstrictions
with local tunable transmission and characterized their electronic properties.
An order of magnitude enhanced gate efficiency is achieved adopting the local
gate geometry with thin dielectric gate oxide. A complete turn off of the
device is demonstrated as a function of the local gate voltage. Such strong
suppression of device conductance was found to be due to both quantum
confinement and Coulomb blockade effects in the constricted graphene
nanostructures.Comment: 3 pages 3 figures; separated and expanded from arXiv:0705.3044v
Electronic Transport in Dual-gated Bilayer Graphene at Large Displacement Fields
We study the electronic transport properties of dual-gated bilayer graphene
devices. We focus on the regime of low temperatures and high electric
displacement fields, where we observe a clear exponential dependence of the
resistance as a function of displacement field and density, accompanied by a
strong non-linear behavior in the transport characteristics. The effective
transport gap is typically two orders of magnitude smaller than the optical
band gaps reported by infrared spectroscopy studies. Detailed temperature
dependence measurements shed light on the different transport mechanisms in
different temperature regimes.Comment: 4 pages, 3 figure
Excited state spectroscopy in carbon nanotube double quantum dots
We report on low temperature measurements in a fully tunable carbon nanotube
double quantum dot. A new fabrication technique has been used for the top-gates
in order to avoid covering the whole nanotube with an oxide layer as in
previous experiments. The top-gates allow us to form single dots, control the
coupling between them and we observe four-fold shell filling. We perform
inelastic transport spectroscopy via the excited states in the double quantum
dot, a necessary step towards the implementation of new microwave-based
experiments.Comment: 16 pages, 6 figures, submitted to nanoletter
Pressure dependence of the magic twist angle in graphene superlattices
The recently demonstrated unconventional superconductivity in twisted bilayer
graphene (tBLG) opens the possibility for interesting applications of
two-dimensional layers that involve correlated electron states. Here we explore
the possibility of modifying electronic correlations by the application of
uniaxial pressure on the weakly interacting layers, which results in increased
interlayer coupling and a modification of the magic angle value and associated
density of states. Our findings are based on first-principles calculations that
accurately describe the height-dependent interlayer coupling through the
combined use of Density Functional Theory and Maximally localized Wannier
functions. We obtain the relationship between twist angle and external pressure
for the magic angle flat bands of tBLG. This may provide a convenient method to
tune electron correlations by controlling the length scale of the superlattice.Comment: 6 pages, 4 figure
Long wavelength local density of states oscillations near graphene step edges
Using scanning tunneling microscopy and spectroscopy, we have studied the
local density of states (LDOS) of graphene over step edges in boron nitride.
Long wavelength oscillations in the LDOS are observed with maxima parallel to
the step edge. Their wavelength and amplitude are controlled by the energy of
the quasiparticles allowing a direct probe of the graphene dispersion relation.
We also observe a faster decay of the LDOS oscillations away from the step edge
than in conventional metals. This is due to the chiral nature of the Dirac
fermions in graphene.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
Quantum Hall Effect, Screening and Layer-Polarized Insulating States in Twisted Bilayer Graphene
We investigate electronic transport in dual-gated twisted bilayer graphene.
Despite the sub-nanometer proximity between the layers, we identify independent
contributions to the magnetoresistance from the graphene Landau level spectrum
of each layer. We demonstrate that the filling factor of each layer can be
independently controlled via the dual gates, which we use to induce Landau
level crossings between the layers. By analyzing the gate dependence of the
Landau level crossings, we characterize the finite inter-layer screening and
extract the capacitance between the atomically-spaced layers. At zero filling
factor, we observe magnetic and displacement field dependent insulating states,
which indicate the presence of counter-propagating edge states with inter-layer
coupling.Comment: 4 pages, 3 figure
Electronic transport and quantum Hall effect in bipolar graphene p-n-p junction
We have developed a device fabrication process to pattern graphene into
nanostructures of arbitrary shape and control their electronic properties using
local electrostatic gates. Electronic transport measurements have been used to
characterize locally gated bipolar graphene -- junctions. We observe a
series of fractional quantum Hall conductance plateaus at high magnetic fields
as the local charge density is varied in the and regions. These
fractional plateaus, originating from chiral edge states equilibration at the
- interfaces, exhibit sensitivity to inter-edge backscattering which is
found to be strong for some of the plateuas and much weaker for other plateaus.
We use this effect to explore the role of backscattering and estimate disorder
strength in our graphene devices.Comment: 4 pages 4 figures, to appear in Phys. Rev. Lett. Original version
arXiv:0705.3044v1 was separated and expanded to this current version and
arXiv:0709.173
Nearly flat Chern bands in moiré superlattices
Topology and electron interactions are two central themes in modern condensed matter physics. Here, we propose graphene-based systems where both the band topology and interaction effects can be simply controlled with electric fields. We study a number of systems of twisted double layers with small twist angle where a moiré superlattice is formed. Each layer is chosen to be either AB-stacked bilayer graphene, ABC-stacked trilayer graphene, or hexagonal boron nitride. In these systems, a vertical applied electric field enables control of the bandwidth, and interestingly also the Chern number. We find that the Chern numbers of the bands associated with each of the two microscopic valleys can be ±0,±1,±2,±3 depending on the specific system and vertical electrical field. We show that these graphene moiré superlattices are promising platforms to realize a number of fascinating many-body phenomena, including (fractional) quantum anomalous Hall effects. We also discuss conceptual similarities and implications for modeling twisted bilayer graphene systems.National Science Foundation (U.S.) (Grant DMR-1608505)Simons Foundation (Simons Investigator Award)Gordon and Betty Moore Foundation (Grant GBMF4541)STC Center for Integrated Quantum Materials (Grant DMR-1231319
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