892 research outputs found
Conductance of p-n-p graphene structures with 'air-bridge' top gates
We have fabricated graphene devices with a top gate separated from the
graphene layer by an air gap--a design which does not decrease the mobility of
charge carriers under the gate. This gate is used to realise p-n-p structures
where the conducting properties of chiral carriers are studied. The band
profile of the structures is calculated taking into account the specifics of
the graphene density of states and is used to find the resistance of the p-n
junctions expected for chiral carriers. We show that ballistic p-n junctions
have larger resistance than diffusive ones. This is caused by suppressed
transmission of chiral carriers at angles away from the normal to the junction.Comment: to be published in Nano Letter
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
Impurities as a source of flicker noise in graphene
We experimentally study the effect of different scattering potentials on the
flicker noise observed in graphene devices on silica substrates. The noise in
nominally identical devices is seen to behave in two distinct ways as a
function of carrier concentration, changing either monotonically or
nonmonotonically. We attribute this to the interplay between long- and
short-range scattering mechanisms. Water is found to significantly enhance the
noise magnitude and change the type of the noise behaviour. By using a simple
model, we show that water is a source of long-range scattering.Comment: 4 pages, 4 figure
Giant Nonlocality near the Dirac Point in Graphene
Transport measurements have been a powerful tool for uncovering new
electronic phenomena in graphene. We report nonlocal measurements performed in
the Hall bar geometry with voltage probes far away from the classical path of
charge flow. We observe a large nonlocal response near the Dirac point in
fields as low as 0.1T, which persists up to room temperature. The nonlocality
is consistent with the long-range flavor currents induced by lifting of
spin/valley degeneracy. The effect is expected to contribute strongly to all
magnetotransport phenomena near the neutrality point
Atomically thin boron nitride: a tunnelling barrier for graphene devices
We investigate the electronic properties of heterostructures based on
ultrathin hexagonal boron nitride (h-BN) crystalline layers sandwiched between
two layers of graphene as well as other conducting materials (graphite, gold).
The tunnel conductance depends exponentially on the number of h-BN atomic
layers, down to a monolayer thickness. Exponential behaviour of I-V
characteristics for graphene/BN/graphene and graphite/BN/graphite devices is
determined mainly by the changes in the density of states with bias voltage in
the electrodes. Conductive atomic force microscopy scans across h-BN terraces
of different thickness reveal a high level of uniformity in the tunnel current.
Our results demonstrate that atomically thin h-BN acts as a defect-free
dielectric with a high breakdown field; it offers great potential for
applications in tunnel devices and in field-effect transistors with a high
carrier density in the conducting channel.Comment: 7 pages, 5 figure
How close can one approach the Dirac point in graphene experimentally?
The above question is frequently asked by theorists who are interested in
graphene as a model system, especially in context of relativistic quantum
physics. We offer an experimental answer by describing electron transport in
suspended devices with carrier mobilities of several 10^6 cm^2V^-1s^-1 and with
the onset of Landau quantization occurring in fields below 5 mT. The observed
charge inhomogeneity is as low as \approx10^8 cm^-2, allowing a neutral state
with a few charge carriers per entire micron-scale device. Above liquid helium
temperatures, the electronic properties of such devices are intrinsic, being
governed by thermal excitations only. This yields that the Dirac point can be
approached within 1 meV, a limit currently set by the remaining charge
inhomogeneity. No sign of an insulating state is observed down to 1 K, which
establishes the upper limit on a possible bandgap
Prospects for the discovery of the next new element: Influence of projectiles with Z > 20
The possibility of forming new superheavy elements with projectiles having Z
> 20 is discussed. Current research has focused on the fusion of 48Ca with
actinides targets, but these reactions cannot be used for new element
discoveries in the future due to a lack of available target material. The
influence on reaction cross sections of projectiles with Z > 20 have been
studied in so-called analog reactions, which utilize lanthanide targets
carefully chosen to create compound nuclei with energetics similar to those
found in superheavy element production. The reactions 48Ca, 45Sc, 50Ti, 54Cr +
159Tb, 162Dy have been studied at the Cyclotron Institute at Texas A&M
University using the Momentum Achromat Recoil Spectrometer. The results of
these experimental studies are discussed in terms of the influence of
collective enhancements to level density for compound nuclei near closed
shells, and the implications for the production of superheavy elements. We have
observed no evidence to contradict theoretical predictions that the maximum
cross section for the 249Cf(50Ti, 4n)295120 and 248Cm(54Cr, 4n)298120 reactions
should be in the range of 10-100 fb.Comment: An invited talk given by Charles M. Folden III at the 11th
International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio,
Texas, USA, May 27-June 1, 2012. Also contains information presented by
Dmitriy A. Mayorov and Tyler A. Werke in separate contributions to the
conference. This contribution will appear in the NN2012 Proceedings in
Journal of Physics: Conference Series (JPCS
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