313 research outputs found
Intrinsic ripples in graphene
The stability of two-dimensional (2D) layers and membranes is subject of a
long standing theoretical debate. According to the so called Mermin-Wagner
theorem, long wavelength fluctuations destroy the long-range order for 2D
crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be
crumpled. These dangerous fluctuations can, however, be suppressed by
anharmonic coupling between bending and stretching modes making that a
two-dimensional membrane can exist but should present strong height
fluctuations. The discovery of graphene, the first truly 2D crystal and the
recent experimental observation of ripples in freely hanging graphene makes
these issues especially important. Beside the academic interest, understanding
the mechanisms of stability of graphene is crucial for understanding electronic
transport in this material that is attracting so much interest for its unusual
Dirac spectrum and electronic properties. Here we address the nature of these
height fluctuations by means of straightforward atomistic Monte Carlo
simulations based on a very accurate many-body interatomic potential for
carbon. We find that ripples spontaneously appear due to thermal fluctuations
with a size distribution peaked around 70 \AA which is compatible with
experimental findings (50-100 \AA) but not with the current understanding of
stability of flexible membranes. This unexpected result seems to be due to the
multiplicity of chemical bonding in carbon.Comment: 14 pages, 6 figure
Valley filter and valley valve in graphene
It is known that the lowest propagating mode in a narrow ballistic ribbon of
graphene may lack the twofold valley degeneracy of higher modes. Depending on
the crystallographic orientation of the ribbon axis, the lowest mode mixes both
valleys or lies predominantly in a single valley (chosen by the direction of
propagation). We show, using a tight-binding model calculation, that a
nonequilibrium valley polarization can be realized in a sheet of graphene, upon
injection of current through a ballistic point contact with zigzag edges. The
polarity can be inverted by local application of a gate voltage to the point
contact region. Two valley filters in series may function as an
electrostatically controlled ``valley valve'', representing a
zero-magnetic-field counterpart to the familiar spin valve.Comment: RevTeX, 4 pages, 5 figure
Large Representation Recurrences in Large N Random Unitary Matrix Models
In a random unitary matrix model at large N, we study the properties of the
expectation value of the character of the unitary matrix in the rank k
symmetric tensor representation. We address the problem of whether the standard
semiclassical technique for solving the model in the large N limit can be
applied when the representation is very large, with k of order N. We find that
the eigenvalues do indeed localize on an extremum of the effective potential;
however, for finite but sufficiently large k/N, it is not possible to replace
the discrete eigenvalue density with a continuous one. Nonetheless, the
expectation value of the character has a well-defined large N limit, and when
the discreteness of the eigenvalues is properly accounted for, it shows an
intriguing approximate periodicity as a function of k/N.Comment: 24 pages, 11 figure
Broken symmetry states and divergent resistance in suspended bilayer graphene
Graphene [1] and its bilayer have generated tremendous excitement in the
physics community due to their unique electronic properties [2]. The intrinsic
physics of these materials, however, is partially masked by disorder, which can
arise from various sources such as ripples [3] or charged impurities [4].
Recent improvements in quality have been achieved by suspending graphene flakes
[5,6], yielding samples with very high mobilities and little charge
inhomogeneity. Here we report the fabrication of suspended bilayer graphene
devices with very little disorder. We observe fully developed quantized Hall
states at magnetic fields of 0.2 T, as well as broken symmetry states at
intermediate filling factors , , and . The
devices exhibit extremely high resistance in the state that grows
with magnetic field and scales as magnetic field divided by temperature. This
resistance is predominantly affected by the perpendicular component of the
applied field, indicating that the broken symmetry states arise from many-body
interactions.Comment: 23 pages, including 4 figures and supplementary information; accepted
to Nature Physic
Bipolar supercurrent in graphene
Graphene -a recently discovered one-atom-thick layer of graphite- constitutes
a new model system in condensed matter physics, because it is the first
material in which charge carriers behave as massless chiral relativistic
particles. The anomalous quantization of the Hall conductance, which is now
understood theoretically, is one of the experimental signatures of the peculiar
transport properties of relativistic electrons in graphene. Other unusual
phenomena, like the finite conductivity of order 4e^2/h at the charge
neutrality (or Dirac) point, have come as a surprise and remain to be
explained. Here, we study the Josephson effect in graphene. Our experiments
rely on mesoscopic superconducting junctions consisting of a graphene layer
contacted by two closely spaced superconducting electrodes, where the charge
density can be controlled by means of a gate electrode. We observe a
supercurrent that, depending on the gate voltage, is carried by either
electrons in the conduction band or by holes in the valence band. More
importantly, we find that not only the normal state conductance of graphene is
finite, but also a finite supercurrent can flow at zero charge density. Our
observations shed light on the special role of time reversal symmetry in
graphene and constitute the first demonstration of phase coherent electronic
transport at the Dirac point.Comment: Under review, 12 pages, 4 Figs., suppl. info (v2 identical, resolved
file problems
Ripple Texturing of Suspended Graphene Atomic Membranes
Graphene is the nature's thinnest elastic membrane, with exceptional
mechanical and electrical properties. We report the direct observation and
creation of one-dimensional (1D) and 2D periodic ripples in suspended graphene
sheets, using spontaneously and thermally induced longitudinal strains on
patterned substrates, with control over their orientations and wavelengths. We
also provide the first measurement of graphene's thermal expansion coefficient,
which is anomalously large and negative, ~ -7x10^-6 K^-1 at 300K. Our work
enables novel strain-based engineering of graphene devices.Comment: 15 pages, 4 figure
Application of Graphene within Optoelectronic Devices and Transistors
Scientists are always yearning for new and exciting ways to unlock graphene's
true potential. However, recent reports suggest this two-dimensional material
may harbor some unique properties, making it a viable candidate for use in
optoelectronic and semiconducting devices. Whereas on one hand, graphene is
highly transparent due to its atomic thickness, the material does exhibit a
strong interaction with photons. This has clear advantages over existing
materials used in photonic devices such as Indium-based compounds. Moreover,
the material can be used to 'trap' light and alter the incident wavelength,
forming the basis of the plasmonic devices. We also highlight upon graphene's
nonlinear optical response to an applied electric field, and the phenomenon of
saturable absorption. Within the context of logical devices, graphene has no
discernible band-gap. Therefore, generating one will be of utmost importance.
Amongst many others, some existing methods to open this band-gap include
chemical doping, deformation of the honeycomb structure, or the use of carbon
nanotubes (CNTs). We shall also discuss various designs of transistors,
including those which incorporate CNTs, and others which exploit the idea of
quantum tunneling. A key advantage of the CNT transistor is that ballistic
transport occurs throughout the CNT channel, with short channel effects being
minimized. We shall also discuss recent developments of the graphene tunneling
transistor, with emphasis being placed upon its operational mechanism. Finally,
we provide perspective for incorporating graphene within high frequency
devices, which do not require a pre-defined band-gap.Comment: Due to be published in "Current Topics in Applied Spectroscopy and
the Science of Nanomaterials" - Springer (Fall 2014). (17 pages, 19 figures
Snap-through instability of graphene on substrates
We determine the graphene morphology regulated by substrates with herringbone
and checkerboard surface corrugations. As the graphene/substrate interfacial
bonding energy and the substrate surface roughness vary, the graphene
morphology snaps between two distinct states: 1) closely conforming to the
substrate and 2) remaining nearly flat on the substrate. Such a snapthrough
instability of graphene can potentially lead to desirable electronic properties
to enable graphene-based devices.Comment: 13 pages, 4 figures; Nanoscale Research Letters, in press, 200
Boron nitride substrates for high-quality graphene electronics
Graphene devices on standard SiO2 substrates are highly disordered,
exhibiting characteristics far inferior to the expected intrinsic properties of
graphene[1-12]. While suspending graphene above the substrate yields
substantial improvement in device quality[13,14], this geometry imposes severe
limitations on device architecture and functionality. Realization of
suspended-like sample quality in a substrate supported geometry is essential to
the future progress of graphene technology. In this Letter, we report the
fabrication and characterization of high quality exfoliated mono- and bilayer
graphene (MLG and BLG) devices on single crystal hexagonal boron nitride (h-BN)
substrates, by a mechanical transfer process. Variable-temperature
magnetotransport measurements demonstrate that graphene devices on h-BN exhibit
enhanced mobility, reduced carrier inhomogeneity, and reduced intrinsic doping
in comparison with SiO2-supported devices. The ability to assemble crystalline
layered materials in a controlled way sets the stage for new advancements in
graphene electronics and enables realization of more complex graphene
heterostructres.Comment: 20 pages (includes supplementary info), 7 figure
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