94 research outputs found
Mapping Dirac quasiparticles near a single Coulomb impurity on graphene
The response of Dirac fermions to a Coulomb potential is predicted to differ significantly from how non-relativistic electrons behave in traditional atomic and impurity systems. Surprisingly, many key theoretical predictions for this ultra-relativistic regime have not been tested. Graphene, a two-dimensional material in which electrons behave like massless Dirac fermions, provides a unique opportunity to test such predictions. Graphene’s response to a Coulomb potential also offers insight into important material characteristics, including graphene’s intrinsic dielectric constant, which is the primary factor determining the strength of electron–electron interactions in graphene. Here we present a direct measurement of the nanoscale response of Dirac fermions to a single Coulomb potential placed on a gated graphene device. Scanning tunnelling microscopy was used to fabricate tunable charge impurities on graphene, and to image electronic screening around them for a Q = +1|e| charge state. Electron-like and hole-like Dirac fermions were observed to respond differently to a Coulomb potential. Comparing the observed electron–hole asymmetry to theoretical simulations has allowed us to test predictions for how Dirac fermions behave near a Coulomb potential, as well as extract graphene’s intrinsic dielectric constant: ε[subscript g] = 3.0±1.0. This small value of ε[subscript g] indicates that electron–electron interactions can contribute significantly to graphene properties.United States. Office of Naval Research. Multidisciplinary University Research Initiative (Award N00014-09-1-1066)United States. Dept. of Energy. Office of Science (Contract DE-AC02-05CH11231)National Science Foundation (U.S.) (Award DMR-0906539
Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride
The Schr\"odinger equation dictates that the propagation of nearly free
electrons through a weak periodic potential results in the opening of band gaps
near points of the reciprocal lattice known as Brillouin zone boundaries.
However, in the case of massless Dirac fermions, it has been predicted that the
chirality of the charge carriers prevents the opening of a band gap and instead
new Dirac points appear in the electronic structure of the material. Graphene
on hexagonal boron nitride (hBN) exhibits a rotation dependent Moir\'e pattern.
In this letter, we show experimentally and theoretically that this Moir\'e
pattern acts as a weak periodic potential and thereby leads to the emergence of
a new set of Dirac points at an energy determined by its wavelength. The new
massless Dirac fermions generated at these superlattice Dirac points are
characterized by a significantly reduced Fermi velocity. The local density of
states near these Dirac cones exhibits hexagonal modulations indicating an
anisotropic Fermi velocity.Comment: 16 pages, 6 figure
Origin of the energy bandgap in epitaxial graphene
We studied the effect of quantum confinement on the size of the band gap in
single layer epitaxial graphene. Samples with different graphene terrace sizes
are studied by using low energy electron microscopy (LEEM) and angle-resolved
photoemission spectroscopy (ARPES). The direct correlation between the terrace
size extracted from LEEM and the gap size extracted from ARPES shows that
quantum confinement alone cannot account for the large gap observed in
epitaxial graphene samples
Giant Phonon-induced Conductance in Scanning Tunneling Spectroscopy of Gate-tunable Graphene
The honeycomb lattice of graphene is a unique two-dimensional (2D) system
where the quantum mechanics of electrons is equivalent to that of relativistic
Dirac fermions. Novel nanometer-scale behavior in this material, including
electronic scattering, spin-based phenomena, and collective excitations, is
predicted to be sensitive to charge carrier density. In order to probe local,
carrier-density dependent properties in graphene we have performed
atomically-resolved scanning tunneling spectroscopy measurements on
mechanically cleaved graphene flake devices equipped with tunable back-gate
electrodes. We observe an unexpected gap-like feature in the graphene tunneling
spectrum which remains pinned to the Fermi level (E_F) regardless of graphene
electron density. This gap is found to arise from a suppression of electronic
tunneling to graphene states near E_F and a simultaneous giant enhancement of
electronic tunneling at higher energies due to a phonon-mediated inelastic
channel. Phonons thus act as a "floodgate" that controls the flow of tunneling
electrons in graphene. This work reveals important new tunneling processes in
gate-tunable graphitic layers
Graphene plasmonics
Two rich and vibrant fields of investigation, graphene physics and
plasmonics, strongly overlap. Not only does graphene possess intrinsic plasmons
that are tunable and adjustable, but a combination of graphene with noble-metal
nanostructures promises a variety of exciting applications for conventional
plasmonics. The versatility of graphene means that graphene-based plasmonics
may enable the manufacture of novel optical devices working in different
frequency ranges, from terahertz to the visible, with extremely high speed, low
driving voltage, low power consumption and compact sizes. Here we review the
field emerging at the intersection of graphene physics and plasmonics.Comment: Review article; 12 pages, 6 figures, 99 references (final version
available only at publisher's web site
Spatially Resolving Spin-split Edge States of Chiral Graphene Nanoribbons
A central question in the field of graphene-related research is how graphene
behaves when it is patterned at the nanometer scale with different edge
geometries. Perhaps the most fundamental shape relevant to this question is the
graphene nanoribbon (GNR), a narrow strip of graphene that can have different
chirality depending on the angle at which it is cut. Such GNRs have been
predicted to exhibit a wide range of behaviour (depending on their chirality
and width) that includes tunable energy gaps and the presence of unique
one-dimensional (1D) edge states with unusual magnetic structure. Most GNRs
explored experimentally up to now have been characterized via electrical
conductivity, leaving the critical relationship between electronic structure
and local atomic geometry unclear (especially at edges). Here we present a
sub-nm-resolved scanning tunnelling microscopy (STM) and spectroscopy (STS)
study of GNRs that allows us to examine how GNR electronic structure depends on
the chirality of atomically well-defined GNR edges. The GNRs used here were
chemically synthesized via carbon nanotube (CNT) unzipping methods that allow
flexible variation of GNR width, length, chirality, and substrate. Our STS
measurements reveal the presence of 1D GNR edge states whose spatial
characteristics closely match theoretical expectations for GNR's of similar
width and chirality. We observe width-dependent splitting in the GNR edge state
energy bands, providing compelling evidence of their magnetic nature. These
results confirm the novel electronic behaviour predicted for GNRs with
atomically clean edges, and thus open the door to a whole new area of
applications exploiting the unique magnetoelectronic properties of chiral GNRs
All-Optical Generation of Surface Plasmons in Graphene
27 pages, 12 figures, includes supplementary materialarXiv is an e-print service in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics.Here we present an all-optical plasmon coupling scheme, utilising the intrinsic nonlinear optical response of graphene. We demonstrate coupling of free-space, visible light pulses to the surface plasmons in a planar, un-patterned graphene sheet by using nonlinear wave mixing to match both the wavevector and energy of the surface wave. By carefully controlling the phase-matching conditions, we show that one can excite surface plasmons with a defined wavevector and direction across a large frequency range, with an estimated photon efficiency in our experiments approaching
Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial
Hexagonal boron nitride (h-BN) is a natural hyperbolic material1, in which the dielectric constants are the same in the basal plane (ε[superscript t] ≡ ε[superscript x] = ε[superscript y]) but have opposite signs (ε[superscript t] ε[superscript z ]< 0) in the normal plane (ε[superscript z]). Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons—collective modes that originate from the coupling between photons and electric dipoles in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene with hyperbolic phonon polaritons in h-BN so that the eigenmodes of the graphene/h-BN heterostructure are hyperbolic plasmon–phonon polaritons. The hyperbolic plasmon–phonon polaritons in graphene/h-BN suffer little from ohmic losses, making their propagation length 1.5–2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon–phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial as the resulting properties of these devices are not present in its constituent elements alone
Electrical Control of Optical Emitter Relaxation Pathways enabled by Graphene
Controlling the energy flow processes and the associated energy relaxation
rates of a light emitter is of high fundamental interest, and has many
applications in the fields of quantum optics, photovoltaics, photodetection,
biosensing and light emission. While advanced dielectric and metallic systems
have been developed to tailor the interaction between an emitter and its
environment, active control of the energy flow has remained challenging. Here,
we demonstrate in-situ electrical control of the relaxation pathways of excited
erbium ions, which emit light at the technologically relevant telecommunication
wavelength of 1.5 m. By placing the erbium at a few nanometres distance
from graphene, we modify the relaxation rate by more than a factor of three,
and control whether the emitter decays into either electron-hole pairs, emitted
photons or graphene near-infrared plasmons, confined to 15 nm to the sheet.
These capabilities to dictate optical energy transfer processes through
electrical control of the local density of optical states constitute a new
paradigm for active (quantum) photonics.Comment: 9 pages, 4 figure
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