2,663 research outputs found
Experimental observation of nanoscale radiative heat flow due to surface plasmons in graphene and doped silicon
Owing to its two dimensional electronic structure, graphene exhibits many
unique properties. One of them is a wave vector and temperature dependent
plasmon in the infrared range. Theory predicts that due to these plasmons,
graphene can be used as a universal material to enhance nanoscale radiative
heat exchange for any dielectric substrate. Here we report on radiative heat
transfer experiments between SiC and a SiO2 sphere which have non matching
phonon polariton frequencies, and thus only weakly exchange heat in near field.
We observed that the heat flux contribution of graphene epitaxially grown on
SiC dominates at short distances. The influence of plasmons on radiative heat
transfer is further supported with measurements for doped silicon. These
results highlight graphenes strong potential in photonic nearfield and energy
conversion devices.Comment: 4 pages, 3 figure
Weak antilocalization in epitaxial graphene: evidence for chiral electrons
Transport in ultrathin graphite grown on silicon carbide is dominated by the
electron-doped epitaxial layer at the interface. Weak anti-localization in 2D
samples manifests itself as a broad cusp-like depression in the longitudinal
resistance for magnetic fields 10 mT 5 T. An extremely sharp
weak-localization resistance peak at B=0 is also observed. These features
quantitatively agree with graphene weak-(anti)localization theory implying the
chiral electronic character of the samples. Scattering contributions from the
trapped charges in the substrate and from trigonal warping due to the graphite
layer on top are tentatively identified. The Shubnikov-de Haas oscillations are
remarkably small and show an anomalous Berry's phase.Comment: 5 pages, 4 figures. Minor change
Participatory plant breeding: a way to arrive at better-adapted onion varieties
The search for varieties that are better adapted to organic farming is a current topic in the organic sector. Breeding programmes specific for organic agriculture should solve this problem. Collaborating with organic farmers in such programmes, particularly in the selection process, can potentially result in varieties better adapted to their needs. Here, we assume that organic farmers' perceptive of plant health is broader than that of conventional breeders. Two organic onion farmers and one conventional onion breeder were monitored in their selection activities in 2004 and 2005 in order to verify whether and in which way this broader view on plant health contributes to improvement of organic varieties.
They made selections by positive mass selection in three segregating populations under organic conditions. The monitoring showed that the organic farmers selected in the field for earliness and downy mildew and after storage for bulb characteristics. The conventional breeder selected only after storage. Farmers and breeder applied identical selection directions for bulb traits as a round shape, better hardness and skin firmness. This resulted in smaller bulbs in the breeders’ populations, while the bulbs in the farmer populations were bigger than in the original population. In 2006 and 2007 the new onion populations will be compared with each other and the original populations to determine the selection response
Plasmon assisted transport through disordered array of quantum wires
Phononless plasmon assisted thermally activated transport through a long
disordered array of finite length quantum wires is investigated analytically.
Generically strong electron plasmon interaction in quantum wires results in a
qualitative change of the temperature dependence of thermally activated
resistance in comparison to phonon assisted transport. At high temperatures,
the thermally activated resistance is determined by the Luttinger liquid
interaction parameter of the wires.Comment: 7 pages, 1 figure, final version as publishe
Probing terahertz surface plasmon waves in graphene structures
Epitaxial graphene mesas and ribbons are investigated using terahertz (THz)
nearfield microscopy to probe surface plasmon excitation and THz transmission
properties on the sub-wavelength scale. The THz near-field images show
variation of graphene properties on a scale smaller than the wavelength, and
excitation of THz surface waves occurring at graphene edges, similar to that
observed at metallic edges. The Fresnel reflection at the substrate SiC/air
interface is also found to be altered by the presence of graphene ribbon
arrays, leading to either reduced or enhanced transmission of the THz wave
depending on the wave polarization and the ribbon width.Comment: accepted for publication in Applied Physics Lette
Half integer quantum Hall effect in high mobility single layer epitaxial graphene
The quantum Hall effect, with a Berry's phase of is demonstrated here
on a single graphene layer grown on the C-face of 4H silicon carbide. The
mobility is 20,000 cm/Vs at 4 K and ~15,000 cm/Vs
at 300 K despite contamination and substrate steps. This is comparable to the
best exfoliated graphene flakes on SiO and an order of magnitude larger
than Si-face epitaxial graphene monolayers. These and other properties indicate
that C-face epitaxial graphene is a viable platform for graphene-based
electronics.Comment: Some modifications in the text and figures, 7 pages, 2 figure
Bilayer graphene inclusions in rotational-stacked multilayer epitaxial graphene
Additional component in multi-layer epitaxial graphene grown on the
C-terminated surface of SiC, which exhibits the characteristic electronic
properties of a AB-stacked graphene bilayer, is identified in magneto-optical
response of this material. We show that these inclusions represent a
well-defined platform for accurate magneto-spectroscopy of unperturbed graphene
bilayers.Comment: 5 pages, 2 figures, to appear in Phys. Rev.
Evidence for Interlayer Electronic Coupling in Multilayer Epitaxial Graphene from Polarization Dependent Coherently Controlled Photocurrent Generation
Most experimental studies to date of multilayer epitaxial graphene on C-face
SiC have indicated that the electronic states of different layers are decoupled
as a consequence of rotational stacking. We have measured the third order
nonlinear tensor in epitaxial graphene as a novel approach to probe interlayer
electronic coupling, by studying THz emission from coherently controlled
photocurrents as a function of the optical pump and THz beam polarizations. We
find that the polarization dependence of the coherently controlled THz emission
expected from perfectly uncoupled layers, i.e. a single graphene sheet, is not
observed. We hypothesize that the observed angular dependence arises from weak
coupling between the layers; a model calculation of the angular dependence
treating the multilayer structure as a stack of independent bilayers with
variable interlayer coupling qualitatively reproduces the polarization
dependence, providing evidence for coupling.Comment: submitted to Nano Letter
Microscopic correlation between chemical and electronic states in epitaxial graphene on SiC(000-1)
We present energy filtered electron emission spectromicroscopy with spatial
and wave-vector resolution on few layer epitaxial graphene on SiC$(000-1) grown
by furnace annealing. Low energy electron microscopy shows that more than 80%
of the sample is covered by 2-3 graphene layers. C1s spectromicroscopy provides
an independent measurement of the graphene thickness distribution map. The work
function, measured by photoelectron emission microscopy (PEEM), varies across
the surface from 4.34 to 4.50eV according to both the graphene thickness and
the graphene-SiC interface chemical state. At least two SiC surface chemical
states (i.e., two different SiC surface structures) are present at the
graphene/SiC interface. Charge transfer occurs at each graphene/SiC interface.
K-space PEEM gives 3D maps of the k_|| pi - pi* band dispersion in micron scale
regions show that the Dirac point shifts as a function of graphene thickness.
Novel Bragg diffraction of the Dirac cones via the superlattice formed by the
commensurately rotated graphene sheets is observed. The experiments underline
the importance of lateral and spectroscopic resolution on the scale of future
electronic devices in order to precisely characterize the transport properties
and band alignments
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