483 research outputs found
Photoreceptor spectral tuning by colorful, multilayered facet lenses in long-legged fly eyes (Dolichopodidae)
The facet lenses of the compound eyes of long-legged flies (Dolichopodidae) feature a striking, interlaced coloration pattern, existing of alternating rows of green-yellow and orange-red reflecting facets, due to dielectric multilayers located distally in the facet lenses (Bernard and Miller. Invest Ophthalmol 7:416-434 (1968). We investigated this phenomenon in the dolichopodid Dolichopus nitidus by applying microspectrophotometry, electron microscopy and optical modeling. The measured narrow-band reflectance spectra, peaking at similar to 540 and similar to 590 nm with bandwidth similar to 105 nm, are well explained by a refractive index oscillating sinusoidally in six periods around a mean value of about 1.44 with amplitude 0.6. The facet lens reflectance spectra are associated with a spectrally restricted, reduced transmittance, which causes modified spectral sensitivities of the underlying photoreceptors. Based on the modeling and electroretinography of the dolichopodid Condylostylus japonicus we conjecture that the green and orange facets narrow the spectral bandwidths of blue and green central photoreceptors, respectively, thus possibly improving color and/or polarization vision.</p
Soil toxicities as causes of sugarcane leaf freckle, macadamia leaf chlorosis (Keaau), and Maui sugarcane growth failure
Symmetric Kondo Lattice States in Doped Strained Twisted Bilayer Graphene
We use the topological heavy fermion (THF) model and its Kondo Lattice (KL)
formulation to study the symmetric Kondo state in twisted bilayer graphene. Via
a large-N approximation, we find a symmetric Kondo (SK) state in KL mode at
fillings . In the SK state, all symmetries are preserved and
the local moments are Kondo screened by the conduction electrons. At the
mean-field level of the THF model at , we also find
a similar symmetric state. We study the stability of the symmetric state by
comparing its energy with the ordered states and find the ordered states to
have lower energy. However, moving away from integer fillings by doping holes
to the light bands, we find the energy difference is reduced, which suggests
the loss of ordering and a tendency towards Kondo screening. In order to
include many-body effects beyond the mean-field approximation, we perform
dynamical mean-field theory (DMFT) calculations on the THF model. We find the
spin susceptibility follows a Curie behavior at down to
where the onset of screening of the local moment becomes
visible. This hints to very low Kondo temperatures at these fillings, in
agreement with the outcome of our mean-field calculations. At non-integer
filling DMFT shows deviations from a
-susceptibility at much higher temperatures, suggesting a more effective
screening of local moments with doping. Finally, we study the effect of a
-rotational-symmetry-breaking strain via mean-field approaches and find
that a symmetric phase (that only breaks symmetry) can be stabilized
at sufficiently large strain at . Our results suggest that
a symmetric Kondo phase is strongly suppressed at integer fillings, but could
be stabilized either at non-integer fillings or by applying strain.Comment: 40 pages, 15 figure
Electron waves in chemically substituted graphene
We present exact analytical and numerical results for the electronic spectra
and the Friedel oscillations around a substitutional impurity atom in a
graphene lattice. A chemical dopant in graphene introduces changes in the
on-site potential as well as in the hopping amplitude. We employ a T-matrix
formalism and find that disorder in the hopping introduces additional
interference terms around the impurity that can be understood in terms of
bound, semi-bound, and unbound processes for the Dirac electrons. These
interference effects can be detected by scanning tunneling microscopy.Comment: 4 pages, 7 figure
Atomic Hole Doping of Graphene
Graphene is an excellent candidate for the next generation of electronic
materials due to the strict two-dimensionality of its electronic structure as
well as the extremely high carrier mobility. A prerequisite for the development
of graphene based electronics is the reliable control of the type and density
of the charge carriers by external (gate) and internal (doping) means. While
gating has been successfully demonstrated for graphene flakes and epitaxial
graphene on silicon carbide, the development of reliable chemical doping
methods turns out to be a real challenge. In particular hole doping is an
unsolved issue. So far it has only been achieved with reactive molecular
adsorbates, which are largely incompatible with any device technology. Here we
show by angle-resolved photoemission spectroscopy that atomic doping of an
epitaxial graphene layer on a silicon carbide substrate with bismuth, antimony
or gold presents effective means of p-type doping. Not only is the atomic
doping the method of choice for the internal control of the carrier density. In
combination with the intrinsic n-type character of epitaxial graphene on SiC,
the charge carriers can be tuned from electrons to holes, without affecting the
conical band structure
Electrical and thermoelectrical transport in Dirac fermions through a quantum dot
We investigate the conductance and thermopower of massless Dirac fermions
through a quantum dot using a pseudogap Anderson model in the non-crossing
approximation. When the Fermi level is at the Dirac point, the conductance has
a cusp where the thermopower changes its sign. When the Fermi level is away
from the Dirac point, the Kondo temperature illustrates a quantum impurity
transition between an asymmetric strong coupling Kondo state and a localized
moment state. The conductance shows a peak near this transition and reaches the
unitary limit at low temperatures. The magnitude of the thermopower exceeds
, and the thermoelectric figure of merit exceeds unity.Comment: 5 pages, 4 figure
Room-temperature ferromagnetism in graphite driven by 2D networks of point defects
Ferromagnetism in carbon-based materials is appealing for both applications
and fundamental science purposes because carbon is a light and bio-compatible
material that contains only s and p electrons in contrast to traditional
ferromagnets based on 3d or 4f electrons. Here we demonstrate direct evidence
for ferromagnetic order locally at defect structures in highly oriented
pyrolytic graphite (HOPG) with magnetic force microscopy and in bulk
magnetization measurements at room temperature. Magnetic impurities have been
excluded as the origin of the magnetic signal after careful analysis supporting
an intrinsic magnetic behavior of carbon. The observed ferromagnetism has been
attributed to originate from unpaired electron spins localized at grain
boundaries of HOPG. Grain boundaries form two-dimensional arrays of point
defects, where their spacing depends on the mutual orientation of two grains.
Depending on the distance between these point defects, scanning tunneling
spectroscopy of grain boundaries showed two intense split localized states for
small distances between defects (< 4 nm) and one localized state at the Fermi
level for large distances between defects (> 4 nm).Comment: 19 pages, 5 figure
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