68 research outputs found
Edge effects in graphene nanostructures: II. Semiclassical theory of spectral fluctuations and quantum transport
We investigate the effect of different edge types on the statistical
properties of both the energy spectrum of closed graphene billiards and the
conductance of open graphene cavities in the semiclassical limit. To this end,
we use the semiclassical Green's function for ballistic graphene flakes that we
have derived in Reference 1. First we study the spectral two point correlation
function, or more precisely its Fourier transform the spectral form factor,
starting from the graphene version of Gutzwiller's trace formula for the
oscillating part of the density of states. We calculate the two leading order
contributions to the spectral form factor, paying particular attention to the
influence of the edge characteristics of the system. Then we consider transport
properties of open graphene cavities. We derive generic analytical expressions
for the classical conductance, the weak localization correction, the size of
the universal conductance fluctuations and the shot noise power of a ballistic
graphene cavity. Again we focus on the effects of the edge structure. For both,
the conductance and the spectral form factor, we find that edge induced
pseudospin interference affects the results significantly. In particular
intervalley coupling mediated through scattering from armchair edges is the key
mechanism that governs the coherent quantum interference effects in ballistic
graphene cavities
Detecting the Upturn of the Solar B Neutrino Spectrum with LENA
LENA (Low Energy Neutrino Astronomy) has been proposed as a next generation
50 kt liquid scintillator detector. The large target mass allows a high
precision measurement of the solar B neutrino spectrum, with an
unprecedented energy threshold of 2 MeV. Hence, it can probe the MSW-LMA
prediction for the electron neutrino survival probability in the transition
region between vacuum and matter-dominated neutrino oscillations. Based on
Monte Carlo simulations of the solar neutrino and the corresponding background
spectra, it was found that the predicted upturn of the solar B neutrino
spectrum can be detected with 5 sigma significance after 5 y
Edge effects in graphene nanostructures: I. From multiple reflection expansion to density of states
We study the influence of different edge types on the electronic density of
states of graphene nanostructures. To this end we develop an exact expansion
for the single particle Green's function of ballistic graphene structures in
terms of multiple reflections from the system boundary, that allows for a
natural treatment of edge effects. We first apply this formalism to calculate
the average density of states of graphene billiards. While the leading term in
the corresponding Weyl expansion is proportional to the billiard area, we find
that the contribution that usually scales with the total length of the system
boundary differs significantly from what one finds in semiconductor-based,
Schr\"odinger type billiards: The latter term vanishes for armchair and
infinite mass edges and is proportional to the zigzag edge length, highlighting
the prominent role of zigzag edges in graphene. We then compute analytical
expressions for the density of states oscillations and energy levels within a
trajectory based semiclassical approach. We derive a Dirac version of
Gutzwiller's trace formula for classically chaotic graphene billiards and
further obtain semiclassical trace formulae for the density of states
oscillations in regular graphene cavities. We find that edge dependent
interference of pseudospins in graphene crucially affects the quantum spectrum.Comment: to be published in Phys. Rev.
Symmetry Classes in Graphene Quantum Dots: Universal Spectral Statistics, Weak Localization, and Conductance Fluctuations
We study the symmetry classes of graphene quantum dots, both open and closed,
through the conductance and energy level statistics. For abrupt termination of
the lattice, these properties are well described by the standard orthogonal and
unitary ensembles. However, for smooth mass confinement, special time-reversal
symmetries associated with the sublattice and valley degrees of freedom are
critical: they lead to block diagonal Hamiltonians and scattering matrices with
blocks belonging to the unitary symmetry class even at zero magnetic field.
While the effect of this structure is clearly seen in the conductance of open
dots, it is suppressed in the spectral statistics of closed dots, because the
intervalley scattering time is shorter than the time required to resolve a
level spacing in the closed systems but longer than the escape time of the open
systems.Comment: 4 pages, 4 figures, RevTex, submitted to Phys. Rev. Let
Search for modulations of the solar Be-7 flux in the next-generation neutrino observatory LENA
A next-generation liquid-scintillator detector will be able to perform
high-statistics measurements of the solar neutrino flux. In LENA, solar Be-7
neutrinos are expected to cause 1.7x10^4 electron recoil events per day in a
fiducial volume of 35 kilotons. Based on this signal, a search for periodic
modulations on sub-percent level can be conducted, surpassing the sensitivity
of current detectors by at least a factor of 20. The range of accessible
periods reaches from several minutes, corresponding to modulations induced by
helioseismic g-modes, to tens of years, allowing to study long-term changes in
solar fusion rates.Comment: 15 pages, 9 figure
SchussenAktivplus: reduction of micropollutants and of potentially pathogenic bacteria for further water quality improvement of the river Schussen, a tributary of Lake Constance, Germany
The project focuses on the efficiency of combined technologies to reduce the release of micropollutants and bacteria into surface waters via sewage treatment plants of different size and via stormwater overflow basins of different types. As a model river in a highly populated catchment area, the river Schussen and, as a control, the river Argen, two tributaries of Lake Constance, Southern Germany, are under investigation in this project. The efficiency of the different cleaning technologies is monitored by a wide range of exposure and effect analyses including chemical and microbiological techniques as well as effect studies ranging from molecules to communities
Search for direct photons from S - Au collisions at 200 GeV/u
The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1 < y <y2.65 and 0.4 GeV/c < p^ < 2.0p20 GeV/c the yield and p^p-dependence of the observed photons are well reproduced by hadron decays. Furthermore, their production rate is found to be proportional to the charged particle density. The systematic errors comparing the measured and expected photon yield result in an upper limit of 14% for the emission of direct photons in central S-Au collisions. For a photon source with a yield depending quadratically on the charged particle density the limit can be reduced to 7%
A doublet of 3" cylindrical silicon drift detectors in the CERES/NA45 experiment
We report on the performance of a doublet of 3" cylindrical silicon drift detectors installed as an upgrade of the CERES/NA45 electron pair spectrometer for the Pb-beam at the CERN SPS. The silicon detectors provide external particle tracking and background rejection of conversions and close Dalitz pairs. Results on vertex reconstruction and rejection from Pb test-run in 1994 are presented
Low mass dilepton production at the SPS: probing hot and dense nuclear matter
CERES and HELIOS-3 have detected a significant enhancement of low--mass dileptons in nuclear collisions at 200 GeV/nucleon with respect to the expected ``conventional'' sources. The onset of the excess, starting at a mass of , and the possibility of a quadratic dependence on the event multiplicity suggest the opening of the annihilation channel. This would be the first observation of thermal radiation from dense hadronic matter. Possible interpretations of these results are presented, including the reduction of the mass due to partial restoration of chiral symmetry in the dense fireball formed in the collision
The genomes of two key bumblebee species with primitive eusocial organization
Background: The shift from solitary to social behavior is one of the major evolutionary transitions. Primitively eusocial bumblebees are uniquely placed to illuminate the evolution of highly eusocial insect societies. Bumblebees are also invaluable natural and agricultural pollinators, and there is widespread concern over recent population declines in some species. High-quality genomic data will inform key aspects of bumblebee biology, including susceptibility to implicated population viability threats. Results: We report the high quality draft genome sequences of Bombus terrestris and Bombus impatiens, two ecologically dominant bumblebees and widely utilized study species. Comparing these new genomes to those of the highly eusocial honeybee Apis mellifera and other Hymenoptera, we identify deeply conserved similarities, as well as novelties key to the biology of these organisms. Some honeybee genome features thought to underpin advanced eusociality are also present in bumblebees, indicating an earlier evolution in the bee lineage. Xenobiotic detoxification and immune genes are similarly depauperate in bumblebees and honeybees, and multiple categories of genes linked to social organization, including development and behavior, show high conservation. Key differences identified include a bias in bumblebee chemoreception towards gustation from olfaction, and striking differences in microRNAs, potentially responsible for gene regulation underlying social and other traits. Conclusions: These two bumblebee genomes provide a foundation for post-genomic research on these key pollinators and insect societies. Overall, gene repertoires suggest that the route to advanced eusociality in bees was mediated by many small changes in many genes and processes, and not by notable expansion or depauperation
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