5,777 research outputs found
Flashing annihilation term of a logistic kinetic as a mechanism leading to Pareto distributions
It is shown analytically that the flashing annihilation term of a Verhulst
kinetic leads to the power--law distribution in the stationary state. For the
frequency of switching slower than twice the free growth rate this provides the
quasideterministic source of a Levy noises at the macroscopic level.Comment: 1 fi
Direct evidence for efficient ultrafast charge separation in epitaxial WS<sub>2</sub>/graphene heterostructures
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene
Complex networks: new trends for the analysis of brain connectivity
Today, the human brain can be studied as a whole. Electroencephalography,
magnetoencephalography, or functional magnetic resonance imaging techniques
provide functional connectivity patterns between different brain areas, and
during different pathological and cognitive neuro-dynamical states. In this
Tutorial we review novel complex networks approaches to unveil how brain
networks can efficiently manage local processing and global integration for the
transfer of information, while being at the same time capable of adapting to
satisfy changing neural demands.Comment: Tutorial paper to appear in the Int. J. Bif. Chao
Measurement of the production of a W boson in association with a charm quark in pp collisions at √s = 7 TeV with the ATLAS detector
The production of a W boson in association with a single charm quark is studied using 4.6 fb−1 of pp collision data at s√ = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96+0.26−0.30 at Q 2 = 1.9 GeV2, which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio σ(W + +c¯¯)/σ(W − + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s−s¯¯¯ quark asymmetry
Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene
The pseudospin of Dirac electrons in graphene manifests itself in a peculiar
momentum anisotropy for photo-excited electron-hole pairs. These interband
excitations are in fact forbidden along the direction of the light
polarization, and are maximum perpendicular to it. Here, we use time- and
angle-resolved photoemission spectroscopy to investigate the resulting
unconventional hot carrier dynamics, sampling carrier distributions as a
function of energy and in-plane momentum. We first show that the
rapidly-established quasi-thermal electron distribution initially exhibits an
azimuth-dependent temperature, consistent with relaxation through collinear
electron-electron scattering. Azimuthal thermalization is found to occur only
at longer time delays, at a rate that depends on the substrate and the static
doping level. Further, we observe pronounced differences in the electron and
hole dynamics in n-doped samples. By simulating the Coulomb- and
phonon-mediated carrier dynamics we are able to disentangle the influence of
excitation fluence, screening, and doping, and develop a microscopic picture of
the carrier dynamics in photo-excited graphene. Our results clarify new aspects
of hot carrier dynamics that are unique to Dirac materials, with relevance for
photo-control experiments and optoelectronic device applications.Comment: 23 pages, 12 figure
Tracking primary thermalization events in graphene with photoemission at extreme timescales
Direct and inverse Auger scattering are amongst the primary processes that
mediate the thermalization of hot carriers in semiconductors. These two
processes involve the annihilation or generation of an electron-hole pair by
exchanging energy with a third carrier, which is either accelerated or
decelerated. Inverse Auger scattering is generally suppressed, as the
decelerated carriers must have excess energies higher than the band gap itself.
In graphene, which is gapless, inverse Auger scattering is instead predicted to
be dominant at the earliest time delays. Here, femtosecond
extreme-ultraviolet pulses are used to detect this imbalance, tracking both the
number of excited electrons and their kinetic energy with time- and
angle-resolved photoemission spectroscopy. Over a time window of approximately
25 fs after absorption of the pump pulse, we observe an increase in conduction
band carrier density and a simultaneous decrease of the average carrier kinetic
energy, revealing that relaxation is in fact dominated by inverse Auger
scattering. Measurements of carrier scattering at extreme timescales by
photoemission will serve as a guide to ultrafast control of electronic
properties in solids for PetaHertz electronics.Comment: 16 pages, 8 figure
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