554 research outputs found
Experimentally accessible signatures of Auger scattering in graphene
The gapless and linear electronic band structure of graphene opens up Auger scattering channels bridging the valence and the conduction band and changing the charge carrier density. Here, we reveal experimentally accessible signatures of Auger scattering in optically excited graphene. To be able to focus on signatures of Auger scattering, we apply a low excitation energy, weak pump fluences, and a cryostatic temperature, so that all relevant processes lie energetically below the optical phonon threshold. In this regime, carrier-phonon scattering is strongly suppressed and Coulomb processes govern the carrier dynamics. Depending on the excitation regime, we find an accumulation or depletion of the carrier occupation close to the Dirac point. This reflects well the behavior predicted from Auger-dominated carrier dynamics. Based on this observation, we propose a multicolor pump-probe experiment to uncover the extreme importance of Auger channels for the nonequilibrium dynamics in graphene
Optische und elektronische Eigenschaften von AlGaN/GaN-Heterostrukturen
The electronic material properties of AlGaN/GaN heterostructures were
investigated. The analysis of optical spectra by complex models allowed for
the first time to confirm the theoretically predicted dependence of the
polarisation discontinuity (also called polarisation charge) on the Al
content by reliable experiments. Furthermore, it is shown that the
polarisation discontinuity is constant over the temperature range from 5 K
up to room temperature.
The method employed here is based on the analysis of electroreflectance
(ER) spectra and exploits the specific dependence of the electric field
strength within a layer on the applied electric voltage. In this work this
method is consequently refined to surpass all alternative methods in
accuracy.
ER spectra of group-III-nitrides posses some general peculiarities: (i) In
direct proximity to the band gap they can not be described by constant
Seraphin coefficients in contrast to small gap semiconductors (e.g. GaAs).
(ii) Although, the analysis of the Franz-Keldysh oscillations by Aspnesâ
method yields the correct values of the electric field strength as for
small gap semiconductors.
Optical and especially ER spectra of group-III-nitrides can only be
described completely by taking into account for excitons in electric
fields. For this a model proposed by Blossey was applied to nitride
semiconductors and implemented into a software program. The approach
presented is unique since it allows for a quantitative description of
excitons in inhomogeneous electric fields. The good agreement between
experiment and simulation supports the reliability of the material
properties presented in this work. Furthermore, it was found that the
energetic position of the exciton main resonance as well as its spectral
width depend linearly on the electric field strength.
The fuction of AlGaN/GaN heterostructures as chemical sensors was
investigated too. If Pt contacted samples were exposed to hydrogen the
density of their two dimensional electron gases were raised by some 10^{12}
e/cm^2 while the Schottky barrier heights were lowered by up to 0.85 V.
Wetting of the free surface of not contacted samples by a polar liquid
(acetone) led to an increase of the surface potential by 30 mV and a
decrease of the net surface charge density by 10^{11} e/cm^2.Die vorliegende Arbeit behandelt die elektronischen Materialeigenschaften
von AlGaN/GaN-Heterostrukturen. Die Auswertung optischer Spektren mit
komplexen Modellen ermöglichte erstmals die BestÀtigung des theoretisch
vorausgesagten Verlaufes der PolarisationsdiskontinuitÀt (auch
Polarisationsladung genannt) in AbhÀngigkeit vom Al-Gehalt durch
zuverlÀssige Experimente. Weiterhin wurde festgestellt, dass die
PolarisationsdiskontinuitÀt im Bereich von 5 K bis Raumtemperatur konstant
ist.Das hier verwendete Verfahren basiert auf der Auswertung von
Elektroreflexions- (ER-)spektren und nutzt die spezifische AbhÀngigkeit der
SchichtfeldstÀrke von der angelegten elektrischen Spannung. In dieser
Arbeit wurde das Verfahren konsequent weiterentwickelt und ĂŒbertrifft so
alle alternativen Methoden in der Genauigkeit.
ER-Spektren von Gruppe-III-Nitriden weiĂen generelle Besonderheiten auf. In
unmittelbarer NÀhe zur Bandkante können ER-Spektren im Gegensatz zu
schmallĂŒckigen Halbleitern (z.B. GaAs) nicht durch konstante
Seraphinkoeffizienten beschrieben werden. Jedoch ergibt die Analyse der
Franz-Keldysh-Oszillationen nach der Aspnesâschen Methode wie bei
schmallĂŒckigen Halbleitern die korrekten FeldstĂ€rkebetrĂ€ge.
Optische und insbesondere ER-Spektren von Gruppe-III-Nitridschichten lassen
sich nur vollstĂ€ndig durch BerĂŒcksichtigung der Exzitonen im elektrischen
Feld beschreiben. Dazu wurde in dieser Arbeit ein von Blossey
vorgeschlagenes Modell auf die Nitride angewandt und in einer Software
umgesetzt. Der dargestellte Ansatz zur Spektrensimulation ist dadurch
einzigartig, dass man mit ihm Exzitonen in inhomogenen elektrischen Feldern
quantitativ beschreiben kann. Die gute Ăbereinstimmung von berechneten und
experimentellen Spektren bekrÀftigt die ZuverlÀssigkeit der in dieser
Arbeit bestimmten MaterialgröĂen. Weiterhin wurde festgestellt, dass die
energetische Position der Exzitonenhauptresonanz und deren spektrale Breite
nÀherungsweise einer linearen AbhÀngigkeit von der elektrischen FeldstÀrke
folgen.
Die Wirkungsweise von AlGaN/GaN-Heterostrukturen als chemische Sensoren
wurde ebenfalls untersucht. Werden Pt-kontaktierten Proben Wasserstoff
ausgesetzt, erhöht sich die Dichte des zweidimensionalen Elektronengases um
einige 10^{12} e/cm^2 und die Schottkybarriere verringert sich um bis zu
0,85 V. Bei Proben mit unkontaktierter OberflĂ€che fĂŒhrt die Benetzung mit
einer polaren FlĂŒssigkeit (Azeton) zu einer Potenzialerhöhung um 30 mV und
zu einer Verringerung der OberflÀchennettoladung um 10^{11} e/cm^2
Numerical Investigation of a Mesoscopic Vehicular Traffic Flow Model Based on a Stochastic Acceleration Process
In this paper a spatial homogeneous vehicular traffic flow model based on a
stochastic master equation of Boltzmann type in the acceleration variable is
solved numerically for a special driver interaction model. The solution is done
by a modified direct simulation Monte Carlo method (DSMC) well known in non
equilibrium gas kinetic. The velocity and acceleration distribution functions
in stochastic equilibrium, mean velocity, traffic density, ACN, velocity
scattering and correlations between some of these variables and their car
density dependences are discussed.Comment: 23 pages, 10 figure
Anisotropic thermal expansion and magnetostriction of YNiBC single crystals
We present results of anisotropic thermal expansion and low temperature
magnetostriction measurements on YNiBC single crystals grown by high
temperature flux and floating zone techniques. Quantum oscillations of
magnetostriction were observed at low temperatures for starting at
fields significantly below (). Large irreversible,
longitudinal magnetostriction was seen in both, in-plane and along the c-axis,
directions of the applied magnetic field in the intermediate superconducting
state. Anisotropic uniaxial pressure dependencies of were evaluated using
results of zero field, thermal expansion measurements
A Vehicular Traffic Flow Model Based on a Stochastic Acceleration Process
A new vehicular traffic flow model based on a stochastic jump process in
vehicle acceleration and braking is introduced. It is based on a master
equation for the single car probability density in space, velocity and
acceleration with an additional vehicular chaos assumption and is derived via a
Markovian ansatz for car pairs. This equation is analyzed using simple driver
interaction models in the spatial homogeneous case. Velocity distributions in
stochastic equilibrium, together with the car density dependence of their
moments, i.e. mean velocity and scattering and the fundamental diagram are
presented.Comment: 27 pages, 6 figure
Torque magnetometry study of metamagnetic transitions in single-crystal HoNi2B2C at T\approx 1.9 K
Metamagnetic transitions in single-crystal rare-earth nickel borocarbide
HoNiBC have been studied at T\approx 1.9 K with a Quantum Design torque
magnetometer. This compound is highly anisotropic with a variety of
metamagnetic states at low temperature which includes antiferromagnetic,
ferrimagnetic, non-collinear and ferromagnetic-like (saturated paramagnet)
states. The critical fields of the transitions depend crucially on the angle
between applied field and the easy axis [110]. Measurements of torque
along the c-axis have been made while changing the angular direction of the
magnetic field (parallel to basal tetragonal -planes) and with changing
field at fixed angle over a wide angular range. Two new phase boundaries in the
region of the non-collinear phase have been observed, and the direction of the
magnetization in this phase has been precisely determined. At low field the
antiferromagnetic phase is observed to be multidomain. In the angular range
very close to the hard axis [100] (, where is the angle between field and the hard axis) the
magnetic behavior is found to be ``frustrated'' with a mixture of phases with
different directions of the magnetization.Comment: submitted to Phys. Rev. B, 12 pages, 12 figure
Hot Carrier Transport and Photocurrent Response in Graphene
Strong electron-electron interactions in graphene are expected to result in
multiple-excitation generation by the absorption of a single photon. We show
that the impact of carrier multiplication on photocurrent response is enhanced
by very inefficient electron cooling, resulting in an abundance of hot
carriers. The hot-carrier-mediated energy transport dominates the photoresponse
and manifests itself in quantum efficiencies that can exceed unity, as well as
in a characteristic dependence of the photocurrent on gate voltages. The
pattern of multiple photocurrent sign changes as a function of gate voltage
provides a fingerprint of hot-carrier-dominated transport and carrier
multiplication.Comment: 4 pgs, 2 fg
A genetic assay for gene essentiality in Clostridium
Essential genes of pathogens are potential therapeutic targets, but are difficult to verify. Here, gene essentiality was determined by targeted knockout following engineered gene duplication. Null mutants of candidate essential genes of Clostridium difficile were viable only in the presence of a stable second copy of the gene
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