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 Aspnes’ 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 Bestätigung des theoretisch vorausgesagten Verlaufes der Polarisationsdiskontinuität (auch Polarisationsladung genannt) in Abhängigkeit vom Al-Gehalt durch zuverlässige Experimente. Weiterhin wurde festgestellt, dass die Polarisationsdiskontinuität 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 Abhängigkeit der Schichtfeldstärke von der angelegten elektrischen Spannung. In dieser Arbeit wurde das Verfahren konsequent weiterentwickelt und übertrifft so alle alternativen Methoden in der Genauigkeit. ER-Spektren von Gruppe-III-Nitriden weißen generelle Besonderheiten auf. In unmittelbarer Nähe zur Bandkante können ER-Spektren im Gegensatz zu schmallückigen Halbleitern (z.B. GaAs) nicht durch konstante Seraphinkoeffizienten beschrieben werden. Jedoch ergibt die Analyse der Franz-Keldysh-Oszillationen nach der Aspnes’schen Methode wie bei schmallückigen Halbleitern die korrekten Feldstärkebeträge. Optische und insbesondere ER-Spektren von Gruppe-III-Nitridschichten lassen sich nur vollständig durch Berücksichtigung 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 Übereinstimmung von berechneten und experimentellen Spektren bekräftigt die Zuverlässigkeit der in dieser Arbeit bestimmten Materialgrößen. Weiterhin wurde festgestellt, dass die energetische Position der Exzitonenhauptresonanz und deren spektrale Breite näherungsweise einer linearen Abhängigkeit von der elektrischen Feldstärke 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 Oberfläche führt die Benetzung mit einer polaren Flüssigkeit (Azeton) zu einer Potenzialerhöhung um 30 mV und zu einer Verringerung der Oberflächennettoladung um 10^{11} e/cm^2

Anisotropic thermal expansion and magnetostriction of YNi2_2B2_2C single crystals

We present results of anisotropic thermal expansion and low temperature magnetostriction measurements on YNi$_2$B$_2$C single crystals grown by high temperature flux and floating zone techniques. Quantum oscillations of magnetostriction were observed at low temperatures for $H \| c$ starting at fields significantly below $H_{c2}$ ($H < 0.7 H_{c2}$). 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 $T_c$ were evaluated using results of zero field, thermal expansion measurements

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 HoNi$_2$B$_2$C 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 $\theta$ 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 $ab$-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] ($-6^{\circ} \lesssim\phi \lesssim 6^{\circ}$, where $\phi$ 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

Slow Noncollinear Coulomb Scattering in the Vicinity of the Dirac Point in Graphene

The Coulomb scattering dynamics in graphene in energetic proximity to the Dirac point is investigated by polarization resolved pump-probe spectroscopy and microscopic theory. Collinear Coulomb scattering rapidly thermalizes the carrier distribution in k directions pointing radially away from the Dirac point. Our study reveals, however, that, in almost intrinsic graphene, full thermalization in all directions relying on noncollinear scattering is much slower. For low photon energies, carrier-optical-phonon processes are strongly suppressed and Coulomb mediated noncollinear scattering is remarkably slow, namely on a ps time scale. This effect is very promising for infrared and THz devices based on hot carrier effects

Mutant generation by allelic exchange and genome resequencing of the biobutanol organism Clostridium acetobutylicum ATCC 824

BackgroundClostridium acetobutylicum represents a paradigm chassis for the industrial production of the biofuel biobutanol and a focus for metabolic engineering. We have previously developed procedures for the creation of in-frame, marker-less deletion mutants in the pathogen Clostridium difficile based on the use of pyrE and codA genes as counter selection markers. In the current study we sought to test their suitability for use in C. acetobutylicum.ResultsBoth systems readily allowed the isolation of in-frame deletions of the C. acetobutylicum ATCC 824 spo0A and the cac824I genes, leading to a sporulation minus phenotype and improved transformation, respectively. The pyrE-based system was additionally used to inactivate a putative glycogen synthase (CA_C2239, glgA) and the pSOL1 amylase gene (CA_P0168, amyP), leading to lack of production of granulose and amylase, respectively. Their isolation provided the opportunity to make use of one of the key pyrE system advantages, the ability to rapidly complement mutations at appropriate gene dosages in the genome. In both cases, their phenotypes were restored in terms of production of granulose (glgA) and amylase (amyP). Genome re-sequencing of the ATCC 824 COSMIC consortium laboratory strain used revealed the presence of 177 SNVs and 49 Indels, including a 4916-bp deletion in the pSOL1 megaplasmid. A total of 175 SNVs and 48 Indels were subsequently shown to be present in an 824 strain re-acquired (Nov 2011) from the ATCC and are, therefore, most likely errors in the published genome sequence, NC_003030 (chromosome) and NC_001988 (pSOL1).ConclusionsThe codA or pyrE counter selection markers appear equally effective in isolating deletion mutants, but there is considerable merit in using a pyrE mutant as the host as, through the use of ACE (Allele-Coupled Exchange) vectors, mutants created (by whatever means) can be rapidly complemented concomitant with restoration of the pyrE allele. This avoids the phenotypic effects frequently observed with high copy number plasmids and dispenses with the need to add antibiotic to ensure plasmid retention. Our study also revealed a surprising number of errors in the ATCC 824 genome sequence, while at the same time emphasising the need to re-sequence commonly used laboratory strains