Deep electron traps in organometallic vapor phase grown AlGaAs

Journal ArticleDeep electron traps have been studied by means of deep level transient spectrosocopy in type nominally undoped and interntionally te doped Al Ga As epitaxial layers which were grown by vapor phase epitaxy from organometallic compounds (OMVPE). Three main deep electron levels are present in undoped material: a trap with an activation energy of 0.8 v, which is also foundin GaAs grown by conventional VPE, and two levels specific to OMVPE with activation energies of 0.32 and 0.38 eV, respectively. The concentration of the 0.8 V level is found to be independent of the aluminum content x, supporting the assumption that it is not related to substitutional oxygen. The other levels however, exhibit a very strong dependence of concentration on the composition, varying by four orders of magnitude in the range of in Te-doped samples, a level with an activation energy of 0.23 V has been identified, which is thought to be related to an IR emission found in photoluminescence in OMVPE as well as in liquid phase epitaxial material

Thermodynamic properties of Uranium-Bismuth alloys

Thermodynamic properties of uranium-bismuth alloys were determined by measuring the vapor pressure of bismuth in equilibrium with the condensed phase. Classical methods based on the rate of sublimation, rate of evaporation or rate of effusion could not be used since each method requires an accurate knowledge of the molecular weight of the vapor. The molecular weight of bismuth is not accurately known, but the presence of Bi and Bi2 species has been established.An optical absorption technique was used to determine the concentration of each species independently. Briefly, this method consists of measuring concentrations in a vapor by the quantity of light absorbed at certain characteristic frequencies by the species in the vapor. The amount of each species present, which is related to the pressure, was determined by measuring the diminution of intensity at 3067 and 2731 A. The amount of radiation absorbed was found to be dependent on the thickness of the vapor space and the concentration of the bismuth vapor.The thermodynamic activity of bismuth was measured at temperatures from 725 to 875[deg]C in the regions: U3Bi4 + UBi2, UBi2 + liquid and the one-phase, liquid region. In order to obtain measurable quantities in the regions UBi + U and UBi + U3Bi4 it was necessary to work at temperatures from 800 to 1000[deg]C. From a measure of the activity of bismuth, the activity of uranium was calculated for the entire system. The liquid uranium--bismuth alloys were found not to be regular solutions. The Henry's law parameter (3.49 x 10-3 at 1064[deg]K) was found to be valid for uranium concentrations of less than 2 mole % uranium. From a complete knowledge of the activities of uranium and bismuth in the system the partial molar quantities and integral molar quantities were calculated at five temperatures: 1018, 1041, 1064, 1089 and 1115[deg]K.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32379/1/0000454.pd

Influence of Impact Parameter on Thermal Description of Relativistic Heavy Ion Collisions at GSI/SIS

Attention is drawn to the role played by the size of the system in the thermodynamic analysis of particle yields in relativistic heavy ion collisions at SIS energies. This manifests itself in the non-linear dependence of K+ and K- yields in $AA$ collisions at 1 -- 2 A.GeV on the number of participants. It is shown that this dependence can be quantitatively well described in terms of a thermal model with a canonical strangeness conservation. The measured particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also explained in the context of a thermal model with a common freeze-out temperature and chemical potential. Including the concept of collective flow a consistent picture of particle energy distributions is derived with the flow velocity being strongly impact-parameter dependent.Comment: revtex, 20 figure

Hyperon production in Ar+KCl collisions at 1.76A GeV

We present transverse momentum spectra, rapidity distribution and multiplicity of Lambda-hyperons measured with the HADES spectrometer in the reaction Ar(1.76A GeV)+KCl. The yield of Xi- is calculated from our previously reported Xi-/(Lambda+Sigma0) ratio and compared to other strange particle multiplicities. Employing a strangeness balance equation the multiplicities of the yet unmeasured charged Sigma hyperons can be estimated. Finally a statistical hadronization model is used to fit the yields of pi-, K+, K0s, K-, phi, Lambda and Xi-. The resulting chemical freeze-out temperature of T=(76+-2) MeV is compared to the measured slope parameters obtained from fits to the transverse mass distributions of the particles

Global Search for New Physics with 2.0/fb at CDF

Data collected in Run II of the Fermilab Tevatron are searched for indications of new electroweak-scale physics. Rather than focusing on particular new physics scenarios, CDF data are analyzed for discrepancies with the standard model prediction. A model-independent approach (Vista) considers gross features of the data, and is sensitive to new large cross-section physics. Further sensitivity to new physics is provided by two additional algorithms: a Bump Hunter searches invariant mass distributions for "bumps" that could indicate resonant production of new particles; and the Sleuth procedure scans for data excesses at large summed transverse momentum. This combined global search for new physics in 2.0/fb of ppbar collisions at sqrt(s)=1.96 TeV reveals no indication of physics beyond the standard model.Comment: 8 pages, 7 figures. Final version which appeared in Physical Review D Rapid Communication

Observation of Orbitally Excited B_s Mesons

We report the first observation of two narrow resonances consistent with states of orbitally excited (L=1) B_s mesons using 1 fb^{-1} of ppbar collisions at sqrt{s} = 1.96 TeV collected with the CDF II detector at the Fermilab Tevatron. We use two-body decays into K^- and B^+ mesons reconstructed as B^+ \to J/\psi K^+, J/\psi \to \mu^+ \mu^- or B^+ \to \bar{D}^0 \pi^+, \bar{D}^0 \to K^+ \pi^-. We deduce the masses of the two states to be m(B_{s1}) = 5829.4 +- 0.7 MeV/c^2 and m(B_{s2}^*) = 5839.7 +- 0.7 MeV/c^2.Comment: Version accepted and published by Phys. Rev. Let

Anthropogenic activities in the páramo trigger ecological shifts in Tropical Andean lakes

A robust understanding of the impact of anthropogenic activities on high-altitude tropical aquatic ecosystems is key for the conservation and protection of the Tropical Andean biodiversity hot spot. We present the results of a multiproxy study of lake sediments from the high Andean páramo of El Cajas National Park, a UNESCO biosphere reserve in Ecuador. The main site, Laguna Pallcacocha, is well known for recording El Niño-driven clastic flood layers that are triggered by high-intensity rainfall anomalies from the eastern Pacific. The second site, Laguna El Ocho, does not contain clastic laminations, providing a control. The records show abrupt shifts in diatom assemblages ca. AD 1991 in both high-elevation Andean lakes accompanied by local changes in páramo composition that suggest a sudden nutrient enrichment of the environment. The diatom assemblages from Laguna Pallcacocha, in relation to the clastic input events, are remarkably stable and do not show evident El Niño signals at the analysed resolution. Based on comparison with the nonlaminated El Ocho record, we deduce the main source of this nutrient enrichment to be the construction of a heavily transited road that runs through the park, while climate warming played secondary role by amplifying its effects

Forward pi^0 Production and Associated Transverse Energy Flow in Deep-Inelastic Scattering at HERA

Deep-inelastic positron-proton interactions at low values of Bjorken-x down to x \approx 4.10^-5 which give rise to high transverse momentum pi^0 mesons are studied with the H1 experiment at HERA. The inclusive cross section for pi^0 mesons produced at small angles with respect to the proton remnant (the forward region) is presented as a function of the transverse momentum and energy of the pi^0 and of the four-momentum transfer Q^2 and Bjorken-x. Measurements are also presented of the transverse energy flow in events containing a forward pi^0 meson. Hadronic final state calculations based on QCD models implementing different parton evolution schemes are confronted with the data.Comment: 27 pages, 8 figures and 3 table