2,771 research outputs found

    Sum-over-states vs quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors; femtosecond analogues of multidimensional NMR

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    Two-dimensional correlation spectroscopy (2DCS) based on the nonlinear optical response of excitons to sequences of ultrafast pulses, has the potential to provide some unique insights into carrier dynamics in semiconductors. The most prominent feature of 2DCS, cross peaks, can best be understood using a sum-over-states picture involving the many-body eigenstates. However, the optical response of semiconductors is usually calculated by solving truncated equations of motion for dynamical variables, which result in a quasiparticle picture. In this work we derive Green's function expressions for the four wave mixing signals generated in various phase-matching directions and use them to establish the connection between the two pictures. The formal connection with Frenkel excitons (hard-core bosons) and vibrational excitons (soft-core bosons) is pointed out.Comment: Accepted to Phys. Rev.

    Dynamic extensions of batch systems with cloud resources

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    Compute clusters use Portable Batch Systems (PBS) to distribute workload among individual cluster machines. To extend standard batch systems to Cloud infrastructures, a new service monitors the number of queued jobs and keeps track of the price of available resources. This meta-scheduler dynamically adapts the number of Cloud worker nodes according to the requirement profile. Two different worker node topologies are presented and tested on the Amazon EC2 Cloud service

    Electronic structure study by means of X-ray spectroscopy and theoretical calculations of the "ferric star" single molecule magnet

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    The electronic structure of the single molecule magnet system M[Fe(L)2]3*4CHCl3 (M=Fe,Cr; L=CH3N(CH2CH2O)2) has been studied using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, soft X-ray emission spectroscopy, and density functional calculations. There is good agreement between theoretical calculations and experimental data. The valence band mainly consists of three bands between 2 eV and 30 eV. Both theory and experiments show that the top of the valence band is dominated by the hybridization between Fe 3d and O 2p bands. From the shape of the Fe 2p spectra it is argued that Fe in the molecule is most likely in the 2+ charge state. Its neighboring atoms (O,N) exhibit a magnetic polarisation yielding effective spin S=5/2 per iron atom, giving a high spin state molecule with a total S=5 effective spin for the case of M = Fe.Comment: Fig.2 replaced as it will appear in J. Chem. Phy

    Bandgap engineering of sol-gel synthesized amorphous Zn1-xMgxO films

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    Amorphous Zn1-xMgxO (alpha-Zn1-xMgxO) ternary alloy thin films across the full compositional range were synthesized by a low-cost sol-gel method on quartz substrates. The amorphous property of the alpha-Zn1-xMgxO films was verified by x-ray diffraction, and atomic force microscopy revealed a smooth surface with sub-nanometer root-mean square roughness. The current phase segregation issue limiting application of crystalline Zn1-xMgxO with 38% \u3c x \u3c 75% was completely eliminated by growing amorphous films. Optical transmission measurements showed high transmissivity of more than 90% in the visible and near infrared regions, with optical bandgap tunability from 3.3 eV to more than 6.5 eV by varying the Mg content

    Deep-ultraviolet photodetectors from epitaxially grown NixMg1-xO

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    Deep-ultraviolet (DUV) photodetectors were fabricated from high quality NixMg1-xO epitaxially grown by plasma-assisted molecular beam epitaxy on an approximately lattice matched MgO \u3c 100 \u3e substrate. A mid-range Ni composition (x=0.54) NixMg1-xO film was grown for DUV (lambda(peak) \u3c 300 nm) photoresponse and the film was characterized by reflected high-energy electron diffraction, Rutherford backscattering spectroscopy, x-ray diffraction, and optical transmission measurements. Photoconductive detectors were then fabricated by deposition of symmetric interdigitated contacts (10 nm Pt/150 nm Au) with contact separations of 5, 10, and 15 mu m. The detectors exhibited peak responsivities in the DUV (lambda(peak) approximate to 250 nm) as high as 12 mA/W, low dark currents (I-dark \u3c 25 nA), and DUV:visible ejection ratio of approximately 800:1

    Demokratie auf dem R√ľckzug? Zur Konfrontation zwischen amerikanischem Pr√§sidenten und Kongress √ľber einen Truppenabzug aus dem Irak

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    Im US-Pr√§sidentschaftswahlkampf 2008 ist der Irakkrieg eines der wichtigsten Themen. Die Amerikaner sind allm√§hlich kriegsm√ľde und die Mehrheit bef√ľrwortet einen Truppenr√ľckzug aus dem Irak. Das brachten bereits der Ausgang der Kongresswahlen im Jahr 2006 sowie regelm√§√üige Meinungsumfragen zum Ausdruck. Es ist nun anzunehmen, dass in der √§ltesten Demokratie der Welt Volkes Wille direkten Einfluss auf die Politik haben m√ľsste. Doch dies ist nur bedingt der Fall. Besonders Pr√§sident George W. Bush hat wiederholt klargestellt, dass die Stimmung in der Bev√∂lkerung f√ľr ihn in der Irakfrage nicht ma√ügebend sei. F√ľr ihn z√§hle vielmehr, wie die Gener√§le vor Ort die aktuelle Sicherheitslage und die Fortschritte einsch√§tzen. Dagegen versucht der seit Ende 2006 von Demokraten dominierte Kongress immer wieder, R√ľckzugspl√§ne mit festen Fristen zu etablieren. Bislang ist sein Erfolg jedoch bescheiden. Vor allem die fehlende Unterst√ľtzung der Republikaner im Kongress und die Blockadepolitik des Pr√§sidenten waren daf√ľr verantwortlich. Hinter der Konfrontation steht eine tiefergehende Auseinandersetzung √ľber die Kompetenzverteilung zwischen Pr√§sident und Kongress bei Kriegsentscheidungen, die in der US-Verfassung in einer Grauzone bleibt. Stephanie Sohnius verkn√ľpft vor diesem verfassungsrechtlichen Hintergrund die Aufarbeitung der aktuell wichtigsten USau√üenpolitischen Debatte mit der Frage, ob sich die Demokratie in den USA in der √Ąra des ‚ÄěKrieges gegen den Terrorismus‚Äú (weiterhin) auf dem R√ľckzug befindet

    ELPA: A parallel solver for the generalized eigenvalue problem

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    For symmetric (hermitian) (dense or banded) matrices the computation of eigenvalues and eigenvectors Ax = őĽBx is an important task, e.g. in electronic structure calculations. If a larger number of eigenvectors are needed, often direct solvers are applied. On parallel architectures the ELPA implementation has proven to be very efficient, also compared to other parallel solvers like EigenExa or MAGMA. The main improvement that allows better parallel efficiency in ELPA is the two-step transformation of dense to band to tridiagonal form. This was the achievement of the ELPA project. The continuation of this project has been targeting at additional improvements like allowing monitoring and autotuning of the ELPA code, optimizing the code for different architectures, developing curtailed algorithms for banded A and B, and applying the improved code to solve typical examples in electronic structure calculations. In this paper we will present the outcome of this project

    Low energy measurement of the 7Be(p,gamma)8B cross section

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    We have measured the cross section of the 7Be(p,gamma)8B reaction for E_cm = 185.8 keV, 134.7 keV and 111.7 keV using a radioactive 7Be target (132 mCi). Single and coincidence spectra of beta^+ and alpha particles from 8B and 8Be^* decay, respectively, were measured using a large acceptance spectrometer. The zero energy S factor inferred from these data is 18.5 +/- 2.4 eV b and a weighted mean value of 18.8 +/- 1.7 eV b (theoretical uncertainty included) is deduced when combining this value with our previous results at higher energies.Comment: Accepted for publication in Phys. Rev. Let
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