Resummations in Hot Scalar Electrodynamics

The gauge-boson sector of perturbative scalar electrodynamics is investigated in detail as a testing ground for resummation methods in hot gauge theories. It also serves as a simple non-trivial reference system for the non-Abelian gluon plasma. The complete next-to-leading order contributions to the polarization tensor are obtained within the resummation scheme of Braaten and Pisarski. The simpler scheme proposed recently by Arnold and Espinosa is shown to apply to static quantities only, whereas Braaten-Pisarski resummation turns out to need modification for collective phenomena close to the light-cone. Finally, a recently proposed resummation of quasi-particle damping contributions is assessed critically.Comment: 53 p. LaTeX, 7 figs. (2 in LaTeX, 5 EPS appended as uu-encoded file), ITP-UH-01/94 & DESY 94-03

Wigner Crystal in One Dimension

A one--dimensional gas of electrons interacting with long--range Coulomb forces ($V(r) \approx 1/r$) is investigated. The excitation spectrum consists of separate collective charge and spin modes, with the charge excitation energies in agreement with RPA calculations. For arbitrarily weak Coulomb repulsion density correlations at wavevector $4k_F$ decay extremely slowly and are best described as those of a one--dimensional Wigner crystal. Pinning of the Wigner crystal then leads to the nonlinear transport properties characteristic of CDW. The results allow a consistent interpretation of the plasmon and spin excitations observed in one--dimensional semiconductor structures, and suggest an interpretation of some of the observed features in terms of ``spinons''. A possible explanation for nonlinear transport phenomena is given.Comment: 10 pages, RevTe

Einfluß unterschiedlicher Anbauarten – mineralisch, organisch, biologisch-dynamisch – auf Kartoffeln: Inhaltsstoffe, Sensorik, Festigkeitskennwerte und bildschaffende Methoden

Grundsätzlich sprachen die Kartoffeln der Sorte ´Granola´ sehr schwach auf die eingesetzten Versuchsfaktoren an, sodaß die Unterschiede selten statistisch abgesichert werden konnten. Bei denjenigen Inhaltsstoffen, die deutlicher auf den Anbaufaktor Düngermenge ansprachen, reagierten die Knollen der mineralisch gedüngten Varianten eindeutig stärker (Abnahme der Trockenmasse, der Saccharose-, Stärke- und Clorid-Gehalte, Zunahme der Glukose-, Fruktose-, Fluorid- und Asche-Gehalte) als die Vergleichsvarianten. Ein Grund hierfür könnte die schnellere Verfügbarkeit des mineralischen Düngers gewesen sein. Die biologisch-dynamisch gedüngten Knollen zeigten nur bei drei Parametern dieselbe Reaktion auf die Intensivierung der Düngung wie die organisch gedüngten auf (Zunahme der Chlorid- und Asche-Gehalte, gleichbleibende Fluorid-Gehalte). Bei vier Merkmalen reagierten die biologisch-dynamischen Knollen überhaupt nicht oder sehr gering, wogegen die organisch gedüngten Kartoffeln eine Abnahme im Trockenmasse- und Saccharosegehalt bzw. keine gerichteten Tendenzen in den Glukose- und Fruktosegehalten aufwiesen. Die Festigkeitskennwerte der mineralisch gedüngten Kartoffeln wurde, im Vergleich zu den beiden anderen Varianten, schwächer bewertet. Die sensorischen Untersuchungen ergaben keine Bevorzugung einzelner Varianten, jedoch, besonders bei der minera-lischen Variante, eine tendenzielle Abnahme der sensorischen Qualität mit zunehmender Düngung. Die Ergebnisse der Untersuchungen zur „Vitalqualität“ mit Hilfe bildschaf-fender Methoden bestätigte die prinzipielle Eignung dieser Methode für ergänzende Qualitätsuntersuchungen. Eine abschließende Beurteilung der Qualität der unterschiedlichen Anbauvarianten wird erst in Verbindung der bisherigen Daten mit noch zur Analyse anstehenden Parametern (Nitrat, Aminosäuren, Ascorbinsäure, Kalium) möglich sein. Des weiteren wird in laufenden Sortenversuchen geprüft, ob andere Sorten stärker auf die verschiedenen Versuchsfaktoren reagieren

Visual exploration and retrieval of XML document collections with the generic system X2

This article reports on the XML retrieval system X2 which has been developed at the University of Munich over the last five years. In a typical session with X2, the user first browses a structural summary of the XML database in order to select interesting elements and keywords occurring in documents. Using this intermediate result, queries combining structure and textual references are composed semiautomatically. After query evaluation, the full set of answers is presented in a visual and structured way. X2 largely exploits the structure found in documents, queries and answers to enable new interactive visualization and exploration techniques that support mixed IR and database-oriented querying, thus bridging the gap between these three views on the data to be retrieved. Another salient characteristic of X2 which distinguishes it from other visual query systems for XML is that it supports various degrees of detailedness in the presentation of answers, as well as techniques for dynamically reordering and grouping retrieved elements once the complete answer set has been computed

Step by step capping and strain state of GaN/AlN quantum dots studied by grazing incidence diffraction anomalous fine structure

The investigation of small size embedded nanostructures, by a combination of complementary anomalous diffraction techniques, is reported. GaN Quantum Dots (QDs), grown by molecular beam epitaxy in a modified Stranski-Krastanow mode, are studied in terms of strain and local environment, as a function of the AlN cap layer thickness, by means of grazing incidence anomalous diffraction. That is, the X-ray photons energy is tuned across the Ga absorption K-edge which makes diffraction chemically selective. Measurement of \textit{hkl}-scans, close to the AlN (30-30) Bragg reflection, at several energies across the Ga K-edge, allows the extraction of the Ga partial structure factor, from which the in-plane strain of GaN QDs is deduced. From the fixed-Q energy-dependent diffracted intensity spectra, measured for diffraction-selected iso-strain regions corresponding to the average in-plane strain state of the QDs, quantitative information regarding composition and the out-of-plane strain has been obtained. We recover the in-plane and out-of-plane strains in the dots. The comparison to the biaxial elastic strain in a pseudomorphic layer indicates a tendency to an over-strained regime.Comment: submitted to PR

Aging in the glass phase of a 2D random periodic elastic system

Using RG we investigate the non-equilibrium relaxation of the (Cardy-Ostlund) 2D random Sine-Gordon model, which describes pinned arrays of lines. Its statics exhibits a marginal ($\theta=0$) glass phase for $T<T_g$ described by a line of fixed points. We obtain the universal scaling functions for two-time dynamical response and correlations near $T_g$ for various initial conditions, as well as the autocorrelation exponent. The fluctuation dissipation ratio is found to be non-trivial and continuously dependent on $T$.Comment: 5 pages, RevTex, Modified Versio

Quality Comparison of Mineral, Organic and Biodynamic Cultivation of Potatoes: Contents, Strength Criteria, Sensory Investigations and Picture-Creating Methods

Nutritional quality of food can be characterized by the content of beneficial and harmful substances. For today’s consumers the most important criteria for buying potatoes are taste, quality and shelf-life. Biodynamic preparations are used by about 20% of all organic farms in Germany. These preparations do not primarily serve to increase the yield but are said to support the production of high quality food, having balanced nutrient contents and improved shelf-life characteristics. Since 1993 investigations have been carried out to evaluate the effect of mineral fertilization and organic manuring at varied intensities, combined with application of biodynamic preparations, on some of these parameters. The potatoes of the variety ´Granola´ responded rather slightly to the factors of the trial, hence statistical differences could be confirmed only in few cases. Tubers deriving from the mineral fertilized plots showed the strongest response to elevated fertilization (decrease in dry matter, sucrose, chloride content and density, and an increase in glucose, fructose, fluoride and ash content). Only in three parameters tubers from the biodynamic plots responded in the same way as the organic ones to elevated fertilization (increase in the chloride and ash contents, stagnating fluoride contents). In four parameters, no or only a weak response of the biodynamic tubers has been found, whereas organic potatoes showed decreasing dry matter and sucrose contents, but no distinctive tendency in the parameters glucose and fructose. The parameter strength criteria has been assessed as the weakest in mineral fertilized potatoes. The sensory investigations did not allow to differentiate between the three cropping systems but there was a tendency of worsening sensoric quality at increased fertilization, particularly in the tubers of the mineral plots. The results of the determination of vital quality following picture-creating methods confirmed the applicability of this assay for additional quality determinations. A final assessment of the quality according to the different cropping systems cannot be made at this point of time. Therefore, further parameters (nitrate, potassium, amino acids, ascorbic acid) will be analyzed which might allow a final assessment in combination with the present data. Furthermore, current trials might reveal if other varieties respond stronger to the relevant factors

Drift of a polymer chain in disordered media

We consider the drift of a polymer chain in a disordered medium, which is caused by a constant force applied to the one end of the polymer, under neglecting the thermal fluctuations. In the lowest order of the perturbation theory we have computed the transversal fluctuations of the centre of mass of the polymer, the transversal and the longitudinal size of the polymer, and the average velocity of the polymer. The corrections to the quantities under consideration, which are due to the interplay between the motion and the quenched forces, are controlled by the driving force and the degree of polymerization. The transversal fluctuations of the Brownian particle and of the centre of mass of the polymer are obtained to be diffusive. The transversal fluctuations studied in the present Letter may also be of relevance for the related problem of the drift of a directed polymer in disordered media and its applications.Comment: 11 pages, RevTex, Accepted for publication in Europhysics Letter

Vanadium centers in ZnTe crystals. II. Electron paramagnetic resonance

Four V-related electron-paramagnetic-resonance (EPR) spectra are observed in Bridgman-grown ZnTe doped with vanadium. Two of them are attributed to the charge states VZn3+(A+) and VZn2+(A0) of the isolated V impurity. For the ionized donor, VZn3+(A+), the spectrum reveals the typical behavior of the expected 3A2(F) ground state in tetrahedral symmetry. The incorporation on a cation lattice site could be proved by the resolved superhyperfine interaction with four Te ions. The second spectrum showing triclinic symmetry and S=3/2 is interpreted as the neutral donor state VZn2+(A0). The origin of the triclinic distortion of the cubic (Td) crystal field could be a static Jahn-Teller effect. The two additionally observed EPR spectra are attributed to nearest-neighbor V-related defect pairs. The spectrum of the first one, V2+Zn-YTe, shows trigonal symmetry and can be explained by the S=3/2 manifold of an orbital singlet ground state. An associated defect "YTe" is responsible for the trigonal distortion of the tetrahedral crystal field of V2+Zn. The spectrum of the second pair defect also shows trigonal symmetry and can be described by S=1/2. The ground-state manifold implies a VZn3+−XTe pair as the most probable origin of this spectrum. The S=1/2 ground state is produced by a dominating isotropic exchange interaction coupling the S=1 ground-state manifold of V3+Zn to an assumed S=1/2 ground state of "XTe" in antiferromagnetic orientation. The nature of the associated defects "YTe" and "XTe" remains unknown for both pairs since no hyperfine structure has been observed, but most probably acceptorlike defects are involved

Vanadium centers in ZnTe crystals. I. Optical properties

In ZnTe:V bulk crystals with nominal vanadium concentrations between 1000 and 7000 ppm three vanadium-ion states V+, V2+, and V3+ were found in low-temperature optical measurements. No-phonon lines of the internal emissions were detected for the 5E(D)→5T2(D) transition of V+(d4) at 3401 cm−1 (0.422 eV), for 4T2(F)→4T1(F) of V2+(d3) at 4056 cm−1 (0.503 eV), and for 3T2(F)→3A2(F) of V3+(d2) at 4726 cm−1 (0.586 eV). The energies of the internal transitions are reduced with respect to the corresponding transitions in ZnS:V and ZnSe:V. The respective excitation spectra display, in addition to broad charge-transfer bands, higher excited levels of the individual charge states. Crystal-field calculations of the detected transition energies based on the Tanabe-Sugano scheme are presented. With the help of sensitization experiments, a one-electron model is designed, in which the donor level (V2+/V3+) is situated 12 500 cm−1 (1.55 eV) below the conduction-band edge and the acceptor level (V2+/V+) 9400 cm−1 (1.17 eV) above the valence-band edge. The dynamical behavior of the three infrared lurainescence bands was measured. Decay time constants of 43 μs (V+), 120 μs (V2+), and 420 μs (V3+) were found. Electron-paramagnetic-resonance (EPR) results measured on the same samples are presented in an accompanying paper and confirm the optical detection of isolated substitutional V2+(d3) and V3+(d2) ions. Relations between the EPR and optical results are discussed