Space-Time Uncertainty and Noncommutativity in String Theory

We analyze the nature of space-time nonlocality in string theory. After giving a brief overview on the conjecture of the space-time uncertainty principle, a (semi-classical) reformulation of string quantum mechanics, in which the dynamics is represented by the noncommutativity between temporal and spatial coordinates, is outlined. The formalism is then compared to the space-time noncommutative field theories associated with nonzero electric B-fields.Comment: 10 pages with one figure. Written version of talks presented at the Strings 2000 (Michigan, July, 2000) and the Summer Workshop on Branes (KIAS, Seoul, June, 2000

Graphene formed on SiC under various environments: Comparison of Si-face and C-face

The morphology of graphene on SiC {0001} surfaces formed in various environments including ultra-high vacuum, 1 atm of argon, and 10^-6 to 10^-4 Torr of disilane is studied by atomic force microscopy, low-energy electron microscopy, and Raman spectroscopy. The graphene is formed by heating the surface to 1100 - 1600 C, which causes preferential sublimation of the Si atoms. The argon atmosphere or the background of disilane decreases the sublimation rate so that a higher graphitization temperature is required, thus improving the morphology of the films. For the (0001) surface, large areas of monolayer-thick graphene are formed in this way, with the size of these areas depending on the miscut of the sample. Results on the (000-1) surface are more complex. This surface graphitizes at a lower temperature than for the (0001) surface and consequently the growth is more three-dimensional. In an atmosphere of argon the morphology becomes even worse, with the surface displaying markedly inhomogeneous nucleation, an effect attributed to unintentional oxidation of the surface during graphitization. Use of a disilane environment for the (000-1) surface is found to produce improved morphology, with relatively large areas of monolayer-thick graphene.Comment: 22 pages, 11 figures, Proceedings of STEG-2 Conference; eliminated Figs. 4 and 7 from version 1, for brevity, and added Refs. 18, 29, 30, 31 together with associated discussio

Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB2_2 samples

The real $R_s(T)$ and imaginary $X_s(T)$ parts of the surface impedance $Z_s(T)=R_s(T)+iX_s(T)$ in polycrystalline MgB$_2$ samples of different density with the critical temperature $T_c\approx 38$ K are measured at the frequency of 9.4 GHz and in the temperature range $5\le T<200$ K. The normal skin-effect condition $R_s(T)=X_s(T)$ at $T\ge T_c$ holds only for the samples of the highest density with roughness sizes not more than 0.1 $\mu$m. For such samples extrapolation $T\to 0$ of the linear at $T<T_c/2$ temperature dependences $\lambda_L(T)=X_s(T)/\omega\mu_0$ and $R_s(T)$ results in values of the London penetration depth $\lambda_L(0)\approx 600$ \AA and residual surface resistance $R_{res}\approx 0.8$ m$\Omega$. In the entire temperature range the dependences $R_s(T)$ and $X_s(T)$ are well described by the modified two-fluid model.Comment: 7 pages, 3 figures. Europhysics Letters, accepted for publicatio

Heavy Mesons in Two Dimensions

The large mass limit of QCD uncovers symmetries that are not present in the QCD lagrangian. These symmetries have been applied to physical (finite mass) systems, such as B and D mesons. We explore the validity of this approximation in the 't Hooft model (two-dimensional QCD in the large-N approximation). We find that the large mass approximation is good, even at the charm mass, for form factors, but it breaks down for the pseudoscalar decay constant.Comment: 4 pages, 3 figures inc

Propagating modes of non-Abelian tensor gauge field of second rank

In the recently proposed extension of the YM theory, non-Abelian tensor gauge field of the second rank is represented by a general tensor whose symmetric part describes the propagation of charged gauge boson of helicity two and its antisymmetric part - the helicity zero charged gauge boson. On the non-interacting level these polarizations are similar to the polarizations of the graviton and of the Abelian antisymmetric B field, but the interaction of these gauge bosons carrying non-commutative internal charges cannot be directly identified with the interaction of gravitons or B field. Our intention here is to illustrate this result from different perspectives which would include Bianchi identity for the corresponding field strength tensor and the analysis of the second-order partial differential equation which describes in this theory the propagation of non-Abelian tensor gauge field of the second rank.Comment: 22 pages, Latex fil

TIMED Imaging Photometer Experiment (TIPE)

This document contains a summary of the TIMED Imaging Photometer Experiment (TIPE) instrument study at the time of the termination of project due to TIPE being de-selected from the Thermosphere, Ionosphere and Mesosphere Energetics and Dynamics (TIMED) mission

Polaron effects in electron channels on a helium film

Using the Feynman path-integral formalism we study the polaron effects in quantum wires above a liquid helium film. The electron interacts with two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one dimension (1D) by an harmonic potential. The obtained results are valid for arbitrary temperature ($T$), electron-phonon coupling strength ($\alpha$), and lateral confinement ($\omega_{0}$). Analytical and numerical results are obtained for limiting cases of $T$, $\alpha$, and $\omega_{0}$. We found the surprising result that reducing the electron motion from 2D to quasi-1D makes the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.

Ganzheitliche Untersuchungsmethoden zur Erfassung und Prüfung der Qualität ökologischer Lebensmittel: Stand der Entwicklung und Validierung

In dem wachsenden Markt ökologischer Lebensmittel werden Methoden zur produktorientierten Qualitätserfassung gefordert. Dabei geht es u.a. um die Unterscheidung von Produkten aus unterschiedlichen Anbauverfahren. Die Ziele des Projektes waren daher: 1. ausgewählte ganzheitliche Methoden gemäß ISO 17025 zu validieren, d.h. Laborprozesse festzulegen, sowie Einflussgrößen und Verfahrensmerkmale zu bestimmen, 2. zu testen, ob diese Verfahren eine Differenzierung von definierten Proben statistisch abgesichert zeigen können. . Diese Ziele konnten erreicht werden. Es wurde bestätigt, dass einige der Methoden auf Grundlage dokumentierter Prozeduren Lebensmittel aus definierten Anbauversuchen (u.a. aus dem DOK-Versuch am FIBL/CH) reproduzierbar unterscheiden können. Die Koordination und die Validierung der Kupferchlorid-Kristallisation sowie die Messung der Polyphenole lag bei der Universität Kassel, FG Ökologische Lebensmittelqualität und Ernährungskultur. Die KWALIS GmbH, Dipperz, validierte die Fluoreszenz-Anregungsspektroskopie und die Bestimmung des Physiologischen Aminosäurestatus, die EQC GmbH, Weidenbach die elektrochemischen Messungen. Dr. Kromidas, Saarbrücken übernahm die Beratung der Validierungsprozeduren. . An Blindproben wurde untersucht, ob die Verfahren für Weizen- und Möhrenproben aus definierten Anbau- und Sortenversuchen geeignet sind (Fragestellung der Validierung). Die Proben wurden von unabhängiger Stelle (OEL-FAL, Trenthorst) codiert. Die Proben wurden gleichzeitig an alle Partner versandt; dadurch konnten die Methoden auch untereinander verglichen werden. Die Methoden Kupferchlorid-Kristallisation, Fluoreszenz-Anregungsspektroskopie und Physiologischer Aminosäurestatus sind für die Fragestellung geeignet. Mit allen drei Methoden konnten die Proben differenziert und gruppiert werden. Darüber hinaus konnten mit der Fluoreszenz-Anregungsspektroskopie und über den physiologischen Aminosäurestatus die Proben auch den Anbauweisen richtig zugeordnet werden. Allerdings ist damit noch keine Aussage über die Fähigkeit dieser Verfahren möglich, generell Proben aus ökologischer und konventioneller Herkunft zu unterscheiden. Dafür sind weitere Untersuchungen sowohl an Proben definierter Herkunft als auch an Marktproben notwendig