46 research outputs found
Three-dimensional effects on extended states in disordered models of polymers
We study electronic transport properties of disordered polymers in the
presence of both uncorrelated and short-range correlated impurities. In our
procedure, the actual physical potential acting upon the electrons is replaced
by a set of nonlocal separable potentials, leading to a Schr\"odinger equation
that is exactly solvable in the momentum representation. We then show that the
reflection coefficient of a pair of impurities placed at neighboring sites
(dimer defect) vanishes for a particular resonant energy. When there is a
finite number of such defects randomly distributed over the whole lattice, we
find that the transmission coefficient is almost unity for states close to the
resonant energy, and that those states present a very large localization
length. Multifractal analysis techniques applied to very long systems
demonstrate that these states are truly extended in the thermodynamic limit.
These results reinforce the possibility to verify experimentally theoretical
predictions about absence of localization in quasi-one-dimensional disordered
systems.Comment: 16 pages, REVTeX 3.0, 5 figures on request from FDA
([email protected]). Submitted to Phys. Rev. B. MA/UC3M/09/9
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Improving the hyperpolarization of (31)p nuclei by synthetic design
Traditional (31)P NMR or MRI measurements suffer from low sensitivity relative to (1)H detection and consequently require longer scan times. We show here that hyperpolarization of (31)P nuclei through reversible interactions with parahydrogen can deliver substantial signal enhancements in a range of regioisomeric phosphonate esters containing a heteroaromatic motif which were synthesized in order to identify the optimum molecular scaffold for polarization transfer. A 3588-fold (31)P signal enhancement (2.34% polarization) was returned for a partially deuterated pyridyl substituted phosphonate ester. This hyperpolarization level is sufficient to allow single scan (31)P MR images of a phantom to be recorded at a 9.4 T observation field in seconds that have signal-to-noise ratios of up to 94.4 when the analyte concentration is 10 mM. In contrast, a 12 h 2048 scan measurement under standard conditions yields a signal-to-noise ratio of just 11.4. (31)P-hyperpolarized images are also reported from a 7 T preclinical scanner
Phlebologie an deutschen Hautkliniken: eine Bestandsaufnahme im Auftrag der Deutschen Gesellschaft für Phlebologie
Reich-Schupke S, Alm J, Altmeyer P, et al. Phlebologie an deutschen Hautkliniken: Eine Bestandsaufnahme im Auftrag der Deutschen Gesellschaft für Phlebologie. Hautarzt. 2013;64(9):685-694.Phlebologic diseases have become extremely common and have major socio-economic impact. However, the percentage of dermatologists working in phlebology appears to be decreasing according to the data of the German Society of Phlebology (DGP). To investigate the reasons for this development, we-on behalf of the DGP-sent a questionnaire to 120 German Departments of Dermatology in autumn 2012. In 76 returned questionnaires, the number of physicians with additional fellowship training in phlebology averaged 1.5; the average number of those who fulfill the criteria for training fellows in phlebology was 0.9. In 71.1 % of the departments there was a phlebologist. A special phlebologic outpatient clinic existed in 73.7 % of the departments. Sonography with Doppler (89.5 %) and duplex (86.8 %) was used as the most frequent diagnostic tool. For therapy, compression (94.7 %), sclerotherapy (liquid 78.9 %, foam 63.2 %, catheter 18.4 %), endoluminal thermic procedures (radio wave 28.9 %, laser 17.1 %) and surgery (especially crossectomy and stripping 67.1 %, phlebectomy of tributaries 75 %) were used. The average number of treatments was very heterogenous in the different departments. Phlebology definitely plays an important role in dermatology. Most departments fulfill the formal criteria for the license to conduct advanced training in phlebology. A wide spectrum of phlebological diagnostic and therapeutic procedures is available.Hintergrund
Venenleiden sind eine Volkskrankheit und haben eine enorme sozioökonomische Bedeutung. Dennoch scheint der Anteil der phlebologisch tätigen Dermatologen entsprechend den Zahlen der Deutschen Gesellschaft für Phlebologie (DGP) abzunehmen.
Methoden
Um die Ursachen dieser Entwicklung zu untersuchen, führten wir im Herbst 2012 im Auftrag der DGP eine Befragungsstudie zur Phlebologie an 120 deutschen Hautkliniken durch.
Ergebnisse
In 76 antwortenden Klinien gab es im Mittel 1,5 Phlebologen und 0,9 Ärzte mit Voraussetzungen zur Weiterbildungsermächtigung Phlebologie. In 71,1 % der Kliniken gab es einen Phlebologen, in 73,7 % eine phlebologische Sprechstunde. Zur Diagnostik wurden besonders die Doppler- (89,5 %) und Duplexsonographie (86,8 %) eingesetzt. In der Therapie führte die Kompressionstherapie (94,7 %), gefolgt von Sklerosierungstherapien (flüssig 78,9 %, Schaum 63,2 %, Katheter 18,4 %), den endoluminalen thermischen Verfahren (Radiowelle 28,9 %, Laser 17,1 %) und den operativen Eingriffen (vor allem Cross- und Saphenektomie 67,1 %, Seitenastexhairese 75 %). Die durchschnittlichen Behandlungszahlen waren sehr heterogen.
Schlussfolgerung
Die Phlebologie spielt in der Dermatologie weiterhin eine wichtige Rolle. Mehrheitlich sind in den Kliniken die Möglichkeiten zur Weiterbildung Phlebologie gegeben. Es wird ein breites Spektrum an phlebologischen Techniken zur Diagnostik und Therapie angeboten