388 research outputs found
The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems
The magnetisation relaxations of three different types of geometrically
frustrated magnetic systems have been studied with the same experimental
procedures as previously used in spin glasses. The materials investigated are
YMoO (pyrochlore system), SrCrGaO (piled
pairs of Kagom\'e layers) and (HO)Fe(SO)(OH) (jarosite
compound). Despite a very small amount of disorder, all the samples exhibit
many characteristic features of spin glass dynamics below a freezing
temperature , much smaller than their Curie-Weiss temperature .
The ageing properties of their thermoremanent magnetization can be well
accounted for by the same scaling law as in spin glasses, and the values of the
scaling exponents are very close. The effects of temperature variations during
ageing have been specifically investigated. In the pyrochlore and the
bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a
certain temperature re-initializes ageing and the evolution at the new
temperature is the same as if the system were just quenched from above .
However, as the temperature is raised back to , the sample recovers the
state it had previously reached at that temperature. These features are known
in spin glasses as rejuvenation and memory effects. They are clear signatures
of the spin glass dynamics. In the Kagom\'e compound, there is also some
rejuvenation and memory, but much larger temperature changes are needed to
observe the effects. In that sense, the behaviour of this compound is
quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings
of the International Conference on Highly Frustrated Magnetism (HFM2003),
August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France
The only known cyclopygid–‘atheloptic’ trilobite fauna from North America: the upper Ordovician fauna of the Pyle Mountain Argillite and its palaeoenvironmental significance
The trilobite fauna of the upper Ordovician (middle Katian) Pyle Mountain Argillite comprises a mixture of abundant mesopelagic cyclopygids and other pelagic taxa and a benthic fauna dominated by trilobites lacking eyes. Such faunas were widespread in deep water environments around Gondwana and terranes derived from that continent throughout Ordovician time but this is the only known record of such a fauna from North America and thus from Laurentia. It probably reflects a major sea level rise (the ‘Linearis drowning events’) as does the development of coeval cyclopygid-dominated deep water trilobite faunas in terranes that were marginal to Laurentia and are now preserved in Ireland and Scotland. The Pyle Mountain Argillite trilobite fauna occurs with a deep water Foliomena brachiopod fauna and comprises 22 species. Pelagic trilobites (mostly cyclopygids) constitute 36% of the preserved sclerites, and 45% of the fauna is the remains of trilobites lacking eyes, including one new species, Dindymene whittingtoni sp. nov. Three species of cyclopygid are present, belonging in Cyclopyge, Symphysops and Microparia (Heterocyclopyge). Cyclopygids are widely thought to have been stratified in the water column in life and thus their taxonomic diversity reflects the relative depths of the sea-beds on which their remains accumulated. A tabulation of middle and upper Katian cyclopygid-bearing faunas from several palaeoplates and terranes arranged on the basis of increasing numbers of cyclopygid genera allows an assessment of the relative depth ranges of the associated benthic taxa. The Pyle Mountain Argillite fauna lies towards the deeper end of this depth spectrum
La sédimentation carbonatée ordovicienne : un des principaux facteurs déclencheur de la glaciation hirnantienne
Une nouvelle approche concernant le déroulement de la glaciation hirnantienne est proposée dans ce travail.
Elle s’intéresse aux principaux facteurs clés de cette dernière et associe les effets d’une glaciation continentale à une période
de bas niveau du CO2 atmosphérique. L’accumulation d’un important volume de carbonates au cours de l’Ordovicien
terminal pré-Hirnantien dans des régions où ces derniers étaient antérieurement absents est considérée comme un
important puits de CO2 atmosphérique. Cette accumulation pourrait être la cause d’une baisse importante de la température
moyenne au début de l’Hirnantien à laquelle s’ajoute un autre processus de rétroaction tel que la météorisation des
carbonates. Une augmentation du flux de CO2 de l’atmosphère vers les océans par dissolution devrait avoir été favorisée
par la précipitation de carbonates. L’importante inondation du continent Laurentia, situé à basse latitude au cours du
Cincinnatien, et l’implantation d’une sédimentation carbonatée tempérée sur la marge nord-gondwanienne au cours de
l’Ashgill (pre-Hirnantien), ont favorisé l’enfouissement de plus de 840 × 1015 kg (1,9 × 1019 mol) de CO2 dissous. Cette
masse représente environ 350 fois la valeur actuelle du CO2 atmosphérique. Cette précipitation devrait avoir altéré fortement
l’équilibre entre le CO2 dissous dans les océans et la pression partielle de CO2 dans l’air, entraînant éventuellement
une réduction de cette dernière. L’approche développée dans ce travail offre une explication simple pour la fin
accélérée de la glaciation. La baisse du niveau marin relatif, attribuée au glacio-eustatisme associée au recul de la ligne
de rivage des océans sur les plates-formes, devrait avoir provoqué l’arrêt de la production de sédiments carbonatés et de
l’absorption de CO2. Le niveau de CO2 préglaciaire devrait dès lors se rétablir à la faveur du dégazage de CO2 par volcanisme.
Toutefois, après la fonte des glaces, les circulations océaniques ne reprennent pas et l’absence des courants instaurés
lors de l’Ashgill (pré-Hirnantien) par une importante stratification des eaux océaniques empêche la reprise d’une
importante sédimentation carbonatée. Les pics positifs bien connus du δ13C à la base de l’Hirnantien sont attribués au
lessivage et à la dissolution des carbonates enrichis en 13C lors de l’importante émersion des plate-formes
Photoproduction of eta mesons from the neutron: cross sections and double polarization observable E
Photoproduction of mesons from neutrons} \abstract{Results from
measurements of the photoproduction of mesons from quasifree protons and
neutrons are summarized. The experiments were performed with the CBELSA/TAPS
detector at the electron accelerator ELSA in Bonn using the
decay. A liquid deuterium target was used for the
measurement of total cross sections and angular distributions. The results
confirm earlier measurements from Bonn and the MAMI facility in Mainz about the
existence of a narrow structure in the excitation function of . The current angular distributions show a forward-backward
asymmetry, which was previously not seen, but was predicted by model
calculations including an additional narrow state. Furthermore, data
obtained with a longitudinally polarized, deuterated butanol target and a
circularly polarized photon beam were analyzed to determine the double
polarization observable . Both data sets together were also used to extract
the helicity dependent cross sections and . The
narrow structure in the excitation function of
appears associated with the helicity-1/2 component of the reaction
Experimental constraints on the -nucleus real potential
In a search for mesic states, the production of -mesons in
coincidence with forward going protons has been studied in photon induced
reactions on C for incident photon energies of 1250 - 3100 MeV. The
pairs from decays of bound or quasi-free -mesons have
been measured with the CBELSA/TAPS detector system in coincidence with protons
registered in the MiniTAPS forward array. Structures in the total energy
distribution of the pairs, which would indicate the population
and decay of bound B states, are not observed. The
cross section of 0.3 nb/MeV/sr observed in the bound state energy regime
between -100 and 0 MeV may be accounted for by yield leaking into the bound
state regime because of the large in-medium width of the -meson. A
comparison of the measured total energy distribution with calculations suggests
the real part of the B potential to be small and only
weakly attractive with 35(stat) 20(syst) MeV
in contrast to some theoretical predictions of attractive potentials with a
depth of 100 - 150 MeV.Comment: 13 pages, 8 figure
Slow dynamics and aging in spin-glasses
Contribution presented by Eric Vincent in the Conference `Complex Behaviour
of Glassy Systems', Sitges, Barcelona, Spain, June, 1996. It contains a review
of the experimental results on Slow dynamics and aging in spin-glasses. It also
presents their comparison with recent theoretical developments in the
description of the out of equilibrium dynamics of disordered systems; namely,
the trap model and the mean-field theory.Comment: 35 pages, 12 figures, macro lmamult.sty (included
Pharmacokinetics of oxycodone/naloxone and its metabolites in patients with end-stage renal disease during and between haemodialysis sessions
The pharmacokinetics of oxycodone in patients with end-stage renal disease (ESRD) requiring haemodialysis are largely unknown. Therefore, we investigated the pharmacokinetics of oxycodone/naloxone prolonged release and their metabolites in patients with ESRD during and between haemodialysis sessions.; Single doses of oxycodone/naloxone (5/2.5 or 10/5 mg) were administered in nine patients with ESRD using a cross-over design on the day of dialysis and on a day between dialysis sessions. Plasma, dialysate and urine concentrations of oxycodone, naloxone and their metabolites were determined up to 48 h post-dosing using a liquid chromatography-tandem mass spectrometry system.; Haemodialysis performed 6-10 h after dosing removed ∼10% of the administered dose of oxycodone predominantly as unconjugated oxycodone and noroxycodone or conjugated oxymorphone and noroxymorphone. The haemodialysis clearance of oxycodone based on its recovery in dialysate was (mean ± SD) 8.4 ± 2.1 L/h. The geometric mean (coefficient of variation) plasma elimination half-life of oxycodone during the 4-h haemodialysis period was 3.9 h (39%) which was significantly shorter than the 5.7 h (22%) without haemodialysis. Plasma levels of the active metabolite oxymorphone in its unconjugated form were very low.; Oxycodone is removed during haemodialysis. The pharmacokinetics including the relatively short half-life of oxycodone in patients with ESRD with or without haemodialysis and the absence of unconjugated active metabolites indicate that oxycodone can be used at usual doses in patients requiring dialysis
On the scaling and ageing behaviour of the alternating susceptibility in spin glasses and local scale-invariance
The frequency-dependent scaling of the dispersive and dissipative parts of
the alternating susceptibility is studied for spin glasses at criticality. An
extension of the usual -scaling is proposed. Simulational data from
the three-dimensional Ising spin glass agree with this new scaling form and
moreover reproduce well the scaling functions explicitly calculated for systems
satisfying local scale-invariance. There is also a qualitative agreement with
existing experimental data.Comment: 19 pages, 2 figures, to appear in special issue of J. Phys. Cond.
Matt. dedicated to Lothar Schaefer on the occasion of his 60th birthday,
final form with IOP macro
Temperature shifts in the Sinai model: static and dynamical effects
We study analytically and numerically the role of temperature shifts in the
simplest model where the energy landscape is explicitely hierarchical, namely
the Sinai model. This model has both attractive features (there are valleys
within valleys in a strict self similar sense), but also one important
drawback: there is no phase transition so that the model is, in the large size
limit, effectively at zero temperature. We compute various static chaos
indicators, that are found to be trivial in the large size limit, but exhibit
interesting features for finite sizes. Correspondingly, for finite times, some
interesting rejuvenation effects, related to the self similar nature of the
potential, are observed. Still, the separation of time scales/length scales
with temperatures in this model is much weaker that in experimental
spin-glasses.Comment: 19 pages, Revtex4, eps figure
Mean-field theory of temperature cycling experiments in spin-glasses
We study analytically the effect of temperature cyclings in mean-field
spin-glasses. In accordance with real experiments, we obtain a strong
reinitialization of the dynamics on decreasing the temperature combined with
memory effects when the original high temperature is restored. The same
calculation applied to mean-field models of structural glasses shows no such
reinitialization, again in accordance with experiments. In this context, we
derive some relations between experimentally accessible quantities and propose
new experimental protocols. Finally, we briefly discuss the effect of field
cyclings during isothermal aging.Comment: Some misprints corrected, references updated, final version to apper
in PR
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