Late Cretaceous Plesiosaur Teeth from Axel Heiberg Island, Nunavut, Canada

We report the discovery of Late Cretaceous plesiosaur teeth from non-marine strata on Axel Heiberg Island in the Canadian High Arctic. In comparison to other plesiosaur teeth, these specimens are most similar to the teeth of elasmosaurs: they have a smooth outer surface and crenulated inner surface, with crenulations that extend nearly to the tip of the tooth. Comparisons with elasmosaurid fossils elsewhere indicate that the Axel Heiberg teeth are from juveniles. The presence of a plesiosaur in nonmarine strata on Axel Heiberg Island supports the suggestion that juvenile elasmosaurs frequently inhabited freshwater environments. The temporal distribution of the Axel Heiberg specimens and other occurrences from the High Arctic suggests that elasmosaurids may have expanded their range during a time of extreme climatic warmth.On signale la découverte de dents de plésiosaure du Crétacé supérieur d’une strate non marine à l’île Axel Heiberg, dans l’Extrême-Arctique canadien. Comparativement aux autres dents de plésiosaures, ces spécimens ressemblent beaucoup aux dents d’elasmosaures : leur surface extérieure est lisse et leur surface intérieure est crénelée, les crénulations s’étendant presque jusqu’à la pointe de la dent. Après avoir comparé ces spécimens aux fossiles d’elasmosaures trouvés ailleurs, on a remarqué que les dents trouvées à Axel Heiberg sont les dents de juvéniles. La présence d’un plésiosaure dans une strate non marine de l’île Axel Heiberg vient étayer la suggestion selon laquelle des elasmosaures juvéniles évoluaient souvent dans les milieux dulçaquicoles. La répartition temporelle des spécimens d’Axel Heiberg et d’autres occurrences de l’Extrême-Arctique laissent suggérer que les elasmosaures auraient pu étendre leur parcours au cours d’une période d’extrême chaleur climatique

Correlated enhancement of Hc2 and Jc in carbon nanotube-doped MgB2

The use of MgB2 in superconducting applications still awaits for the development of a MgB2-based material where both current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc is in the range 2.5-10%at depending on field and temperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with extrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT content. The measured Hc2 vs T in all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich et al.Comment: 12 pages, 3 figure

Active Microrheology of Networks Composed of Semiflexible Polymers. II. Theory and comparison with simulations

Building on the results of our computer simulation (ArXiv cond-mat/0503573)we develop a theoretical description of the motion of a bead, embedded in a network of semiflexible polymers, and responding to an applied force. The theory reveals the existence of an osmotic restoring force, generated by the piling up of filaments in front of the moving bead and first deduced through computer simulations. The theory predicts that the bead displacement scales like x ~ t^alfa with time, with alfa=0.5 in an intermediate- and alfa=1 in a long-time regime. It also predicts that the compliance varies with concentration like c^(-4/3) in agreement with experiment.Comment: 18 pages and 2 figure

Shock waves in the dissipative Toda lattice

We consider the propagation of a shock wave (SW) in the damped Toda lattice. The SW is a moving boundary between two semi-infinite lattice domains with different densities. A steadily moving SW may exist if the damping in the lattice is represented by an ``inner'' friction, which is a discrete analog of the second viscosity in hydrodynamics. The problem can be considered analytically in the continuum approximation, and the analysis produces an explicit relation between the SW's velocity and the densities of the two phases. Numerical simulations of the lattice equations of motion demonstrate that a stable SW establishes if the initial velocity is directed towards the less dense phase; in the opposite case, the wave gradually spreads out. The numerically found equilibrium velocity of the SW turns out to be in a very good agreement with the analytical formula even in a strongly discrete case. If the initial velocity is essentially different from the one determined by the densities (but has the correct sign), the velocity does not significantly alter, but instead the SW adjusts itself to the given velocity by sending another SW in the opposite direction.Comment: 10 pages in LaTeX, 5 figures available upon regues

A WZW model based on a non-semi-simple group

We present a conformal field theory which desribes a homogeneous four dimensional Lorentz-signature space-time. The model is an ungauged WZW model based on a central extension of the Poincar\'e algebra. The central charge of this theory is exactly four, just like four dimensional Minkowski space. The model can be interpreted as a four dimensional monochromatic plane wave. As there are three commuting isometries, other interesting geometries are expected to emerge via $O(3,3)$ duality.Comment: 8 pages, phyzzx, IASSNS-HEP-93/61 Texable versio

The establishment of a future NESP dredging research investment framework: NESP TWQ Hub Project 1.9

The purpose of this project was to conduct a facilitated workshop with key researchers and stakeholders to establish priorities for future research investment of the NESP Tropical Water Quality Hub (NESP TWQ) into dredging and disposal of dredged sediments in the GBR. A recent independent review of potential impacts of dredging on the biological values of the Great Barrier Reef (GBR) identified a number of key knowledge gaps that need to be addressed to improve management of dredging1 activities. That review, together with the findings of the currently underway dredging science node of the Western Australian Marine Institution (WAMSI), informed the subsequent prioritisation of research topics to address the most important knowledge gaps

Strong magnetic pair breaking in Mn substituted MgB_2 single crystals

Magnetic ions (Mn) were substituted in MgB_2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, T_c, in Mg_{1-x}Mn_xB_2 has been found to be rapidly suppressed at an initial rate of 10 K/%Mn, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the 3d local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g. with Al or C, suppress T_c much less effectively (e.g. 0.5 K/%Al). The magnitude of the magnetic moment, derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state (S = 1/2). This has been found also in X-ray absorption spectroscopy measurements. Isovalent Mn^{2+} substitution for Mg^{2+} mainly affects superconductivity through spin-flip scattering reducing T_c rapidly and lowering the upper critical field anisotropy H_{c2}^{ab}/H_{c2}^c at T = 0 from 6 to 3.3 (x = 0.88% Mn), while leaving the initial slope dH_{c2}/dT near T_c unchanged for both field orientations.Comment: 9 pages, 9 figure

Resistivity saturation revisited: results from a dynamical mean field theory

We use the dynamical mean field method to study the high-temperature resistivity of electrons strongly coupled to phonons. The results reproduce the qualtiative behavior of the temperature and disorder dependence of the resistivity of the 'A-15' materials, which is commonly described in terms of saturation, but imply that the resistivity does not saturate. Rather, a change in temperature dependence occurs when the scattering becomes strong enough to cause a breakdown of the Migdal approximation.Comment: Minor revisions in response to referee report; latex error corrected so paper prints properl