CARNIVORES PLEISTOCENES EN MÂCONNAIS. EXCuRSION du 16e SYMPOSIUM INTERNATIONAL DE L’OuRS ETDU LION DES CAVERNES (Azé, 2011)

International audienceThe excursion day of the 16th ICBLS provided participants with the opportunity to visit three major palaeontological sites of the Mâconnais: the Azé and Blanot Caves, and the Breccia site of Château, especially rich in big Carnivores, mainly ursids and felids

Exact diagonalization studies of doped Heisenberg spin rings

Motivated by magnetization studies of the "telephon number compound" Sr_14Cu_24O_41 we investigate doped Heisenberg spin rings by means of complete numerical diagonalization of a Heisenberg Hamiltonian that depends parametrically on hole positions. A comparison with experimental magnetization data reveals rather accurate information about the screened electrostatic interaction between the charged holes on the ring which appears to be astonishingly strong.Comment: 3 pages, 2 figures, submitted to JMM

Internal structure and fragmentation kinetics of silica granules

International audienceTo improve the mechanical properties of tires, silica granules can be incorporated into the elastomer as well as carbon black. Ideally, the fragmentation of the granules in the elastomer must be obtained with low mechanical stresses and lead to very small fragments distributed homogeneously in the material. On the other hand, granules must present a sufficient cohesion, in order to avoid the generation of fine particles during handling operations. Thus it appears necessary to control the mechanical strength of granules and the mechanism of their fragmentation. In this experimental study, we investigated the fragmentation of silica granules of 250 microns produced by spray drying. For this, we characterized by granulometry the evolution of the Particle Size Distribution of silica powder in water. The granules were suspended in water and submitted to ultrasounds. This treatment is used to create the fragmentation that occurs by viscous shearing in industrial rubber processing. A core-shell structure, characteristic of granules obtained by atomization process, was observed by SEM. Furthermore, by varying the intensity of mechanical stress, the multi-scale structure of granules was evidenced as well as the existence of different regimes of fragmentation. The kinetics of fragmentation was experimentally followed on two grades of silica that showed significant differences in their behavior during the fragmentation process

Strong Coulomb effects in hole-doped Heisenberg chains

Substances such as the ``telephone number compound'' Sr14Cu24O41 are intrinsically hole-doped. The involved interplay of spin and charge dynamics is a challenge for theory. In this article we propose to describe hole-doped Heisenberg spin rings by means of complete numerical diagonalization of a Heisenberg Hamiltonian that depends parametrically on hole positions and includes the screened Coulomb interaction among the holes. It is demonstrated that key observables like magnetic susceptibility, specific heat, and inelastic neutron scattering cross section depend sensitively on the dielectric constant of the screened Coulomb potential.Comment: 5 pages, 6 figures, to be published in Eur. Phys. J.

Hole-Pairs in a Spin Liquid: Influence of Electrostatic Hole-Hole Repulsion

The stability of hole bound states in the t-J model including short-range Coulomb interactions is analyzed using computational techniques on ladders with up to $2 \times 30$ sites. For a nearest-neighbors (NN) hole-hole repulsion, the two-holes bound state is surprisingly robust and breaks only when the repulsion is several times the exchange $J$. At $\sim 10$ hole doping the pairs break only for a NN-repulsion as large as $V \sim 4J$. Pair-pair correlations remain robust in the regime of hole binding. The results support electronic hole-pairing mechanisms on ladders based on holes moving in spin-liquid backgrounds. Implications in two dimensions are also presented. The need for better estimations of the range and strength of the Coulomb interaction in copper-oxides is remarked.Comment: Revised version with new figures. 4 pages, 5 figure

Tidal, Geological, and Biological Impacts to Humboldt Bay\u27s pH

This research examines factors that control pH in Humboldt Bay – a shallow, tidally-driven estuary in northern California (USA) that supports shellfisheries which are economically important to the state. Time-series data from hydrographic sensors at two Central and Northern California Ocean Observing System (CenCOOS) stations, as well as multiple 2021 undergraduate cruises, were used to understand the role of tides, biological productivity and carbonate dissolution in controlling pH on various timescales. Differences in pH, dissolved oxygen, chlorophyll, and temperature between an in-bay sensor and a coastal sensor indicate that the tidal flux exerts a long term, seasonal control on pH, but biological productivity substantially modifies carbon and oxygen thereby controlling pH on daily and weekly timescales. Sediment samples were also collected from the bay in 2021 to study carbonate dissolution. Sediments were incubated for three days in both stirred and unstirred conditions (to mimic tidal mixing and no tidal mixing respectively) and DO, pH and alkalinity were monitored. For all stirred incubations, large increases in pH and alkalinity suggested considerable carbonate sediment dissolution. When scaled to the bay’s in-situ suspended sediment concentrations, carbonate dissolution may exert a supplementary control on pH at similar time scales as biological productivity, but the magnitude of its effect is less

Spatial and Temporal Variations of Microplastics within Humboldt Bay, California

This study aimed to quantify microplastic (MP) concentration and analyze the spatial and temporal variabilities of the concentrations during the tidal cycle in Humboldt Bay, California. To get an approximation of MP concentration, both water and sediment samples were taken at five different stations, twice during one tidal cycle. Sampling was conducted during two different cruises, on the 19th and 21st of September 2020. The samples were processed in the lab using a density separation procedure and filtration. MP concentrations in the different samples were determined using an average optical microscopy count. Comparison of the water column MP concentrations during ebb and flood tides shows higher concentrations during flood tide, 49.0 particles/L ± 32.37 (flood) vs 34.4 particles/L ± 16.32 (ebb), indicating that MPs are brought into Humboldt Bay from the ocean. The comparison of the MP concentrations during lower energy and higher energy conditions indicates that concentrations in the water column were elevated when there was greater tidal kinetic energy, approximated by the covariance of the measured velocity in North Bay Channel. This result was assumed to be caused by the strong tidal currents stirring up both sediments and the settled MPs into the water column. Due to lower tidal kinetic energy on the sediment sampling cruise day, we could not confirm that assumption. Water samples indicated that MPs are heterogeneously distributed in the bay, with higher concentrations found near the Entrance Channel and lower concentrations found further north in the bay. Sediment samples also indicate a heterogeneous distribution of MPs in the bay, with the lowest concentrations near the Entrance Channel, 15 particles/kg, where high tidal currents inhibit settling of particles

Hidden Symmetries and their Consequences in t2gt_{2g} Cubic Perovskites

The five-band Hubbard model for a $d$ band with one electron per site is a model which has very interesting properties when the relevant ions are located at sites with high (e. g. cubic) symmetry. In that case, if the crystal field splitting is large one may consider excitations confined to the lowest threefold degenerate $t_{2g}$ orbital states. When the electron hopping matrix element ($t$) is much smaller than the on-site Coulomb interaction energy ($U$), the Hubbard model can be mapped onto the well-known effective Hamiltonian (at order $t^{2}/U$) derived by Kugel and Khomskii (KK). Recently we have shown that the KK Hamiltonian does not support long range spin order at any nonzero temperature due to several novel hidden symmetries that it possesses. Here we extend our theory to show that these symmetries also apply to the underlying three-band Hubbard model. Using these symmetries we develop a rigorous Mermin-Wagner construction, which shows that the three-band Hubbard model does not support spontaneous long-range spin order at any nonzero temperature and at any order in $t/U$ -- despite the three-dimensional lattice structure. Introduction of spin-orbit coupling does allow spin ordering, but even then the excitation spectrum is gapless due to a subtle continuous symmetry. Finally we showed that these hidden symmetries dramatically simplify the numerical exact diagonalization studies of finite clusters.Comment: 26 pages, 3 figures, 520 KB, submitted Phys. Rev.

Magnetic anisotropies and general on--site Coulomb interactions in the cuprates

This paper derives the anisotropic superexchange interactions from a Hubbard model for excitations within the copper 3d band and the oxygen 2p band of the undoped insulating cuprates. We extend the recent calculation of Yildirim et al. [Phys. Rev. B {\bf VV}, pp, 1995] in order to include the most general on--site Coulomb interactions (including those which involve more than two orbitals) when two holes occupy the same site. Our general results apply when the oxygen ions surrounding the copper ions form an octahedron which has tetragonal symmetry (but may be rotated as in lanthanum cuprate). For the tetragonal cuprates we obtain an easy--plane anisotropy in good agreement with experimental values. We predict the magnitude of the small in--plane anisoComment: 25 pages, revte