Interfaces, strings, and a soft mode in the square lattice quantum dimer model

The quantum dimer model on the square lattice is a U(1) gauge theory that addresses aspects of the physics of high-Tc superconductors. Using a quantum Monte Carlo method, we show that the theory exists in a confining columnar valence bond solid phase. The interfaces separating distinct columnar phases display plaquette order, which, however, is not realized as a bulk phase. Static “electric” charges are confined by flux tubes that consist of multiple strands, each carrying a fractionalized flux ¼. A soft pseudo-Goldstone mode (which becomes exactly massless at the Rokhsar-Kivelson point) extends deep into the columnar phase, with potential implications for high-Tc physics.

Fourier Series by Least Square Minimization

An introduction to Fourier Series based on the minimization of the least square error between an approximate series representation and the exact function.

Statue de Berlioz du Square Bintmille, à Paris et à La Cote-Saint-André

Signatur des Originals: S 36/F03013

Decomposition of $\rm R\times C$ contingency table chi -square: Application to binned DNA fragment size data and population structure analysis

The purpose of this research is to develop a new statistical method to determine the minimum set of rows (R) in a R x C contingency table of discrete data that explains the dependence of observations. The statistical power of the method will be empirically determined by computer simulation to judge its efficiency over the presently existing methods. The method will be applied to data on DNA fragment length variation at six VNTR loci in over 72 populations from five major racial groups of human (total sample size is over 15,000 individuals; each sample having at least 50 individuals). DNA fragment lengths grouped in bins will form the basis of studying inter-population DNA variation within the racial groups are significant, will provide a rigorous re-binning procedure for forensic computation of DNA profile frequencies that takes into account intra-racial DNA variation among populations. ^

Gas hydrates at DSDP Leg 84

On DSDP Leg 84, gas hydrates were found at three sites (565, 568, and 570) and were inferred, on the basis of inorganic and organic geochemical evidence, to be present at two sites (566 and 569); no evidence for gas hydrates was observed at Site 567. Recovered gas hydrates appeared as solid pieces of white, icelike material occupying fractures in mudstone or as coarse-grained sediment in which the pore space exhibited rapid outgassing. Also a 1.05-m-long core of massive gas hydrate was obtained at Site 570. Downhole logging indicated that this hydrate was actually 3 to 4 m thick. Measurements of the amount of methane released during the decomposition of these recovered samples clearly showed that gas hydrates had been found. The distribution of evolved hydrocarbon gases indicated that Structure I gas hydrates were present because of the apparent inclusion of methane and ethane and exclusion of propane and higher molecular weight gases. The water composing the gas hydrates was fresh, having chlorinities ranging from 0.5 to 3.2 per mil. At Sites 565, 568, and 570, where gas hydrates were observed, the chlorinity of pore water squeezed from the sediment decreased with sediment depth. The chlorinity profiles may indicate that gas hydrates can often occur finely dispersed in sediments but that these gas hydrates are not recovered because they do not survive the drilling and recovery process. Methane in the gas hydrates found on Leg 84 was mainly derived in situ by biogenic processes, whereas the accompanying small amounts of ethane likely resulted from low-temperature diagenetic processes. Finding gas hydrates on Leg 84 expands observations made earlier on Leg 66 and particularly Leg 67. The results of all of these legs show that gas hydrates are common in landward slope sediments of the Middle American Trench from Mexico to Costa Rica.

Gas hydrates of the Gulf of Mexico

Gas hydrates were recovered from eight sites on the Louisiana slope of the Gulf of Mexico. The gas hydrate discoveries ranged in water depths from 530 to 2400 m occurring as small to medium sized (0.5-50 mm) nodules, interspersed layers (1-10 mm thick) or as solid masses (> 150 mm thick). The hydrates have gas:fluid ratios as high as 170:1 at STP, C1/(C2 + C3) ratios ranging from 1.9 to > 1000 and d13C ratios from -43 to -71 per mil. Thermogenic gas hydrates are associated with oil-stained cores containing up to 7% extractable oil exhibiting moderate to severe biodegradation. Biogenic gas hydrates are also associated with elevated bitumen levels (10-700 ppm). All gas hydrate associated cores contain high percentages (up to 65%) of authigenic, isotopically light carbonate. The hydrate-containing cores are associated with seismic "wipeout" zones indicative of gassy sediments. Collapsed structures, diapiric crests, or deep faults on the flanks of diapirs appear to be the sites of the shallow hydrates.

Geochemistry of gas hydrates of DSDP Hole 84-570

During Deep Sea Drilling Project (DSDP) Leg 84 a core 1 m long and 6 cm in diameter of massive gas hydrate was unexpectedly recovered at Site 570 in upper slope sediment of the Middle America Trench offshore of Guatemala. This core contained only 5-7% sediment, the remainder being the solid hydrate composed of gas and water. Samples of the gas hydrate were decomposed under controlled conditions in a closed container maintained at 4°C. Gas pressure increased and asymptotically approached the equilibrium decomposition pressure for an ideal methane hydrate, CH4.5-3/4H2O, of 3930 kPa and approached to this pressure after each time gas was released, until the gas hydrate was completely decomposed. The gas evolved during hydrate decomposition was 99.4% methane, ~0.2% ethane, and ~0.4% CO2. Hydrocarbons from propane to heptane were also present, but in concentrations of less than 100 p.p.m. The carbon-isotopic composition of methane was -41 to -44 per mil, relative to PDB standard. The observed volumetric methane/water ratio was 64 or 67, which indicates that before it was stored and analyzed, the gas hydrate probably had lost methane. The sample material used in the experiments was likely a mixture of methane hydrate and water ice. Formation of this massive gas hydrate probably involved the following processes: (i) upward migration of gas and its accumulation in a zone where conditions favored the growth of gas hydrates, (ii) continued, unusually rapid biological generation of methane, and (iii) release of gas from water solution as pressure decreased due to sea level lowering and tectonic uplift.

Sedimentology and gas hydrates of IODP Expedition 311 samples

Expedition 311 of the Integrated Ocean Drilling Program (IODP) to northern Cascadia recovered gas-hydrate bearing sediments along a SW-NE transect from the first ridge of the accretionary margin to the eastward limit of gas-hydrate stability. In this study we contrast the gas gas-hydrate distribution from two sites drilled ~ 8 km apart in different tectonic settings. At Site U1325, drilled on a depositional basin with nearly horizontal sedimentary sequences, the gas-hydrate distribution shows a trend of increasing saturation toward the base of gas-hydrate stability, consistent with several model simulations in the literature. Site U1326 was drilled on an uplifted ridge characterized by faulting, which has likely experienced some mass wasting events. Here the gas hydrate does not show a clear depth-distribution trend, the highest gas-hydrate saturation occurs well within the gas-hydrate stability zone at the shallow depth of ~ 49 mbsf. Sediments at both sites are characterized by abundant coarse-grained (sand) layers up to 23 cm in thickness, and are interspaced within fine-grained (clay and silty clay) detrital sediments. The gas-hydrate distribution is punctuated by localized depth intervals of high gas-hydrate saturation, which preferentially occur in the coarse-grained horizons and occupy up to 60% of the pore space at Site U1325 and > 80% at Site U1326. Detailed analyses of contiguous samples of different lithologies show that when enough methane is present, about 90% of the variance in gas-hydrate saturation can be explained by the sand (> 63 µm) content of the sediments. The variability in gas-hydrate occupancy of sandy horizons at Site U1326 reflects an insufficient methane supply to the sediment section between 190 and 245 mbsf.