2,286 research outputs found
Siderite concretions from nonmarine shales (Westphalian A) of the Pennines, England: Controls on their growth and composition
Back-scattered electron microscopy has been used to examine the microstructure of nonmarine-shale-hosted siderite concretions. The concretions are composed of 50-100 mu m, zoned crystallites, which exhibit no noticeable center-to-edge variation within any individual concretion. This indicates that siderite crystallites nucleated at virtually the same time across the entire concretion and that the concretions did not grow by radial addition of siderite layers around a central nucleus. Further siderite precipitation took place by crystal growth onto the nuclei. The total proportion of siderite in any part of the concretion bears no simple relationship to the porosity of the enclosing shale at the time of precipitation, and growth by passive precipitation in pore space is unlikely. Integration of microprobe data with bulk mineral-chemical and stable-isotope data suggests that the siderite crystallites are composed of an Fe-Mn-rich end member with a delta(13)C value of similar to +10 parts per thousand and a Mg-Ca-rich end member with a delta(13)C value of similar to 0 parts per thousand to -5 parts per thousand. The mineral-chemical and stable-isotope compositions of these concretions resulted from microbially mediated processes operating close (< 10 m) to the sediment-water interface, during methanogenesis. Methanogenesis can generate low-delta(13)C as well as high-delta(13)C carbonate cements, hence deep-burial diagenetic reactions, such as decarboxylation of organic matter, need not be invoked to generate solutes for siderite precipitation
Numerical studies of the two- and three-dimensional gauge glass at low temperature
We present results from Monte Carlo simulations of the two- and
three-dimensional gauge glass at low temperature using the parallel tempering
Monte Carlo method. Our results in two dimensions strongly support the
transition being at T_c=0. A finite-size scaling analysis, which works well
only for the larger sizes and lower temperatures, gives the stiffness exponent
theta = -0.39 +/- 0.03. In three dimensions we find theta = 0.27 +/- 0.01,
compatible with recent results from domain wall renormalization group studies.Comment: 7 pages, 10 figures, submitted to PR
Nature of the Spin-glass State in the Three-dimensional Gauge Glass
We present results from simulations of the gauge glass model in three
dimensions using the parallel tempering Monte Carlo technique. Critical
fluctuations should not affect the data since we equilibrate down to low
temperatures, for moderate sizes. Our results are qualitatively consistent with
earlier work on the three and four dimensional Edwards-Anderson Ising spin
glass. We find that large scale excitations cost only a finite amount of energy
in the thermodynamic limit, and that those excitations have a surface whose
fractal dimension is less than the space dimension, consistent with a scenario
proposed by Krzakala and Martin, and Palassini and Young.Comment: 5 pages, 7 figure
Monte Carlo simulations of the four-dimensional XY spin glass at low temperatures
We report results for simulations of the four-dimensional XY spin glass using
the parallel tempering Monte Carlo method at low temperatures for moderate
sizes. Our results are qualitatively consistent with earlier work on the
three-dimensional gauge glass as well as three- and four-dimensional
Edwards-Anderson Ising spin glass. An extrapolation of our results would
indicate that large-scale excitations cost only a finite amount of energy in
the thermodynamic limit. The surface of these excitations may be fractal,
although we cannot rule out a scenario compatible with replica symmetry
breaking in which the surface of low-energy large-scale excitations is space
filling.Comment: 6 pages, 8 figure
Dry matter uptake and digestion of alfalfa harvested at sunset and sunrise
The preference exhibited by animals in selecting one feed over another is important only if the preferred diet is consumed daily in larger quantities, digested to a greater extent, or both. Six alfalfa (Medicago sativa L.) hays were harvested in pairs at sunset (PM) and sunrise (AM) on consecutive days at three harvest dates. A previous study of these hays demonstrated differences in ruminant preference favoring PM harvests. This study evaluated the effects of time of cutting and harvest date on voluntary DMI and nutrient digestibility. The hays were field-cured, baled, and chopped before evaluation for intake and digestibility. Studies were conducted for sheep (Ovis aries), goats (Capra hircus), and cattle (Bos taurus). Goats, but not steers or sheep, demonstrated differences in nutrient digestibility between PM- and AM-cut hays. Goats consumed more PM than AM hay (2.97 vs. 2.83 kg/100 kg of BW; P = 0.07) and digested it to a greater extent (0.710 vs. 0.696; P = 0.03), resulting in greater digestible DMI (2.11 vs. 1.97 kg/100 kg of BW; P = 0.03). Sheep consumed (mean = 2.52 kg/100 kg of BW; P = 0.59) and digested (mean = 0.681; P = 0.25) PM- and AM-cut hays similarly. Steers consumed larger quantities of PM-than AM-cut hay (2.90 vs. 2.62 kg/100 kg of BW; P = 0.11), but digestion did not differ with cutting time (mean = 0.660; P = 0.75). Difference values (composition of fed hay minus composition of orts) indicated that sheep and goats selected from the feed offered similarly, whereas steers selected differently. Difference values for CP averaged 94 and 101 g/kg for goats and sheep and 32 g/kg for steers (P < 0.01), and difference values for NDF averaged 185 and 196 g/kg for goats and sheep and 73 g/kg for steers (P <or= 0.01). Steer DMI and digestible DMI were associated with preference (r = +0.83, P <or= 0.05; and r = +0.89, P <or= 0.05) and with coordinates for preference criteria (dimension 1; r = +0.90, P <or= 0.05; and r = +0.89, P <or= 0.05) from a previous preference trial. Intake and digestion responses for goats and sheep showed no relationship with the previous preference trial measurements. For cattle and goats, the management strategy of mowing in the afternoon seems to take advantage of small, but influential diurnal changes in the soluble carbohydrate fraction and offers the potential to improve forage quality
Calculation of ground states of four-dimensional +or- J Ising spin glasses
Ground states of four-dimensional (d=4) EA Ising spin glasses are calculated
for sizes up to 7x7x7x7 using a combination of a genetic algorithm and
cluster-exact approximation. The ground-state energy of the infinite system is
extrapolated as e_0=-2.095(1). The ground-state stiffness (or domain wall)
energy D is calculated. A D~L^{\Theta} behavior with \Theta=0.65(4) is found
which confirms that the d=4 model has an equilibrium spin-glass-paramagnet
transition for non-zero T_c.Comment: 5 pages, 3 figures, 31 references, revtex; update of reference
Bose-Einstein condensation and superfluidity of dilute Bose gas in a random potential
We develop the dilute Bose gas model with random potential in order to
understand the Bose system in random media such as 4He in porous glass. Using
the random potential taking account of the pore size dependence, we can compare
quantitatively the calculated specific heat with the experimental results,
without free parameters. The agreement is excellent at low temperatures, which
justifies our model. The relation between Bose condensation and superfluidity
is discussed. Our model can predict some unobserved phenomena in this system.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
Disordered Bosons: Condensate and Excitations
The disordered Bose Hubbard model is studied numerically within the
Bogoliubov approximation. First, the spatially varying condensate wavefunction
in the presence of disorder is found by solving a nonlinear Schrodinger
equation. Using the Bogoliubov approximation to find the excitations above this
condensate, we calculate the condensate fraction, superfluid density, and
density of states for a two-dimensional disordered system. These results are
compared with experiments done with adsorbed in porous media.Comment: RevTeX, 26 pages and 10 postscript figures appended (Figure 9 has
three separate plots, so 12 postcript files altogether
Nature of the vortex-glass order in strongly type-II superconductors
The stability and the critical properties of the three-dimensional
vortex-glass order in random type-II superconductors with point disorder is
investigated in the unscreened limit based on a lattice {\it XY} model with a
uniform field. By performing equilibrium Monte Carlo simulations for the system
with periodic boundary conditions, the existence of a stable vortex-glass order
is established in the unscreened limit. Estimated critical exponents are
compared with those of the gauge-glass model.Comment: Error in the reported value of the exponent eta is correcte
Zero Temperature Glass Transition in the Two-Dimensional Gauge Glass Model
We investigate dynamic scaling properties of the two-dimensional gauge glass
model for the vortex glass phase in superconductors with quenched disorder.
From extensive Monte Carlo simulations we obtain static and dynamic finite
size scaling behavior, where the static simulations use a temperature exchange
method to ensure convergence at low temperatures. Both static and dynamic
scaling of Monte Carlo data is consistent with a glass transition at zero
temperature. We study a dynamic correlation function for the superconducting
order parameter, as well as the phase slip resistance. From the scaling of
these two functions, we find evidence for two distinct diverging correlation
times at the zero temperature glass transition. The longer of these time scales
is associated with phase slip fluctuations across the system that lead to
finite resistance at any finite temperature, while the shorter time scale is
associated with local phase fluctuations.Comment: 8 pages, 10 figures; v2: some minor correction
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