1,361 research outputs found
Nutrient Content of Runoff Water From Rice Fields
Current perception is that nutrient runoff from croplands is a significant contributor to poor water quality in some areas. While extensive research has been conducted to survey and ameliorate this problem for several upland crops, little work has been done to evaluate the problem with flooded rice (Oryza sativa, L.) soils. Since rice production utilizes a major portion of the total irrigation water usage for certain areas, it is important to understand the contribution of rice production to non-point source N and P in surface water. Several production fields were selected to evaluate the concentrations of nutrients in the floodwater at selected distances across the field, including inlet and exit. The fields were evaluated in either 1990, 1991, or 1992 and were managed by the individual rice producer. Water samples were collected from several locations within each field weekly following establishment of the permanent flood and analyzed for inorganic N (NH4-N, No3-N, and NO2-N) and soluble P. The N concentrations in the floodwater normally peaked following N fertilizer application but rapidly declined and remained below 1 mg N L-1. Water management resulted in some variation among locations with respect to the timing and magnitude of these peaks. The P concentrations were usually highest near the well and declined to less than 0.05 mg P L-1 as the water moved across the field. This was attributed to plant uptake, uptake by algae, and sediment deposition. The data indicates that rice fields have the potential to be utilized as a filtration system to reduce the nutrient load of irrigation water similar to constructed wetlands . Use of catfish pondwater, in comparison to well water, resulted in only slightly higher total N and total P levels with higher amounts of the nutrients in the organic form. Although the P levels were high enough to potentially contribute to eutrophication of surface water, the water exiting the field was lower than at the entry point irrespective of the source. Also, the total P (organic + inorganic) concentration was less than 0.05 mg P L- 1
Control of Superconducting Correlations in High-Tc Cuprates
A strategy to enhance d-wave superconducting correlations is proposed based
on our numerical study for correlated electron models for high-Tc cuprates. We
observe that the pairing is enhanced when the single-electron level around
(pi,0) is close to the Fermi level E_F, while the d-wave pairing interaction
itself contains elements to disfavor the pairing due to shift of the
(pi,0)-level. Angle-resolved photoemission results in the cuprates are
consistently explained in the presence of the d-wave pairing interaction. Our
proposal is the tuning of the (pi,0)-level under the many-body effects to E_F
by optimal design of band structure.Comment: 4 pages, 6 eps figure
Superconducting Gap Anisotropy and Quasiparticle Interactions: a Doping Dependent ARPES Study
Comparing ARPES measurements on Bi2212 with penetration depth data, we show
that a description of the nodal excitations of the d-wave superconducting state
in terms of non-interacting quasiparticles is inadequate, and we estimate the
magnitude and doping dependence of the Landau interaction parameter which
renormalizes the linear T contribution to the superfluid density. Furthermore,
although consistent with d-wave symmetry, the gap with underdoping cannot be
fit by the simple coskx-cosky form, which suggests an increasing importance of
long range interactions as the insulator is approached.Comment: 4 pages, 3 eps figs, manuscript and Fig. 3 significantly revise
Electron momentum distribution in underdoped cuprates
We investigate the electron momentum distribution function (EMD) in a weakly
doped two-dimensional quantum antiferromagnet (AFM) as described by the t-J
model. Our analytical results for a single hole in an AFM based on the
self-consistent Born approximation (SCBA) indicate an anomalous momentum
dependence of EMD showing 'hole pockets' coexisting with a signature of an
emerging large Fermi surface. The position of the incipient Fermi surface and
the structure of the EMD is determined by the momentum of the ground state. Our
analysis shows that this result remains robust in the presence of next-nearest
neighbor hopping terms in the model. Exact diagonalization results for small
clusters are with the SCBA reproduced quantitatively.Comment: 5 pages, submitted to PR
Geology and geochronology of the Two-Thirty prospect, Northparkes district, NSW
The Northparkes district, central New South Wales, hosts several economic Cu–Au deposits associated with discrete, thin, porphyry intrusive complexes emplaced in the Late Ordovician during formation of the Macquarie Arc. The recently discovered Two-Thirty Cu–Au–(Mo) prospect is a mineralised magmatic–hydrothermal breccia complex that is hosted by the moderately east-dipping Goonumbla Volcanic Complex on the western limb of the Milpose Syncline ∼15 km south of the Northparkes porphyry district. Generation of the magmatic–hydrothermal breccia complex is interpreted to be related to the 448.0 ± 4.4 Ma emplacement of the Two-Thirty porphyry. However, Re–Os dating of molybdenite from the breccia complex indicates a potential for a ca 440 Ma mineralising event that has similar timing to economic porphyry mineralisation in the Northparkes district. The discovery of the Two-Thirty prospect has important implications for exploration in the Northparkes district and the broader Macquarie Arc. Two-Thirty is only the second known occurrence of magmatic-hydrothermal breccia-hosted mineralisation discovered within the Macquarie Arc, with the other being Cadia Quarry. Mineralisation at Two-Thirty is potentially older than the Northparkes and Cadia deposits, and younger than the epithermal and calc-alkaline deposits at Cowal, Marsden and Ridgeway.KEY POINTS: The Two-Thirty is a polyphase magmatic–hydrothermal breccia complex that hosts Cu–Au (Mo). The Two-Thirty is the first significant breccia-hosted mineralisation found in the Northparkes district. U–Pb zircon crystallisation ages of the causative intrusion at Two-Thirty pre-date mineralisation at Northparkes. Re–Os dates of molybdenite from the Two-Thirty breccia complex are coeval with syn-mineralisation at Northparkes, supporting the model of periodic release of melts and fluids from underlying magma chambers during the formation of porphyry mineralisation in the Northparkes district.This research is funded by Australian Research Council sponsors of the
Lachlan ARC Linkage Project ‘LP160100483’ CMOC-Northparkes, Rio
Tinto, Evolution Mining, IMEx Consulting, Heron Resources, Sandfire
Resources NL, New South Resources, AngloGold Ashanti, Alkane Resources, Geoscience Australia, The University of Tasmania, Australian
National University, University of Melbourne, CCFS, Curtin University,
the New South Wales, Tasmanian and Victorian state governments
Spectral functions, Fermi surface and pseudogap in the t-J model
Spectral functions within the generalized t-J model as relevant to cuprates
are analyzed using the method of equations of motion for projected fermion
operators. In the evaluation of the self energy the decoupling of spin and
single-particle fluctuations is performed. It is shown that in an undoped
antiferromagnet (AFM) the method reproduces the selfconsistent Born
approximation. For finite doping with short range AFM order the approximation
evolves into a paramagnon contribution which retains large incoherent
contribution in the hole part of the spectral function as well as the
hole-pocket-like Fermi surface at low doping. On the other hand, the
contribution of (longitudinal) spin fluctuations, with the coupling mostly
determined predominantly by J and next-neighbor hopping t', is essential for
the emergence of the pseudogap. The latter shows at low doping in the effective
truncation of the large Fermi surface, reduced electron density of states and
at the same time quasiparticle density of states at the Fermi level.Comment: RevTex, 13 pages, 11 figures (5 color
Dispersion of Ordered Stripe Phases in the Cuprates
A phase separation model is presented for the stripe phase of the cuprates,
which allows the doping dependence of the photoemission spectra to be
calculated. The idealized limit of a well-ordered array of magnetic and charged
stripes is analyzed, including effects of long-range Coulomb repulsion.
Remarkably, down to the limit of two-cell wide stripes, the dispersion can be
interpreted as essentially a superposition of the two end-phase dispersions,
with superposed minigaps associated with the lattice periodicity. The largest
minigap falls near the Fermi level; it can be enhanced by proximity to a (bulk)
Van Hove singularity. The calculated spectra are dominated by two features --
this charge stripe minigap plus the magnetic stripe Hubbard gap. There is a
strong correlation between these two features and the experimental
photoemission results of a two-peak dispersion in LaSrCuO, and
the peak-dip-hump spectra in BiSrCaCuO. The
differences are suggestive of the role of increasing stripe fluctuations. The
1/8 anomaly is associated with a quantum critical point, here expressed as a
percolation-like crossover. A model is proposed for the limiting minority
magnetic phase as an isolated two-leg ladder.Comment: 24 pages, 26 PS figure
Magnetic neutron scattering in hole doped cuprate superconductors
A review is presented of the static and dynamic magnetic properties of
hole-doped cuprate superconductors measured with neutron scattering. A wide
variety of experiments are described with emphasis on the monolayer
La_{2-x}(Sr,Ba)_{x}CuO_{4} and bilayer YBa_{2}Cu_{3}O_{6+x} cuprates. At zero
hole doping, both classes of materials are antiferromagnetic insulators with
large superexchange constants of J > 100 meV. For increasing hole doping, the
cuprates become superconducting at a critical hole concentration of
x_{c}=0.055. The development of new instrumentation at neutron beam sources
coupled with the improvement in materials has lead to a better understanding of
these materials and the underlying spin dynamics over a broad range of hole
dopings. We will describe how the spin dispersion changes across the insulating
to superconducting boundary as well as the static magnetic properties which are
directly coupled with the superconductivity. Experiments directly probing the
competing magnetic and superconducting order parameters involving magnetic
fields, impurity doping, and structural order will be examined. Correlations
between superconductivity and magnetism will also be discussed.Comment: 14 pages, 18 figures. To be published in Journal of the Physical
Society of Japa
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
Azimuthal anisotropy and correlations in the hard scattering regime at RHIC
Azimuthal anisotropy () and two-particle angular correlations of high
charged hadrons have been measured in Au+Au collisions at
=130 GeV for transverse momenta up to 6 GeV/c, where hard
processes are expected to contribute significantly. The two-particle angular
correlations exhibit elliptic flow and a structure suggestive of fragmentation
of high partons. The monotonic rise of for GeV/c is
consistent with collective hydrodynamical flow calculations. At \pT>3 GeV/c a
saturation of is observed which persists up to GeV/c.Comment: As publishe
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