The overlap operator as a continued fraction

We use a continued fraction expansion of the sign-function in order to obtain a five dimensional formulation of the overlap lattice Dirac operator. Within this formulation the inverse of the overlap operator can be calculated by a single Krylov space method where nested conjugate gradient procedures are avoided. We show that the five dimensional linear system can be made well conditioned using equivalence transformations on the continued fractions. This is of significant importance when dynamical overlap fermions are simulated.Comment: 3 pages, 1 figure, talk presented by U. Wenger at Lattice2001(chiral

Optical properties of pyrochlore oxide Pb2Ru2O7−δPb_{2}Ru_{2}O_{7-{\delta}}

We present optical conductivity spectra for $Pb_{2}Ru_{2}O_{7-{\delta}}$ single crystal at different temperatures. Among reported pyrochlore ruthenates, this compound exhibits metallic behavior in a wide temperature range and has the least resistivity. At low frequencies, the optical spectra show typical Drude responses, but with a knee feature around 1000 \cm. Above 20000 \cm, a broad absorption feature is observed. Our analysis suggests that the low frequency responses can be understood from two Drude components arising from the partially filled Ru $t_{2g}$ bands with different plasma frequencies and scattering rates. The high frequency broad absorption may be contributed by two interband transitions: from occupied Ru $t_{2g}$ states to empty $e_{g}$ bands and from the fully filled O 2p bands to unoccupied Ru $t_{2g}$ states.Comment: 4 pages, 6 figure

Impact of vertical mixing on sea surface pCO2 in temperate seasonally stratified shelf seas

A key parameter in determining the exchange of CO2 across the ocean-atmosphere interface is the sea surface partial pressure of carbon dioxide (pCO2). Temperate seasonally stratified shelf seas represent a significant sink for atmospheric CO2. Here an analytical model is used to quantify the impact of vertical mixing across the seasonal thermocline on pCO2. The model includes the impacts of the resultant dissolved inorganic carbon, heat, salt, and alkalinity fluxes on the solubility of CO2 and the effect of the inorganic carbon sink created by the primary production fuelled by the flux of limiting nutrient. The results indicate that diapycnal mixing drives a modest but continuous change in pCO2 of order 1–10 µatm d−1. In quantifying the individual impacts of the fluxes of the different parameters, we find that the impact of the fluxes of DIC and nitrate fluxes dominate. In consequence, both the direction and magnitude of the change in pCO2 are strongly dependent on the C:N uptake ratio in primary production. While the smaller impacts of the heat and salt fluxes tend to compensate for each other at midshelf locations, the heat flux dominates close to the shelf break. The analysis highlights the importance of the accurate parameterization of the C:N uptake ratio, the surface-mixed layer depth, and the TKE dissipation rate within the seasonal thermocline in models to be used to predict the air-sea exchange of carbon dioxide in these regimes. The results implicate storms as key periods of pCO2 perturbation

Forage type influences milk yield and ruminal responses to wheat adaptation in late-lactation dairy cows

peer-reviewedThe effects of different wheat adaptation strategies on ruminal fluid pH, dry matter intake (DMI) and energy-corrected milk (ECM) were measured in 28 late-lactation dairy cows. Cows were fed either perennial ryegrass (PRG) hay or alfalfa hay and had no previous wheat adaptation. Wheat was gradually substituted for forage in 3 even increments, over 6 or 11 d, until wheat made up 40% of DMI (∼8 kg of dry matter/cow per day). We found no differences in DMI between adaptation strategies (6 or 11 d) within forage type; however, cows fed alfalfa hay consumed more overall and produced more ECM. The rate of ruminal pH decline after feeding, as well as the decrease in mean, minimum, and maximum ruminal pH with every additional kilogram of wheat was greater for cows fed alfalfa hay. Cows fed alfalfa hay and on the 6-d adaptation strategy had the lowest mean and minimum ruminal fluid pH on 3 consecutive days and were the only treatment group to record pH values below 6.0. Despite ruminal pH declining to levels typically considered low, no other measured parameters indicated compromised fermentation or acidosis. Rather, cows fed alfalfa hay and adapted to wheat over 6 d had greater ECM yields than cows on the 11-d strategy. This was due to the 6-d adaptation strategy increasing the metabolizable energy intake in a shorter period than the 11-d strategy, as substituting wheat for alfalfa hay caused a substantial increase in the metabolizable energy concentration of the diet. We found no difference in ECM between adaptation strategies when PRG hay was fed, as there was no difference in metabolizable energy intake. The higher metabolizable energy concentration and lower intake of the PRG hay meant the increase in metabolizable energy intake with the substitution of wheat was less pronounced for cows consuming PRG hay compared with alfalfa hay. Neither forage type nor adaptation strategy affected time spent ruminating. The higher intakes likely contributed to the lower ruminal pH values from the alfalfa hay treatments. However, both forages allowed the rumen contents to resist the large declines in ruminal pH typically seen during rapid grain adaptation. Depending on the choice of base forage, rapid grain introduction may not result in poor adaptation. In situations where high-energy grains are substituted for a low-energy, high-fiber basal forage, rapid introduction could prove beneficial over gradual strategies

A study of cultivation and sowing systems for cereals.

End of Project ReportDue to the pressure on cereal margins growers are seeking ways of reducing their costs of production. Reduced cultivation may provide cheaper and faster crop establishment, but in the past has been subject to problems with (poor crop establishment) weeds and soil compaction. With modern cultivation and sowing technology and methodology it may be possible to overcome these difficulties. In addition reduced cultivation is considered to be more environmentally friendly and sustainable than the conventional plough-based system. With these considerations in mind it was decided to start an experiment in autumn 2000 to compare the conventional system (PL) with reduced cultivation (RC) for crop establishment, and to examine the effect of incorporating the straw into then soil or baling and removing it. The treatments were assessed on winter wheat, and winter and spring barley. The parameters assessed included – crop establishment, yield and quality effects on disease levels, invertebrate pests and beneficials (aphids, slugs, earthworms, beetles), and soil conditions. The work reported here was conducted on four trial sites at Oak Park, Carlow and Knockbeg, Co. Laois from 2000 to 2003. the objectives were to establish the efficacy of RC compared with PL and to its effects on the parameters outlined above. Plant establishment was lower after RC in most of the experiments. Winter wheat yields were good irrespective of the number of plants established and there were no significant differences between cultivation or straw disposal method. The relative yields of the PL and RC treatments in winter barley varied over the three years; in 2001 there was no difference, in 2002 PL yielded 1 t/ha more than RC, while in 2003 the PL area produced 2 t/ha extra. Spring barley yields were similar on the PL and RC treatments. Broadleaved weeds were not a problem on the PL or RC treatments but there were more annual grass weeds on the RC plots. This was particularly serious in the winter barley experiments where sterile brome (Bromus sterilis) had a big effect on yield by 2003. annual meadow grass (Poa annua) was a problem in both PL and RC areas if not controlled by timely herbicide application; this was particularly so on the headland areas in the spring barley field. Disease assessment on the winter wheat trail showed no significant differences in Take-all or Eyespot levels between cultivation or straw disposal methods, although Take-all levels tended to be lower on the RC plots. Soil strength as measured by cone penetrometer and shear vane was higher on the RC treatments. In the winter wheat there was no difference in slug numbers between any of the treatments. Leaf damage by slugs increased in RC relative to PL but not significantly, in 2003. No below ground slug damage was found. Earthworm numbers in the winter barley increased significantly on the RC treatment relative to the PL as the study progressed. Straw incorporation increased earthworm numbers on both PL and RC treatments. Allolobophora chlorotia was the most common species in 2004. In the winter wheat the RC and straw incorporation increased earthworm numbers. Cultivation method had a greater effect on ground beetle numbers than method of straw disposal. Large species (e.g. Pterostichies malanarius) were favoured by RC while smaller species (e.g. Bembidon species) were more numerous in the PL plots. Molecular studies on virus diseases, vectors and vector-predators are underway in progress. In the invertebrate pests and beneficials investigations the RC winter barley had 11% fewer aphids and 27% less BYDV than the PL. Incorporating straw reduced aphid numbers and virus incidence by 36% and 32% respectively; the comparable values for the PL + straw treatment were 15% and 15%. In the winter wheat there was less BYDV in the RC plots than the PL + less were the straw was incorporated than where the straw was removed. Aphid infestation of wheat ears was low in the three years of the experiment. The insecticide seed treatment imidacloprid significantly reduced aphid numbers and virus incidence in winter barley but was less effective than a single insecticide spray in controlling the disease. The seed treatment was more effective in controlling BYDV in RC than in the PL plots. Estimations of slug numbers in the winter barley showed that the dominant species was the grey field slug (Deroceras reticulatum). Slug numbers increased significantly on all treatments between 2001 and 2004. In November 2004, slugs were significantly more numerous in the RC treatments than in the PL areas. Leaf damage was proportional to slug populations, but the slugs did not damage the barely seed or reduce plant populations in continuous winter barley under RC. Root and stem diseases (Take-all and Eyespot) were not nay worse under RC than after PL. In fact there was a tendency towards lower disease levels on the RC areas and there was significantly less sharp eyespot on the RC treatment

Robust paramagnetism in Bi2-xMxRu2O7 (M=Mn,Fe,Co,Ni,Cu) pyrochlore

We report physical property characterization of Bi2-xMxRu2O7 pyrochlores, including magnetic suseptibility, resistivity, and Seebeck coefficients. The solid solution exists up to x=0.5 for (M=Cu,Ni,Co) and up to x=0.1 for (M=Fe,Mn). None of the doped materials exhibit ferromagnetism or any localized ruthenium moment behavior. Instead we find the Ru-O and Bi-O sublattices to be essentially independent, with any magnetism resulting from the unpaired transition metal dopant spins. Cobalt substitution for bismuth results in localized Co{2+}, and low temperature spin-glass transitions in several cases. Nickel moments on the pyrochlore lattice display properties intermediate to localized and itinerant. Finally, copper doping results in only an enhancement of the Pauli metallic density of states.Comment: submitted, Phys. Rev.

Grassmann Variables and the Jaynes-Cummings Model

This paper shows that phase space methods using a positive P type distribution function involving both c-number variables (for the cavity mode) and Grassmann variables (for the two level atom) can be used to treat the Jaynes-Cummings model. Although it is a Grassmann function, the distribution function is equivalent to six c-number functions of the two bosonic variables. Experimental quantities are given as bosonic phase space integrals involving the six functions. A Fokker-Planck equation involving both left and right Grassmann differentiation can be obtained for the distribution function, and is equivalent to six coupled equations for the six c-number functions. The approach used involves choosing the canonical form of the (non-unique) positive P distribution function, where the correspondence rules for bosonic operators are non-standard and hence the Fokker-Planck equation is also unusual. Initial conditions, such as for initially uncorrelated states, are used to determine the initial distribution function. Transformations to new bosonic variables rotating at the cavity frequency enables the six coupled equations for the new c-number functions (also equivalent to the canonical Grassmann distribution function) to be solved analytically, based on an ansatz from a 1980 paper by Stenholm. It is then shown that the distribution function is the same as that determined from the well-known solution based on coupled equations for state vector amplitudes of atomic and n-photon product states. The treatment of the simple two fermion mode Jaynes-Cummings model is a useful test case for the future development of phase space Grassmann distribution functional methods for multi-mode fermionic applications in quantum-atom optics.Comment: 57 pages, 0 figures. Version

Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory

We use first principles density functional theory to calculate electronic and magnetic properties of TiF3 using the full potential linearized augmented plane wave method. The LDA approximation predicts a fully saturated ferromagnetic metal and finds degenerate energy minima for high and low symmetry structures. The experimentally observed Jahn-Teller phase transition at Tc=370K can not be driven by the electron-phonon interaction alone, which is usually described accurately by LDA. Electron correlations beyond LDA are essential to lift the degeneracy of the singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are important, the direction of the t2g-level splitting is determined by the dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic insulator with an orbitally ordered ground state. The input parameters U=8.1 eV and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on the TiF$_6^{2-}$ ion using the molecular NRLMOL code. We estimate the Heisenberg exchange constant for spin-1/2 on a cubic lattice to be approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.

Sequential design of computer experiments for the estimation of a probability of failure

This paper deals with the problem of estimating the volume of the excursion set of a function $f:\mathbb{R}^d \to \mathbb{R}$ above a given threshold, under a probability measure on $\mathbb{R}^d$ that is assumed to be known. In the industrial world, this corresponds to the problem of estimating a probability of failure of a system. When only an expensive-to-simulate model of the system is available, the budget for simulations is usually severely limited and therefore classical Monte Carlo methods ought to be avoided. One of the main contributions of this article is to derive SUR (stepwise uncertainty reduction) strategies from a Bayesian-theoretic formulation of the problem of estimating a probability of failure. These sequential strategies use a Gaussian process model of $f$ and aim at performing evaluations of $f$ as efficiently as possible to infer the value of the probability of failure. We compare these strategies to other strategies also based on a Gaussian process model for estimating a probability of failure.Comment: This is an author-generated postprint version. The published version is available at http://www.springerlink.co

Industry concentration and strategic trade policy in successive oligopoly

We study a policy game between exporting and importing countries in vertically linked industries. In a successive international Cournot oligopoly, we analyse incentives for using tax instruments strategically to shift rents vertically, between exporting and importing countries, and horizontally, between exporting countries. We show that the equilibrium outcome depends crucially on the relative degree of competitiveness in the upstream and downstream parts of the industry. With respect to national welfare, a more competitive upstream industry may benefit an exporting (upstream) country and harm an importing (downstream) country. On the other hand, a more competitive downstream industry may harm exporting countries.Financial support from the Norwegian Research Council, through the PETROPOL research programme, is gratefully acknowledged. The paper has been greatly improved by the suggestions of two anonymous referees. We also thank Hisashi Hokari and Frode Meland for valuable comments and suggestions