Phase separation near half-filling point in superconducting compounds

We present the model of superconducting ceramics using the single band extended Hubbard Hamiltonian. We investigate the simultaneous presence of antiferromagnetism (AF) and d-wave superconductivity (SC) in the coherent potential (CP) approximation applied to the on-site Coulomb repulsion $U$. We consider the hopping interaction, $\Delta t$, the inter-site charge-charge interaction, $V$, (creating SC), and the single site Hund's type exchange interaction, $F_{in}$, (creating AF). The influence of these interactions on the separation of superconducting and antiferromagnetic phases near the half-filling point is investigated. Results are compared with the experimental data for YBaCuO and NdCeCuO compounds.Comment: 4 pages, 4 figure

Circular scans for CMB anisotropy observation and analysis

A number of experiments for measuring anisotropies of the Cosmic Microwave Background use scanning strategies in which temperature fluctuations are measured along circular scans on the sky. It is possible, from a large number of such intersecting circular scans, to build two-dimensional sky maps for subsequent analysis. However, since instrumental effects --- especially the excess low-frequency 1/f noise --- project onto such two-dimensional maps in a non-trivial way, we discuss the analysis approach which focuses on information contained in the individual circular scans. This natural way of looking at CMB data from experiments scanning on the circles combines the advantages of elegant simplicity of Fourier series for the computation of statistics useful for constraining cosmological scenarios,and superior efficiency in analysing and quantifying most of the crucial instrumental effects.Comment: 10 pages, 5 figures (.ps), submitted to MNRA

Floods and fish : recruitment and distribution of fish in the Volga River floodplain

Natural river floodplains are among the most diverse and productive ecosystems on Earth and provide key habitats for foraging, spawning and as a nursery for many riverine fish species. Periodic flooding plays a principal role in the ecological processes in floodplain systems resulting in high productivity and diversity, as formulated in the Flood Pulse concept (FPC, a major conceptual framework for understanding the hydro–ecological processes operating in river-floodplain systems). However, little progress has been made in exploring the FPC over larger spatial scales due to lack of empirical data. The Volga-Akhtuba floodplain (Russian Federation) is still relatively undisturbed, while in Europe and North-America about 90% of floodplains have effectively been lost. This thesis provides a quantitative assessment of the FPC by (1) analyzing flood pulse dynamics in response to changes in river flow regime on various spatial and temporal scales and (2) relating these to recruitment success of riverine fish species that use the floodplain; and (3) identifying the hydro-geomorphic variables that are most involved shaping fish abundance and distribution in the floodplain. The results obtained demonstrate that the flood pulse magnitude in the lower Volga has noticeably decreased due to damming of the Volga upstream from the Volga-Akhtuba floodplain. Still, in spite of this hydrological control, considerable year-to-year variation in flood magnitude and timing has remained. Therefore fish populations in the floodplain still depend on variations in the year-to-year flow regime. Moreover, spring temperature and its match with flooding control the recruitment of young fish at the end of the growing season. However, the main sources of spawning stocks in the large-scale Volga-Akhtuba floodplain originate from local floodplain populations and not from the main river channels. Spatial heterogeneity of hydro-geomorphic attributes of the floodplain water bodies is highly significant for structuring fish abundance and distribution in the floodplain. Therefore, even under changing environmental and social circumstances, it is essential to preserve flood dynamics, which is fundamental not only for fish recruitment success but also governs fish distribution and diversity in the floodplain. <br/

Power Spectrum Estimators For Large CMB Datasets

Forthcoming high-resolution observations of the Cosmic Microwave Background (CMB) radiation will generate datasets many orders of magnitude larger than have been obtained to date. The size and complexity of such datasets presents a very serious challenge to analysing them with existing or anticipated computers. Here we present an investigation of the currently favored algorithm for obtaining the power spectrum from a sky-temperature map --- the quadratic estimator. We show that, whilst improving on direct evaluation of the likelihood function, current implementations still inherently scale as the equivalent of the cube of the number of pixels or worse, and demonstrate the critical importance of choosing the right implementation for a particular dataset.Comment: 8 pages LATEX, no figures, corrected misaligned columns in table

Measuring Planck beams with planets

Aims. Accurate measurement of the cosmic microwave background (CMB) anisotropy requires precise knowledge of the instrument beam. We explore how well the Planck beams will be determined from observations of planets, developing techniques that are also appropriate for other experiments. Methods. We simulate planet observations with a Planck-like scanning strategy, telescope beams, noise, and detector properties. Then we employ both parametric and non-parametric techniques, reconstructing beams directly from the time-ordered data. With a faithful parameterization of the beam shape, we can constrain certain detector properties, such as the time constants of the detectors, to high precision. Alternatively, we decompose the beam using an orthogonal basis. For both techniques, we characterize the errors in the beam reconstruction with Monte Carlo realizations. For a simplified scanning strategy, we study the impact on estimation of the CMB power spectrum. Finally, we explore the consequences for measuring cosmological parameters, focusing on the spectral index of primordial scalar perturbations, n_s. Results. The quality of the power spectrum measurement will be significantly influenced by the optical modeling of the telescope. In our most conservative case, using no information about the optics except the measurement of planets, we find that a single transit of Jupiter across the focal plane will measure the beam window functions to better than 0.3% for the channels at 100–217 GHz that are the most sensitive to the CMB. Constraining the beam with optical modeling can lead to much higher quality reconstruction. Conclusions. Depending on the optical modeling, the beam errors may be a significant contribution to the measurement systematics for n_s

The scalar perturbation spectral index n_s: WMAP sensitivity to unresolved point sources

Precision measurement of the scalar perturbation spectral index, n_s, from the Wilkinson Microwave Anisotropy Probe temperature angular power spectrum requires the subtraction of unresolved point source power. Here we reconsider this issue. First, we note a peculiarity in the WMAP temperature likelihood's response to the source correction: Cosmological parameters do not respond to increased source errors. An alternative and more direct method for treating this error term acts more sensibly, and also shifts n_s by ~0.3 sigma closer to unity. Second, we re-examine the source fit used to correct the power spectrum. This fit depends strongly on the galactic cut and the weighting of the map, indicating that either the source population or masking procedure is not isotropic. Jackknife tests appear inconsistent, causing us to assign large uncertainties to account for possible systematics. Third, we note that the WMAP team's spectrum was computed with two different weighting schemes: uniform weights transition to inverse noise variance weights at l = 500. The fit depends on such weighting schemes, so different corrections apply to each multipole range. For the Kp2 mask used in cosmological analysis, we prefer source corrections A = 0.012 +/- 0.005 muK^2 for uniform weighting and A = 0.015 +/- 0.005 muK^2 for N_obs weighting. Correcting WMAP's spectrum correspondingly, we compute cosmological parameters with our alternative likelihood, finding n_s = 0.970 +/- 0.017 and sigma_8 = 0.778 +/- 0.045 . This n_s is only 1.8 sigma from unity, compared to the ~2.6 sigma WMAP 3-year result. Finally, an anomalous feature in the source spectrum at l<200 remains, most strongly associated with W-band.Comment: 9 pages, 10 figures, 3 tables. Submitted to Ap

Testing physical models for dipolar asymmetry with CMB polarization

The cosmic microwave background (CMB) temperature anisotropies exhibit a large-scale dipolar power asymmetry. To determine whether this is due to a real, physical modulation or is simply a large statistical fluctuation requires the measurement of new modes. Here we forecast how well CMB polarization data from \Planck\ and future experiments will be able to confirm or constrain physical models for modulation. Fitting several such models to the \Planck\ temperature data allows us to provide predictions for polarization asymmetry. While for some models and parameters \Planck\ polarization will decrease error bars on the modulation amplitude by only a small percentage, we show, importantly, that cosmic-variance-limited (and in some cases even \Planck) polarization data can decrease the errors by considerably better than the expectation of $\sqrt 2$ based on simple $\ell$-space arguments. We project that if the primordial fluctuations are truly modulated (with parameters as indicated by \Planck\ temperature data) then \Planck\ will be able to make a 2$\sigma$ detection of the modulation model with 20--75\% probability, increasing to 45--99\% when cosmic-variance-limited polarization is considered. We stress that these results are quite model dependent. Cosmic variance in temperature is important: combining statistically isotropic polarization with temperature data will spuriously increase the significance of the temperature signal with 30\% probability for \Planck.Comment: 18 pages, 11 figures, 2 tables. Version updated to match PRD versio

Evidence of vorticity and shear at large angular scales in the WMAP data: a violation of cosmological isotropy?

Motivated by the large-scale asymmetry observed in the cosmic microwave background sky, we consider a specific class of anisotropic cosmological models -- Bianchi type VII_h -- and compare them to the WMAP first-year data on large angular scales. Remarkably, we find evidence of a correlation which is ruled out as a chance alignment at the 3sigma level. The best fit Bianchi model corresponds to x=0.55, Omega_0=0.5, a rotation axis in the direction (l,b)=(222degr,-62degr), shear (sigma/H)_0=2.4e-10 and a right--handed vorticity (omega/H)_0=6.1e-10. Correcting for this component greatly reduces the significance of the large-scale power asymmetry, resolves several anomalies detected on large angular scales (ie. the low quadrupole amplitude and quadrupole/octopole planarity and alignment), and can account for a non--Gaussian "cold spot" on the sky. Despite the apparent inconsistency with the best-fit parameters required in inflationary models to account for the acoustic peaks, we consider the results sufficiently provocative to merit further consideration.Comment: 4 pages, 3 figures; emulateapj.cls; ApJL accepted version plus fixed error in vorticity calculation (sqrt(2) off in Table 1, abstract, and conclusions); basic conclusions unchange