Observation of superconductivity (Tc = 50 K) in a new tetragonal alkaline-earth cuprate Sr0.8Ba1.2CuO3+δ, synthesised at ambient pressure

The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr_0.8Ba_1.2CuO_3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition 50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La₂CuO₄-type tetragonal T structure (a = 3.8988(3) Å and C = 12.815(3) Å) with oxygen vacancies located within the CuO₂ planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr_0.8Ba_1.2CuO_3+δ and also in the isostructural superconductor Sr₂CuO_3+δ are of an identical nature.     

Uniform distribution of polynomial-generated sequences inGF[q,x]

Summary Analogs are proved for sequences in Φ=GF[q, x], the ring of polynomials in x over a finite field of q elements, of results of Niven, Zane, and Cavior concerning uniform distribution of sequences of integers generated by polynomials with integer coefficients. The uniform distribution (mod m), m an integer, in these papers is replaced here by a concept called weak uniform distribution (mod M), M ε Φ. Supported byNSF Research GrantGP 6515. Entrata in Redazione il 21 settembre 1968.     

Frequency-stabilized cavity ring-down spectroscopy measurements of carbon dioxide isotopic ratios

Carbon dioxide (CO₂) isotopic ratios on samples of pure CO₂ were measured in the 1.6 μm wavelength region using the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) technique. We present CO₂ absorption spectra with peak signal-to-noise ratios as high as 28,000:1. Measured single-spectrum signal-to-noise ratios were as high as 8900:1, 10,000:1, and 1700:1 for ¹³C/¹²C, ¹⁸O/¹⁶O, and ¹⁷O/¹⁶O, respectively. In addition, we demonstrate the importance of utilizing the Galatry line profile in the spectrum analysis. The use of the Voigt line profile, which neglects the observed collisional narrowing, leads to large systematic errors which are transition-dependent and vary with temperature and pressure. While the relatively low intensities of CO₂ transitions near λ=1.6 μm make this spectral region non-optimal, the sensitivity and stability of FS-CRDS enabled measurement precision of pure CO₂ samples which are comparable to those of other optical techniques which operate at far more propitious wavelengths. These results indicate that a FS-CRDS spectrometer designed to probe CO₂ bands near wavelengths of 2.0 μm or 4.3 μm could achieve significantly improved precision over the present instrument and likely be competitive with mass spectrometric methods.     

Structured Markov Chain Monte Carlo

Abstract

This article introduces a general method for Bayesian computing in richly parameterized models, structured Markov chain Monte Carlo (SMCMC), that is based on a blocked hybrid of the Gibbs sampling and Metropolis—Hastings algorithms. SMCMC speeds algorithm convergence by using the structure that is present in the problem to suggest an appropriate Metropolis—Hastings candidate distribution. Although the approach is easiest to describe for hierarchical normal linear models, we show that its extension to both nonnormal and nonlinear cases is straightforward. After describing the method in detail we compare its performance (in terms of run time and autocorrelation in the samples) to other existing methods, including the single-site updating Gibbs sampler available in the popular BUGS software package. Our results suggest significant improvements in convergence for many problems using SMCMC, as well as broad applicability of the method, including previously intractable hierarchical nonlinear model settings.     

Orthorhombic Polymorphs of 1-Phenyl-3-(3-Hydroxyphenyl)-2-Propen-1-One

Abstract  

Chalcones, 1,3-diphenyl-2-propene-1-ones, exist naturally and synthetically, and are characterized by the linkage of two aromatic rings joining a three carbon α-β-unsaturated carbonyl entity. We report the observation of two new polymorphs of a hydroxy chalcone, C₆H₅–CO–CH=CH–C₆H₄ (m-OH), identified as a result of a Claisen–Schmidt synthesis and manual screening technique. One polymorph of this compound has been reported previously and exists in the monoclinic system, space group P2/n, with unit cell parameters a = 13.295(6) ?, b = 5.659(2) ?, c = 16.144(8) ?, β = 109.73(5)°, V = 1143.3(9) ?³, and Z = 4. Two crystalline forms (II and III) reported herein, are polymorphs of the reported monoclinic form (I). Form II exists in the orthorhombic system, space group Pca2₁, with unit cell constants a = 11.631 (2) ?, b = 13.163 (3) ?, c = 7.3977 (14) ?, V = 1132.6 (4) ?³, and Z = 4. Form III also crystallizes in the orthorhombic system, however in space group Pbca, with unit cell parameters a = 11.8100(8) ?, b = 7.4075(5) ?, c = 25.8729(19) ?, V = 2263.4(3) ?³, and Z = 8. Variations in the hydrogen bonding connectivity help to distinguish these two forms from the monoclinic, whereas crystal packing differentiates the two orthorhombic forms. The conformation of the C=C (C2–C3) is E for both orthorhombic forms.     

Semi-implicit two-level predictor–corrector methods for non-linearly coupled,hydrostatic, barotropic/baroclinic flows

The unsteady shallow-water equations for barotropic/baroclinic (free-surface/density-stratified) flows with non-linear coupling of density transport and momentum are solved using a family of two-time-level, semi-implicit predictor–corrector methods (PC2). The PC2 methods are a general family that includes the popular TRIM method for hydrostatic flows. PC2 is characterised by four ‘θ’ parameters controlling the time ‘n’ and ‘n + 1’ weighting of (1) free surface gradient, (2) predictor step, (3) baroclinic gradient and (4) density temporal interpolation. Stability of the non-linear coupling between momentum and density transport for PC2 is examined in the inviscid limit. Central difference and quadratic (QUICK) spatial interpolation for density are compared. Second-order temporal accuracy for both barotropic and baroclinic flows is simultaneously obtained with appropriate θ parameters, which has previously been shown to be impractical for TRIM. The 2nd-order PC2 method has near-neutral non-linear stability (slightly positive amplification factor) where linear theory predicts exactly neutral stability. QUICK is shown to be preferable to central difference spatial discretisation to reduce the amplification factor. Adjusting the baroclinic weighting or adding small artificial viscosities can stabilise the model for non-linear internal wave simulations.