Poorly characterized phases in C2M carbonaceous chondrites : proposed structures and significance

Poorly characterized phases (PCP's) may constitute up to 30 volume percent of some C2M carbonaceous chondrite matrices [1] and are an important key to an understanding of matrix evolution. PCPs are usually fine-grained (<lµm in size), and are difficult to characterize by conventional optical or X-ray techniques [2]. Fuchs et al. [2] introduced the term PCP in their detailed description of Fe-S-Ni-0 phases in the matrix of Murchison. Ramdohr [3] described a similar Fe-S-C phase with trace amounts of Ni in five carbonaceous chondrites and proposed that this mineral has a layer structure. Both the Fe-S-Ni-0 and Fe-S-Ni-C phases have similar optical properties and are conveniently described by the generic term PCP [1]. On the basis of recent high resolution electron microscopy (HREM) studies [4-9], we propose that these PCP's form at least two ordered, stable structures based upon alternating sequences of mackinawite- and brucite- (or amakinite-) type layers

Towards a complete inventory of stratospheric dust particles, with implications for their classification

Several investigators have recently proposed classification schemes for stratospheric dust particles [1-3]. In addition, extraterrestrial materials within stratospheric dust collections may be used as a measure of micrometeorite flux [4]. However, little attention has been given to the problems of the stratospheric collection as a whole. Some of these problems include: (a) determination of accurate particle abundances at a given point in time; (b) the extent of bias in the particle selection process; (c) the variation of particle shape and chemistry with size; (d) the efficacy of proposed classification schemes and (e) an accurate determination of physical parameters associated with the particle collection process (e.g. minimum particle size collected, collection efficiency, variation of particle density with time). We present here preliminary results from SEM, EDS and, where appropriate, XRD analysis of all of the particles from a collection surface which sampled the stratosphere between 18 and 20km in altitude. Determinations of particle densities from this study may then be used to refine models of the behavior of particles in the stratosphere [5]

Shortening the interval to resumption of ovarian cycles in postpartum beef cows.

End of Project Report• In beef cows the interval from calving to first ovulation, or postpartum interval, is affected by nutrition and by the suckling effect of the calf. • The suckling effect is the biggest determinant of this interval, comprising: (i) physical contact and (ii) maternal bonding between cow and calf. Restricted suckling and calf isolation induce a rapid resumption of oestrous cycles. • Prepartum nutrition is the next most important determinant of the postpartum interval. Cows that calve in poor body condition have a longer interval than cows that calve in good body condition. • Increasing the level of nutrition in the postpartum period has only a limited effect in shortening the postpartum interval. • In most cows the first postpartum ovulation is silent and is succeeded by a short oestrous cycle of approximately 8-10 days. The first observed oestrus occurs prior to the second ovulation. • When used in combination with calf isolation and restricted suckling, progesterone pre-treatment for 6 days induces oestrus in the majority of cows and eliminates the short oestrous cycle. • The prolonged postpartum interval in beef cows is not due to failure of ovarian follicle development but to failure of successive dominant follicles to ovulate due to the inadequate frequency of LH pulses.European Union 3rd Framework Programme (Contract AIR3-CT94-1124

Nutrition and Oestrus and Ovarian Cycles in Cattle

End of Project ReportThe overall objective of this project was to establish the effects of both long- and short-term changes in nutrition on ovarian follicle dynamics and on the systemic concentrations of metabolic and reproductive hormones. In order to avoid the confounding effects of lactation, suckling and maternal–calf bonding, beef heifers were used in a series of three studies.European Union 3rd Framework Programme (Contract AIR3-CT94-1124

Two-Dimensional Diffusion in the Presence of Topological Disorder

How topological defects affect the dynamics of particles hopping between lattice sites of a distorted, two-dimensional crystal is addressed. Perturbation theory and numerical simulations show that weak, short-ranged topological disorder leads to a finite reduction of the diffusion coefficient. Renormalization group theory and numerical simulations suggest that longer-ranged disorder, such as that from randomly placed dislocations or random disclinations with no net disclinicity, leads to subdiffusion at long times.Comment: 10 pages, 6 figure

Ab initio many-body calculations on infinite carbon and boron-nitrogen chains

In this paper we report first-principles calculations on the ground-state electronic structure of two infinite one-dimensional systems: (a) a chain of carbon atoms and (b) a chain of alternating boron and nitrogen atoms. Meanfield results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by second-order M{\o}ller-Plesset perturbation theory and the coupled-cluster approach. The calculations were performed using 6-31$G^{**}$ basis sets, including the d-type polarization functions. Both at the Hartree-Fock (HF) and the correlated levels we find that the infinite carbon chain exhibits bond alternation with alternating single and triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In addition, we also performed density-functional-theory-based local density approximation (LDA) calculations on the infinite carbon chain using the same basis set. Our LDA results, in contradiction to our HF and correlated results, predict a very small bond alternation. Based upon our LDA results for the carbon chain, which are in agreement with an earlier LDA calculation calculation [ E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488 (1998).], we conclude that the LDA significantly underestimates Peierls distortion. This emphasizes that the inclusion of many-particle effects is very important for the correct description of Peierls distortion in one-dimensional systems.Comment: 3 figures (included). To appear in Phys. Rev.

Block-Transitive Designs in Affine Spaces

This paper deals with block-transitive $t$-$(v,k,\lambda)$ designs in affine spaces for large $t$, with a focus on the important index $\lambda=1$ case. We prove that there are no non-trivial 5-$(v,k,1)$ designs admitting a block-transitive group of automorphisms that is of affine type. Moreover, we show that the corresponding non-existence result holds for 4-$(v,k,1)$ designs, except possibly when the group is one-dimensional affine. Our approach involves a consideration of the finite 2-homogeneous affine permutation groups.Comment: 10 pages; to appear in: "Designs, Codes and Cryptography

Crystal structure of solid Oxygen at high pressure and low temperature

Results of X-ray diffraction experiments on solid oxygen at low temperature and at pressures up to 10 GPa are presented.A careful sample preparation and annealing around 240 K allowed to obtain very good diffraction patterns in the orthorhombic delta-phase. This phase is stable at low temperature, in contrast to some recent data [Y. Akahama et al., Phys. Rev. B64, 054105 (2001)], and transforms with decreasing pressure into a monoclinic phase, which is identified as the low pressure alpha-phase. The discontinuous change of the lattice parameters, and the observed metastability of the alpha-phase increasing pressure suggest that the transition is of the first order.Comment: 4 pages with three figure