Hybrid methods for function generation

A method of function generation is discussed based on the use of hybrid digital analogue techniques. A brief description of a quantizer is given together with some experimental results. The purpose of this unit is to give the derivative with respect to time (in quantized form) of a variable. Units of this type used in conjunction with pulse modulators, which are also described, are shown to be capable of performing a wide variety of analytic functions. The application to correlation functions, using magnetic core storage for time delays, is also discussed. Another application is a fast analogue/digital converter with an incremental bit resolution time of one micro-second. The possibility of extending the application to generation of arbitrary functions of one or more variable is considered and one possible form of instrumentation, based on the use of magnetic core storage, is described

Relationships Between Atomic Diffusion Mechanisms and Ensemble Transport Coefficients in Crystalline Polymorphs

Ionic transport in conventional ionic solids is generally considered to proceed via independent diffusion events or "hops''. This assumption leads to well-known Arrhenius expressions for transport coefficients, and is equivalent to assuming diffusion is a Poisson process. Using molecular dynamics simulations of the low-temperature B1, B3, and B4 AgI polymorphs, we have compared rates of ion-hopping with corresponding Poisson distributions to test the assumption of independent hopping in these common structure-types. In all cases diffusion is a non-Poisson process, and hopping is strongly correlated in time. In B1 the diffusion coefficient can be approximated by an Arrhenius expression, though the physical significance of the parameters differs from that commonly assumed. In low temperature B3 and B4 diffusion is characterised by concerted motion of multiple ions in short closed loops. Diffusion coefficients can not be expressed in a simple Arrhenius form dependent on single-ion free-energies, and intrinsic diffusion must be considered a many-body process

Molecular Dynamics Simulation of Coherent Interfaces in Fluorite Heterostructures

The standard model of enhanced ionic conductivities in solid electrolyte heterostructures follows from a continuum mean-field description of defect distributions that makes no reference to crystalline structure. To examine ionic transport and defect distributions while explicitly accounting for ion-ion correlations and lattice effects, we have performed molecular dynamics simulations of a model coherent fluorite heterostructure without any extrinsic defects, with a difference in standard chemical potentials of mobile fluoride ions between phases induced by an external potential. Increasing the offset in fluoride ion standard chemical potentials across the internal interfaces decreases the activation energies for ionic conductivity and diffusion and strongly enhances fluoride ion mobilities and defect concentrations near the heterostructure interfaces. Non-charge-neutral "space-charge" regions, however, extend only a few atomic spacings from the interface, suggesting a continuum model may be inappropriate. Defect distributions are qualitatively inconsistent with the predictions of the continuum mean-field model, and indicate strong lattice-mediated defect-defect interactions. We identify an atomic-scale "Frenkel polarisation" mechanism for the interfacial enhancement in ionic mobility, where preferentially oriented associated Frenkel pairs form at the interface and promote local ion mobility via concerted diffusion processes

Soil organic carbon and root distribution in a temperate arable agroforestry system

Aim To determine, for arable land in a temperate area, the effect of tree establishment and intercropping treatments, on the distribution of roots and soil organic carbon to a depth of 1.5 m. Methods A poplar (Populus sp.) silvoarable agroforestry experiment including arable controls was established on arable land in lowland England in 1992. The trees were intercropped with an arable rotation or bare fallow for the first 11 years, thereafter grass was allowed to establish. Coarse and fine root distributions (to depths of up to 1.5 m and up to 5 m from the trees) were measured in 1996, 2003, and 2011. The amount and type of soil carbon to 1.5 m depth was also measured in 2011. Results The trees, initially surrounded by arable crops rather than fallow, had a deeper coarse root distribution with less lateral expansion. In 2011, the combined length of tree and understorey vegetation roots was greater in the agroforestry treatments than the control, at depths below 0.9 m. Between 0 and 1.5 m depth, the fine root carbon in the agroforestry treatment (2.56 t ha-1) was 79% greater than that in the control (1.43 t ha-1). Although the soil organic carbon in the top 0.6 m under the trees (161 t C ha-1) was greater than in the control (142 t C ha-1), a tendency for smaller soil carbon levels beneath the trees at lower depths, meant that there was no overall tree effect when a 1.5 m soil depth was considered. From a limited sample, there was no tree effect on the proportion of recalcitrant soil organic carbon. Conclusions The observed decline in soil carbon beneath the trees at soil depths greater than 60 cm, if observed elsewhere, has important implication for assessments of the role of afforestation and agroforestry in sequestering carbon

Phylogeny of Miliusa (Magnoliales: Annonaceae: Malmeoideae: Miliuseae), with descriptions of two new species from Malesia

The molecular phylogeny of Miliusa (Annonaceae) is reconstructed, with 27 (of ca. 50) species included, using a combination of seven plastid markers (rbcL exon, trnL intron, trnL-F spacer, matK exon, ndhF exon, psbA-trnH spacer, and ycf1 exon) constituting ca. 7 kb. In addition, two new species of Miliusa are described from the Malesian area: M. butonensis sp. nov. from Buton Island, Indonesia and M. viridifl ora sp. nov. from Papua New Guinea. The former is included in the molecular phylogenetic analysis. The reconstructed phylogeny corresponds well to the informal morphological grouping proposed earlier. A revised key to 13 Austro-Malesian species of Miliusa is provided

Tissue-specific expression from a compound TATA-dependent and TATA-independent promoter

We have found that the mouse metallothionein-I (MT-I) gene promoter functions in an unusual, compound manner. It directs both TATA-dependent and TATA-independent modes of transcription in vivo. The TATA-dependent message is initiated at the previously characterized +1 transcription start site and is the predominant species in most tissues. In many cell types it is metal inducible. The TATA-independent initiation sites are distributed over the 160 bp upstream of the previously characterized +1 start site, and the RNA products are present in all tissues examined. Only in testis, however, do the TATA-independent transcripts predominate, accumulating to highest levels in pachytene-stage meiotic cells and early spermatids. Unlike the TATA-dependent +1 transcript, these RNAs are not induced by metal, even in cultured cells in which the +1 species is induced. Transfection studies of site-directed mutants show that destruction of the TATA element drastically alters the ratio of the two RNA classes in cells in which the +1 transcripts normally dominates. In TATA-minus mutants, the TATA-independent RNAs become the most prevalent, although they remain refractory to metal induction. Thus, the MT-I promoter utilizes two different types of core promoter function within a single cell population. The two different types of core promoter respond very differently to environmental stimuli, and the choice between them appears to be regulated in a tissue-specific fashion