Electronic transport and localization in low mobility solids and liquids

This article is concerned with the electronic transport properties of the large group of solids (and also some liquids) which possess carrier mobilities from 10 cm²V^-1sec^-1 to many orders below that value. Its main aim is to trace the close relation between transport and localization. To this end the physical basis of three important localization models is discussed and the predicted transport mechanisms are reviewed in the light of the experimental information. After a brief summary of the experimental methods, the first subject is the localization arising from strong electron-phonon interaction. Here we consider in some detail the formation of a small polaron in a molecular crystal and its transport by phonon-assisted intermolecular hopping. The second part of the paper deals with the localization of an electron in a so-called electronic bubble, which occurs for example in simple liquids such as Ne or He. The stability and transport of this entity is discussed. The third section is devoted to non-crystalline solids. In these localization is associated with the lack of long-range order and does not depend on phonon interaction. The transport through the various parts of the density of states spectrum is summarized and the suggested models are reviewed on the basis of recent experimental work on amorphous silicon. Finally, the possibility of polaron formation and transport in some chalcogenide alloy glasses is briefly considered. The paper leads to the conclusion that available experimental evidence supports the general validity of the above three models, although it must be emphasized that this is based on an as yet limited range of experimental information.     

Secondary metabolites produced by fungi derived from a microbial mat encountered in an iron-rich natural spring

A collection of fungal isolates was obtained from a complex microbial mat, which occupied an iron-rich freshwater spring that feeds into Clear Creek, Golden, Colorado, USA. Two of the fungal isolates, a Glomeromycete (possibly Entrophospora sp.) and a Dothideomycete (possibly Phaeosphaeria sp.), were investigated for bioactive secondary metabolites. In total, six new compounds consisting of clearanols A-E (5, 6, 10-12) and disulochrin (7) were purified and their structures were determined. Disulochrin exhibited modest antibacterial activity against methicillin-resistant Staphylococcus aureus, whereas clearanol C showed weak inhibitory activity against Candida albicans biofilm formation.     

Cross section of the reaction 6Li(p, γ)7Be

The cross section of the reaction ⁶Li(p, γ)⁷Be has been measured using Ge(Li) γ-ray spectrometers for proton bombarding energies E_p from 200 keV to 1200 keV. At E_p = 800 keV, the total (p, γ) integrated cross section is found to be 3.1 ± 0.4 μb. The cross section adopted from consideration of this and previous measurements is in good agreement with that predicted from the known thermal neutron cross section for ⁶Li(n, γ)⁷Li on the assumption that properties of mirror direct capture reactions can be well described by optical potentials that use the same parmeter values for the two reactions.     

Aluminium

Ohne Zusammenfassung     

Octupole vibrations in the even mercury and lead isotopes

Prominent groups are observed corresponding to excited states in the region of 2.6 MeV in the scattering of 24 and 27 MeV α-particles from ^{198, 200, 202, 204}Hg and ^{204, 206, 208}Pb. For each of the Pb isotopes the state concerned is identified with the known 3^? octupole vibration. Angular distribution measurements for scattering to the Pb and ²⁰⁴Hg states are in agreement with octupole-vibration coupled-channels predictions, and systematic trends suggest that the states observed in the isotopes ^{198, 200, 202}Hg can also be attributed to octupole vibrations.     

Morphology of chemical vapor deposited titanium diboride

Titanium diboride has been chemical vapor deposited from TiCl₄, BCl₃, and H₃ gases on a heated graphite substrate. The morphologies of the deposits were correlated with the deposition parameters of substrate temperature, total pressure, B: Ti atomic ratio, Cl: H atomic ratio, and total flow rate. The diboride deposits exhibited a variety of morphologies which include coherent coatings with nodular or faceted surfaces, plate-like crystallites, dendritic crystallites, and irregularly shaped, faceted crystallites. The deposit morphology was found to be most sensitive to deposition temperature, B: Ti atomic ratio, and Cl: H atomic ratio. Variations in both the total pressure and total flow rate did not significantly affect the diboride morphology. All the TiB₂ deposits except those produced under a large excess of H₂ had a preferred orientation such that the hexagonal c-direction was parallel to the substrate surface.     

Static quadrupole moments of the first excited states of 200Hg and 202Hg

The static quadrupole moments Q_2⁺ and B(E2; 0⁺ → 2⁺) values of the 2⁺ first excited states of ²⁰⁰Hg and ²⁰²Hg have been determined using the reorientation effect in Coulomb excitation. An annular silicon surface-barrier detector was used to detect backscattered ⁴He, ¹²C and ¹⁶O projectiles. It is found that for ²⁰⁰Hg, Q_2⁺ = +1.07 ± 0.19 e · b(+0.98 ± 0.19 e · b) for destructive (constructive) interference from the 2^+′ state, and B(E2; 0⁺ → 2⁺) = 0.853 ± 0.007 e² · b². For ²⁰²Hg, we find Q_2⁺ = +1.01 ± 0.13 e · b (+0.87 ± 0.13 e · b) and B(E2; 0⁺ → 2⁺) = 0.605 ± 0.005 e² · b². The Q_2⁺ value obtained for ²⁰⁰Hg is in agreement with previous work, but that for ²⁰²Hg is not. The results obtained are compared with the predictions of various nuclear models, and the mass dependence of Q_2⁺ in the region 182 ≦ A ≦ 206 is examined.     

Application of amorphous silicon field effect transistors in addressable liquid crystal display panels

It is shown that thin-film field effect transistors (FETs) made from amorphous (a-) silicon deposited by the glow-discharge technique have considerable potential as switching elements in addressable liquid crystal display panels. The fabrication of the elements and their characteristics with steady and pulsed applied potentials are discussed in some detail. Two important points are stressed: (i) a-Si device arrays can be produced by well-established photolithographic techniques, and (ii) satisfactory operation at applied voltages below 15VV is possible. Small experimental 7×5 transistor panels have been investigated and it is shown that with the present design up to 250-way multiplexing could be achieved. The reproducibility of FET characteristics is good and in tests so far no change has been observed after more than 10⁹ switching operations.     

Interaction of human immunodeficiency virus (HIV) glycans with lectins of the human immune system

Approximately half of the molecular mass of gp120, the receptor-binding envelope protein of human immunodeficiency virus (HIV), consists of N-linked glycans. Nearly half of these glycans are of the high mannose type. These high mannose glycans furnish a rich forest of mannose residues on the virus surface making HIV a prime target for interaction with mannose-specific lectins of the immune system. This review focuses on the known interactions between gp120 and immune system lectins some of which HIV appears to exploit. The effect of variation in glycosylation of gp120, especially with respect to clades of HIV, on binding of immune system lectins is highlighted.