An empirical model and an ab initio calculation of the bulk moduli for covalent solids are used to suggest possible new hard materials. The empirical model indicates that hypothetical covalent solids formed between carbon and nitrogen are good candidates for extreme hardness. A prototype system is chosen and a first principles pseudopotential total energy calculation on the system is performed. The results are consistent with the empirical model and show that materials like the prototype can have bulk moduli comparable to or greater than diamond. It may be possible to synthesize such materials in the laboratory.

Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB2 are presented. The E2g phonons, which involve in-plane B displacements, couple strongly to the px,y electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting λ in the clean limit is calculated to be significantly larger. The E2g phonons are strongly anharmonic, and the non-linear contribution to the coupling between the E2g modes and the px,y bands is significant.The recent discovery of superconductivity near 40 K in MgB 2 has generated much interest in the properties of this simple intermetallic compound [1]. A significant B isotope effect strongly suggests phonon-mediated pairing [2]. To explain the large T c , an electron-phonon coupling (EPC) constant of λ ≈ 1 is needed. Yet estimates of the coupling strength based on the latest measurements of the low-temperature specific heat [3], combined with the density of states (DOS) from density-functional calculations [4], yield λ ≈ 0.6-0.7. Further, the measured temperature dependence of the electrical resistivity [5] is consistent with λ tr 0.6. First-principles calculations of the EPC give λ ≈ 0.7 − 0.9 [4,6,7]. Clearly there is a problem in reconciling all these numbers. Another puzzle involves tunneling measurements of the gap. Values of 2∆/k B T c ranging from 1.2 to 4 have been reported. The values below the BCS weak coupling limit of 3.5 have been attributed to surface effects, but the best-quality spectra [8] show a very clean gap with 2∆/k B T c = 1.25. Sharvin contact measurements [9] reveal a gap at 4.3 meV (2∆/k B T c = 2.6), and additional structures at 2∆/k B T c = 1.5 and 3, raising the possibility of multiple gaps. Careful analysis of the temperature and magnetic field dependence of the specific heat suggests anisotropic or multiple gap structure as well [3]. Thus, even if superconductivity in MgB 2 is phonon-mediated, it is likely that an analysis beyond the simple isotropic Eliashberg model is needed.The MgB 2 lattice consists of two parallel systems of flat layers. One layer contains B atoms in a honeycomb lattice, the other Mg atoms in a triangular lattice halfway between the B layers. First-principles calculations [4] find that the electronic states near the Fermi level are primarily B in character and the Fermi surface (FS) comprises four sheets: two nearly cylindrical hole sheets about the Γ-A line arising from quasi-2D p x,y B bands, and two tubular networks arising from 3D p z bonding and antibonding bands [4]. The difference in character between the sheets raises the possibility that each has a distinct gap that could be observed in the clean limit. Such interband anisotropy enhances the effective EPC constant relevant to superconductivity and decreases the coupling constant for transport, compared to the average values [10][11][12]. This could explain the discrepant values of λ deduced from different types of experiments.In this pa...

Semiconducting molybdenum disulfphide has emerged as an attractive material for novel nanoscale optoelectronic devices due to its reduced dimensionality and large direct bandgap. Since optoelectronic devices require electron-hole generation/recombination, it is important to be able to fabricate ambipolar transistors to investigate charge transport both in the conduction band and in the valence band. Although n-type transistor operation for single-layer and few-layer MoS2 with gold source and drain contacts was recently demonstrated, transport in the valence band has been elusive for solid-state devices. Here we show that a multi-layer MoS2 channel can be hole-doped by palladium contacts, yielding MoS2 p-type transistors. When two different materials are used for the source and drain contacts, for example hole-doping Pd and electron-doping Au, the Schottky junctions formed at the MoS2 contacts produce a clear photovoltaic effect.

Auxin regulates various aspects of plant growth and development. The AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) genes encode short-lived transcriptional repressors that are targeted by the TRANSPORT INHIBITOR RESPONSE1/AUXIN RECEPTOR F-BOX proteins. The Aux/IAA proteins regulate auxin-mediated gene expression by interacting with members of the AUXIN RESPONSE FACTOR protein family. Aux/IAA function is poorly understood; herein, we report the identification and characterization of insertion mutants in 12 of the 29 Aux/IAA family members. The mutants show no visible developmental defects compared with the wild type. Double or triple mutants of closely related Aux/IAA genes, such as iaa8-1 iaa9-1 or iaa5-1 iaa6-1 iaa19-1, also exhibit wild-type phenotypes. Global gene expression analysis reveals that the molecular phenotypes of auxin-treated and untreated light-grown seedlings are unaffected in the iaa17-6 and iaa5-1 iaa6-1 iaa19-1 mutants. By contrast, similar analysis with the gain-of-function axr3-1/iaa17-1 mutant seedlings reveals dramatic changes in basal and auxin-induced gene expression compared with the wild type. Expression of several type-A ARABIDOPSIS RESPONSE REGULATOR genes and a number of genes involved in cell wall biosynthesis and degradation is repressed in axr3-1/iaa17-1. The data suggest extensive functional redundancy among Aux/IAA gene family members and that enhanced stability of the AXR3/IAA17 protein severely alters the molecular phenotype, resulting in developmental defects.

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Overall, RYGB produces greater and more predicted favourable changes in gut microbiota functional capacity than SG. An increase in Roseburia species was the only compositional change common to both types of surgery among those achieving diabetes remission.

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