Stacking dependence of carrier transport properties in multilayered black phosphorous

We present the effect of different stacking orders on carrier transport properties of multi-layer black phosphorous. We consider three different stacking orders AAA, ABA and ACA, with increasing number of layers (from 2 to 6 layers). We employ a hierarchical approach in density functional theory (DFT), with structural simulations performed with Generalized Gradient Approximation (GGA) and the bandstructure, carrier effective masses and optical properties evaluated with the Meta-Generalized Gradient Approximation (MGGA). The carrier transmission in the various black phosphorous sheets was carried out with the non-equilibrium Greens function (NEGF) approach. The results show that ACA stacking has the highest electron and hole transmission probabilities. The results show tunability for a wide range of band-gap, carrier effective masses and transmission with a great promise for lattice engineering (stacking order and layers) in black phosphorous.Comment: 18 Pages , 10 figure

Invariance of density correlations with charge density in polyelectrolyte solutions

We present a theory for the equilibrium structure of polyelectrolyte solutions. The main element is a simple, new optimization scheme that allows theories such as the random phase approximation (RPA) to handle the harsh repulsive forces present in such systems. Comparison is made with data from recent neutron scattering experiments of randomly charged, hydrophilic polymers in salt-free, semi-dilute solution at various charge densities. Models with varying degrees of realism are examined. The usual explanation of the invariance observed at high charge density has been counterion condensation. However, when polymer-polymer correlations are treated properly, we find that modeling polymer-counterion correlations at the level of Debye-Huckel theory is sufficient.Comment: 4 pages, 2 figure

Flight/ground sample comparison relating to flight experiment M552, exothermic brazing

Comparisons were made between Skylab and ground-based specimens of nickel and stainless steel which were vacuum brazed using silver-copper-lithium alloy with various joint configurations. It was established that the absence of gravity greatly extends the scope of brazing since capillary flow can proceed without gravity interference. There was also evidence of enhanced transport, primarily in that liquid silver copper alloy dissolves nickel to a much greater extent in the zero gravity environment

The Electronic Structure of Vanadium Oxides as Catalysts in the Selective Oxidation of Small Alkanes

The present work considers vanadium oxides catalysts in the selective oxidation of small alkanes. The dynamics of their (surface) electronic structure modulated by the chemical potential of reaction gases were investigated regarding charge carrier dynamics, surface valence/conduction band structure and work function modifications. The charge carrier dynamics were studied with the in situ microwave cavity perturbation technique allowing the determination of the catalyst conductivity in a contact free manner in a fixed bed reactor geometry. An evaluation program based on the transmission line theory was developed for precise conductivity determination. The validity of the evaluation methods was tested with the n-type semiconducting vanadium pentoxide in the oxidation of n-butane. In agreement with literature, the experiments revealed an n-type conductivity. The addition of n-butane in the reaction feed leads to an increased conductivity corresponding to the abundance of electronically active V4+ defect states (corresponding to oxygen vacancies) in the forbidden bandgap of vanadium pentoxide increasing the mobile electron density. Based on results of the reference study, the selective propane oxidation catalyst MoVNbTeO x M1-phase was investigated in the selective oxidation of ethane, propane and n-butane. Also the impact of water in the propane feed, triggering the abundance of the industrially important key product acrylic acid, on the MoVNbTeOx M1-phase electronic structure was studied. The in situ microwave cavity perturbation studies at ambient pressure were complemented with near ambient pressure X-ray photoelectron and X-ray absorption spectroscopy investigations at 0.25 mbar to understand the charge transfer processes according to semiconductor physics. The conductivity of MoVNbTeOx M1-phase increased with increasing propane to oxygen ratio identifying MoVNbTeOx M1-phase as an n-type semiconductor. In the alkane (ethane, propane and n-butane) exchange experiment, the number of electrons transferred to MoVNbTeOx M1-phase increased from ethane, to propane and finally to n-butane oxidation resulting in an increased conductivity. The X-ray photoelectron spectroscopy reveals that the exchange of the alkane leads to a modulation of the V4+/V5+ redox couple at the surface corresponding to shifts of the valence band edge and electron affinity. Thus the surface of MoVNbTeOx M1-phase, being in dynamic equilibrium with the bulk electronic structure, is modified by the compositions (corresponding to the chemical potential) of the gas phase. The bulk charge carrier density is triggered by the barrier height of the surface induced space charge layer resulting in a modified conductivity. In contrast the modulated electron affinity can be explained by a change of the surface dipole. Water in propane feed leads to a decreased conductivity of MoVNbTeOx M1-phase without a modification of the space charge layer. A drastic change of the surface elemental composition, in particular the abundance of V5+ , is induced by water, observable in the valence band, core level and vanadium L2,3-edges X-ray absorption spectra. The surface modifications were accompanied with a decreased electron affinity corresponding to a decreased surface dipole. The drastically changed valence and conduction band structure likely affects the charge carrier mobility explaining the decreased conductivity in steam containing propane feed. However, results from low pressure in situ photoelectron studies are debated according to their relevance for "real" catalysis at ambient pressures. In particular the oxygen pressure controls the oxidation state of transition metal oxide surfaces. The vanadium L2,3 X-ray absorption edges of vanadyl pyrophosphate were investigated in the selective n-butane oxidation at 10, 100 and 1000 mbar to identify a possible pressure gap using the surface sensitive conversion electron mode. As a result, at low pressures the oxidation of the surface is controlled by the oxygen pressure. In contrast at higher pressures, the surface state of oxidation is triggered by the catalytic reaction providing a steady state between reduction of the catalyst during n-butane conversion and re-oxidation by molecular oxygen

Two-Dimensional Inversion Asymmetric Topological Insulators in Functionalized III-Bi Bilayers

The search for inversion asymmetric topological insulators (IATIs) persists as an effect for realizing new topological phenomena. However, so for only a few IATIs have been discovered and there is no IATI exhibiting a large band gap exceeding 0.6 eV. Using first-principles calculations, we predict a series of new IATIs in saturated Group III-Bi bilayers. We show that all these IATIs preserve extraordinary large bulk band gaps which are well above room-temperature, allowing for viable applications in room-temperature spintronic devices. More importantly, most of these systems display large bulk band gaps that far exceed 0.6 eV and, part of them even are up to ~1 eV, which are larger than any IATIs ever reported. The nontrivial topological situation in these systems is confirmed by the identified band inversion of the band structures and an explicit demonstration of the topological edge states. Interestingly, the nontrivial band order characteristics are intrinsic to most of these materials and are not subject to spin-orbit coupling. Owning to their asymmetric structures, remarkable Rashba spin splitting is produced in both the valence and conduction bands of these systems. These predictions strongly revive these new systems as excellent candidates for IATI-based novel applications.Comment: 17 pages,5figure

The Structure of Barium in the hcp Phase Under High Pressure

Recent experimental results on two hcp phases of barium under high pressure show interesting variation of the lattice parameters. They are here interpreted in terms of electronic structure calculation by using the LMTO method and generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II the dramatic drop in c/a is an instability analogous to that in the group II metals but with the transfer of s to d electrons playing a crucial role in Ba. Meanwhile in phase V, the instability decrease a lot due to the core repulsion at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx, 71.15LaComment: 29 pages, 8 figure

Extension of the universal force field for metal–organic frameworks

We have extended the Universal Force Field for Metal-Organic Frameworks (UFF4MOF) to cover all moieties present in the most extensive framework library to date, i.e. the Computation-Ready Experimental(CoRE) database (Chem. Mater. 26, 6185 (2014)). Thus, we have extended the parameters to include the fourth and fifth row transition metals, lanthanides and an additional atom type for Sulphur, while the parameters of original UFF and of UFF4MOF are not modified. Employing the new parameters signicantly enlarges the number of structures that may be subjected to a UFF calculation, i.e. more than doubling accessible MOFs of the CoRE structures and thus reaching over 99% of CoRE structure coverage. In turn, 95% of optimized cell parameters are within 10% of their experimental values. We contend these parameters will be most useful for the generation and rapid prototyping of hypothetical MOF structures from SBU databases

Vibration Isolation Design for the Micro-X Rocket Payload

Micro-X is a NASA-funded, sounding rocket-borne X-ray imaging spectrometer that will allow high precision measurements of velocity structure, ionization state and elemental composition of extended astrophysical systems. One of the biggest challenges in payload design is to maintain the temperature of the detectors during launch. There are several vibration damping stages to prevent energy transmission from the rocket skin to the detector stage, which causes heating during launch. Each stage should be more rigid than the outer stages to achieve vibrational isolation. We describe a major design effort to tune the resonance frequencies of these vibration isolation stages to reduce heating problems prior to the projected launch in the summer of 2014.Comment: 6 pages, 7 figures, LTD15 Conference Proceeding

Inelastic neutron scattering in random binary alloys : an augmented space approach

Combining the augmented space representation for phonons with a generalized version of Yonezawa-Matsubara diagrammatic technique, we have set up a formalism to seperate the coherent and incoherent part of the total intensity of thermal neutron scattering from disordered alloys. This is done exacly without taking any recourse to mean-field like approximation (as done previously). The formalism includes disorder in masses, force constants and scattering lengths. Implementation of the formalism to realistic situations is performed by an augmented space Block recursion which calculates entire Green matrix and self energy matrix which in turn is needed to evaluate the coherent and incoherent intensities. we apply the formalism to NiPd and NiPt alloys. Numerical results on coherent and incoherent scattering cross sections are presented along the highest symmetry directions. Finally the incoherent intensities are compared with the CPA and also with experiments.Comment: 18 pages, 13 figure