4,934 research outputs found
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
Study of a pair of coupled continuum equations modeling surface growth
In this communication we introduce a pair of coupled continuum equations to
model overlayer growth with evaporation-accretion due to thermal or mechanical
agitations of the substrate. We gain insight into the dynamics of growth via
one-loop perturbative techniques. This allows us to analyze our numerical data.
We conclude that there is a crossover behaviour from a roughening regime to a
very long-time, large length scale smoothening regime.Comment: 21 pages, 13 figures. Submitted to J. Phys.: Condens. Matte
Echolocation by Quasiparticles
It is shown that the local density of states (LDOS), measured in an Scanning
Tunneling Microscopy (STM) experiment, at a single tip position contains
oscillations as a function of Energy, due to quasiparticle interference, which
is related to the positions of nearby scatterers. We propose a method of STM
data analysis based on this idea, which can be used to locate the scatterers.
In the case of a superconductor, the method can potentially distinguish the
nature of the scattering by a particular impurity.Comment: 4+ page
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
Generation of spiral bevel gears with zero kinematical errors and computer aided tooth contact analysis
Kinematic errors in spiral bevel gears are a major source of noise and vibrations in transmissions. A method for the generation of Gleason's spiral bevel gears which provides conjugated gear tooth surfaces and an improved bearing contact was developed. A computer program for the simulation of meshing, misalignment, and bearing contact was written
Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S=1/2 chains in CuCl2*2((CD3)2SO)
Field-dependent specific heat and neutron scattering measurements were used
to explore the antiferromagnetic S=1/2 chain compound CuCl2 * 2((CD3)2SO). At
zero field the system acquires magnetic long-range order below TN=0.93K with an
ordered moment of 0.44muB. An external field along the b-axis strengthens the
zero-field magnetic order, while fields along the a- and c-axes lead to a
collapse of the exchange stabilized order at mu0 Hc=6T and mu0 Hc=3.5T,
respectively (for T=0.65K) and the formation of an energy gap in the excitation
spectrum. We relate the field-induced gap to the presence of a staggered
g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective
staggered fields for magnetic fields applied along the a- and c-axes.
Competition between anisotropy, inter-chain interactions and staggered fields
leads to a succession of three phases as a function of field applied along the
c-axis. For fields greater than mu0 Hc, we find a magnetic structure that
reflects the symmetry of the staggered fields. The critical exponent, beta, of
the temperature driven phase transitions are indistinguishable from those of
the three-dimensional Heisenberg magnet, while measurements for transitions
driven by quantum fluctuations produce larger values of beta.Comment: revtex 12 pages, 11 figure
The dark side of price cap regulation: a laboratory experiment
In a nutshell, price cap regulation is meant to establish a quid pro quo: regulators are obliged by law to intervene only at rare, previously defined points in time, and only by imposing an upper bound on prices; firms are meant to justify regulatory restraint by adopting socially beneficial innovations. In the policy debate, a potential downside of the arrangement has featured less prominently: the economic environment is unlikely to be stable while the cap is in place. If regulators take this into account, they have to decide under uncertainty and also anticipate how regulated firms will react. In a lab experiment, we manipulate the degree of regulatory uncertainty. We compare a baseline when regulators have the same information as firms about demand with treatments wherein they receive only a noisy signal and another when they know only the distribution from which demand realizations are taken. In the face of uncertainty, regulators impose overly generous price caps, which firms exploit. In the experiment, the social damage is severe, and does not disappear with experience
Total energy differences between SiC polytypes revisited
The total energy differences between various SiC polytypes (3C, 6H, 4H, 2H,
15R and 9R) were calculated using the full-potential linear muffin-tin orbital
method using the Perdew-Wang-(91) generalized gradient approximation to the
exchange-correlation functional in the density functional method. Numerical
convergence versus k-point sampling and basis set completeness are demonstrated
to be better than 1 meV/atom. The parameters of several generalized anisotropic
next-nearest-neighbor Ising models are extracted and their significance and
consequences for epitaxial growth are discussed.Comment: 8 pages, 3 figures, Latex, uses epsfig and revte
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