616 research outputs found
On the relation between adjacent inviscid cell type solutions to the rotating-disk equations
Over a large range of the axial coordinate a typical higher-branch solution of the rotating-disk equations consists of a chain of inviscid cells separated from each other by viscous interlayers. In this paper the leading-order relation between two adjacent cells will be established by matched asymptotic expansions for general values of the parameter appearing in the equations. It is found that the relation between the solutions in the two cells crucially depends on the behaviour of the tangential velocity in the viscous interlayer. The results of the theory are compared with accurate numerical solutions and good agreement is obtained
Superconductivity in Mg10Ir19B16
Mg10Ir19B16, a previously unreported compound in the Mg-Ir-B chemical system,
is found to be superconducting at temperatures near 5 K. The fact that the
compound exhibits a range of superconducting temperatures between 4 and 5 K
suggests that a range of stoichiometries is allowed, though no structural
evidence for this is observed. The compound has a large, noncentrosymmetric,
body centered cubic unit cell with a = 10.568 Angstrom, displaying a structure
type for which no previous superconductors have been reported.Comment: submitted to PR
Coupling between electronic and structural degrees of freedom in the triangular lattice conductor NaxCoO2
The determination by powder neutron diffraction of the ambient temperature
crystal structures of compounds in the NaxCoO2 family, for 0.3 < x <= 1.0, is
reported. The structures consist of triangular CoO2 layers with Na ions
distributed in intervening charge reservoir layers. The shapes of the CoO6
octahedra that make up the CoO2 layers are found to be critically dependent on
the electron count and on the distribution of the Na ions in the intervening
layers, where two types of Na sites are available. Correlation of the shapes of
cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram
in NaxCoO2 is made, showing how structural and electronic degrees of freedom
can be coupled in electrically conducting triangular lattice systems.Comment: 15 pages, 1 tables, 6 figures Submitted to Physical Review
Enabling single-mode behavior over large areas with photonic Dirac cones
Many of graphene's unique electronic properties emerge from its Dirac-like
electronic energy spectrum. Similarly, it is expected that a nanophotonic
system featuring Dirac dispersion will open a path to a number of important
research avenues. To date, however, all proposed realizations of a photonic
analog of graphene lack fully omnidirectional out-of-plane light confinement,
which has prevented creating truly realistic implementations of this class of
systems. Here we report on a novel route to achieve all-dielectric
three-dimensional photonic materials featuring Dirac-like dispersion in a
quasi-two-dimensional system. We further discuss how this finding could enable
a dramatic enhancement of the spontaneous emission coupling efficiency (the
\beta-factor) over large areas, defying the common wisdom that the \beta-factor
degrades rapidly as the size of the system increases. These results might
enable general new classes of large-area ultralow-threshold lasers,
single-photon sources, quantum information processing devices and energy
harvesting systems
Growing LaAlO3/SrTiO3 interfaces by sputter deposition
Sputter deposition of oxide materials in a high-pressure oxygen atmosphere is a well-known technique to produce thin films of perovskite oxides in particular. Also interfaces can be fabricated, which we demonstrated recently by growing LaAlO3 on SrTiO3 substrates and showing that the interface showed the same high degree of epitaxy and atomic order as is made by pulsed laser deposition. However, the high pressure sputtering of oxides is not trivial and number of parameters are needed to be optimized for epitaxial growth. Here we elaborate on the earlier work to show that only a relatively small parameter window exists with respect to oxygen pressure, growth temperature, radiofrequency power supply and target to substrate distance. In particular the sensitivity to oxygen pressure makes it more difficult to vary the oxygen stoichiometry at the interface, yielding it insulating rather than conducting
Extreme Sensitivity of Superconductivity to Stoichiometry in FeSe (Fe1+dSe)
The recently discovered iron arsenide superconductors, which display
superconducting transition temperatures as high as 55 K, appear to share a
number of general features with high-Tc cuprates, including proximity to a
magnetically ordered state and robustness of the superconductivity in the
presence of disorder. Here we show that superconductivity in Fe1+dSe, the
parent compound of the superconducting arsenide family, is destroyed by very
small changes in stoichiometry. Further, we show that non-superconducting
Fe1+dSe is not magnetically ordered down to low temperatures. These results
suggest that robust superconductivity and immediate instability against an
ordered magnetic state should not be considered as intrinsic characteristics of
iron-based superconducting systems, and that Fe1+dSe may present a unique
opportunity for determining which materials characteristics are critical to the
existence of superconductivity in high Tc iron arsenide superconductors and
which are not.Comment: Updated to reflect final version and include journal referenc
In-situ STEM imaging of growth and phase change of individual CuAlX precipitates in Al alloy
Age-hardening in Al alloys has been used for over a century to improve its mechanical properties. However, the lack of direct observation limits our understanding of the dynamic nature of the evolution of nanoprecipitates during age-hardening. Using in-situ (scanning) transmission electron microscopy (S/TEM) while heating an Al-Cu alloy, we were able to follow the growth of individual nanoprecipitates at atomic scale. The heat treatments carried out at 140, 160, 180 and 200 degrees C reveal a temperature dependence on the kinetics of precipitation and three kinds of interactions of nano-precipitates. These are precipitate-matrix, precipitate-dislocation, and precipitate-precipitate interactions. The diffusion of Cu and Al during these interactions, results in diffusion-controlled individual precipitate growth, an accelerated growth when interactions with dislocations occur and a size dependent precipitateprecipitate interaction: growth and shrinkage. Precipitates can grow and shrink at opposite ends at the same time resulting in an effective displacement. Furthermore, the evolution of the crystal structure within an individual nanoprecipiate, specifically the mechanism of formation of the strengthening phase,theta', during heat-treatment is elucidated by following the same precipitate through its intermediate stages for the first time using in-situ S/TEM studies
Tuning the Charge Density Wave and Superconductivity in CuxTaS2
We report the characterization of layered, 2H-type CuxTaS2, for x between 0
and 0.12. The charge density wave (CDW), at 70 K for TaS2, is destabilized with
Cu doping. The sub-1K superconducting transition in undoped 2H-TaS2 jumps
quickly to 2.5 K at low x, increases to 4.5 K at the optimal composition
Cu0.04TaS2, and then decreases at higher x. The electronic contribution to the
specific heat, first increasing and then decreasing as a function of Cu
content, is 12 mJ mol-1 K-2 at Cu0.04TaS2. Electron diffraction studies show
that the CDW remains present at the optimal superconducting composition, but
with both a changed q vector and decreased coherence length. We present an
electronic phase diagram for the system.Comment: 7 pages, 9 figures. To be published in Physical Review
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