432 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
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
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
Charge-ordering, commensurability and metallicity in the phase diagram of layered Na(x)CoO(2)
The phase diagram of non-hydrated Na(x)CoO(2) has been determined by changing
the Na content x using a series of chemical reactions. As x increases from 0.3,
the ground state goes from a paramagnetic metal to a charge-ordered insulator
(at x=1/2) to a `Curie-Weiss metal' (around 0.70), and finally to a weak-moment
magnetically ordered state (x>0.75). The unusual properties of the state at 1/2
(including particle-hole symmetry at low T and enhanced thermal conductivity)
are described. The strong coupling between the Na ions and the holes is
emphasized.Comment: 4 pages with 3 figures, changed conten
Ferromagnetism below 10 K in Mn doped BiTe
Ferromagnetism is observed below 10 K in [Bi0.75Te0.125Mn0.125]Te. This
material has the BiTe structure, which is made from the stacking of two
Te-Bi-Te-Bi-Te blocks and one Bi-Bi block per unit cell. Crystal structure
analysis shows that Mn is localized in the Bi2 blocks, and is accompanied by an
equal amount of TeBi anti-site occupancy in the Bi2Te3 blocks. These TeBi
anti-site defects greatly enhance the Mn solubility. This is demonstrated by
comparison of the [Bi1-xMnx]Te and [Bi1-2xTexMnx]Te series; in the former, the
solubility is limited to x = 0.067, while the latter has xmax = 0.125. The
magnetism in [Bi1-xMnx]Te changes little with x, while that for
[Bi1-2xTexMnx]Te shows a clear variation, leading to ferromagnetism for x >
0.067. Magnetic hysteresis and the anomalous Hall Effect are observed for the
ferromagnetic samples.Comment: Accepted for publication in Phys. Rev.
Superconductivity at 2.3 K in the misfit compound (PbSe)1.16(TiSe2)2
The structural misfit compound (PbSe)1.16(TiSe2)2 is reported. It is a
superconductor with a Tc of 2.3 K. (PbSe)1.16(TiSe2)2 derives from a parent
compound, TiSe2, which shows a charge density wave transition and no
superconductivity. The crystal structure, characterized by high resolution
electron microscopy and powder x-ray diffraction, consists of two layers of
1T-TiSe2 alternating with a double layer of (100) PbSe. Transport measurements
suggest that the superconductivity is induced by charge transfer from the PbSe
layers to the TiSe2 layers.Comment: 17 pages, 4 figures. To be published in Physical Review
Two and Three Dimensional Incommensurate Modulation in Optimally-Doped BiSrCaCuO
X-ray scattering measurements on optimally-doped single crystal samples of
the high temperature superconductor BiSrCaCuO reveal
the presence of three distinct incommensurate charge modulations, each
involving a roughly fivefold increase in the unit cell dimension along the {\bf
b}-direction. The strongest scattering comes from the well known (H, K
0.21, L) modulation and its harmonics. However, we also observe broad
diffraction which peak up at the L values complementary to those which
characterize the known modulated structure. These diffraction features
correspond to correlation lengths of roughly a unit cell dimension,
20 in the {\bf c} direction, and of 185
parallel to the incommensurate wavevector. We interpret these features as
arising from three dimensional incommensurate domains and the interfaces
between them, respectively. In addition we investigate the recently discovered
incommensuate modulations which peak up at (1/2, K 0.21, L) and related
wavevectors. Here we explicitly study the L-dependence of this scattering and
see that these charge modulations are two dimensional in nature with weak
correlations on the scale of a bilayer thickness, and that they correspond to
short range, isotropic correlation lengths within the basal plane. We relate
these new incommensurate modulations to the electronic nanostructure observed
in BiSrCaCuO using STM topography.Comment: 8 pages, 8 figure
- …