36 research outputs found
A Reduced Neodymium Titanate with a New Intergrowth Structure Type
Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å)
Origin of the photoemission final-state effects in Bi2Sr2CaCu2O8 by very-low-energy electron diffraction
Very-low-energy electron diffraction with a support of full-potential band
calculations is used to achieve the energy positions, K// dispersions,
lifetimes and Fourier compositions of the photoemission final states in
Bi2Sr2CaCu2O8 at low excitation energies. Highly structured final states
explain the dramatic matrix element effects in photoemission. Intense c(2x2)
diffraction reveals a significant extrinsic contribution to the shadow Fermi
surface. The final-state diffraction effects can be utilized to tune the
photoemission experiment on specific valence states or Fermi surface replicas.Comment: 4 pages, 3 Postscript figures, submitted to Phys. Rev. Lett; major
revision
Structure optimization effects on the electronic properties of BiSrCaCuO
We present detailed first-principles calculations for the normal state
electronic properties of the high T superconductor
BiSrCaCuO, by means of the linearized augmented plane wave
(LAPW) method within the framework of density functional theory (DFT). As a
first step, the body centered tetragonal (BCT) cell has been adopted, and
optimized regarding its volume, ratio and internal atomic positions by
total energy and force minimizations. The full optimization of the BCT cell
leads to small but visible changes in the topology of the Fermi surface,
rounding the shape of CuO barrels, and causing both the BiO bands,
responsible for the pockets near the \textit{\=M} 2D symmetry point, to dip
below the Fermi level. We have then studied the influence of the distortions in
the BiO plane observed in nature by means of a
orthorhombic cell (AD-ORTH) with space group. Contrary to what has been
observed for the Bi-2201 compound, we find that for Bi-2212 the distortion does
not sensibly shift the BiO bands which retain their metallic character. As a
severe test for the considered structures we present Raman-active phonon
frequencies () and eigenvectors calculated within the frozen-phonon
approximation. Focussing on the totally symmetric A modes, we observe
that for a reliable attribution of the peaks observed in Raman experiments,
both - and a-axis vibrations must be taken into account, the latter being
activated by the in-plane orthorhombic distortion.Comment: 22 pages, 4 figure
and Oxygen Stoichiometry: Structure, Resistivity, Fermi Surface Topology and Normal State Properties
(2212) single crystal samples
were studied using transmission electron microscopy (TEM), plane
() and axis () resistivity, and high resolution
angle-resolved ultraviolet photoemission spectroscopy (ARUPS). TEM reveals that
the modulation in the axis for doped 2212 is dominantly
of type that is not sensitive to the oxygen content of the system, and the
system clearly shows a structure of orthorhombic symmetry. Oxygen annealed
samples exhibit a much lower axis resistivity and a resistivity minimum at
K. He-annealed samples exhibit a much higher axis resistivity and
behavior below 300K. The Fermi surface (FS) of oxygen annealed
2212 mapped out by ARUPS has a pocket in the FS around the
point and exhibits orthorhombic symmetry. There are flat, parallel sections of
the FS, about 60\% of the maximum possible along , and about 30\%
along . The wavevectors connecting the flat sections are about
along , and about along , rather than . The symmetry of the near-Fermi-energy dispersing
states in the normal state changes between oxygen-annealed and He-annealed
samples.Comment: APS_REVTEX 3.0, 49 pages, including 11 figures, available upon
request. Submitted to Phys. Rev. B
On the origin and evolution of the material in 67P/Churyumov-Gerasimenko
International audiencePrimitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects
New Reduced Ternary Titanates from Borate Fluxes
Single crystals of the new reduced strontium titanate SrTi11O20 (space group P1, a = 7.1252(8) Å, b = 7.6644(8) Å, c = 13.1577(16) Å, α = 90.21(1)°, β = 92.79(1)°, γ = 103.94(1)°) are formed by slowly cooling SrTiO2.5 in molten SrO · 2B2O3 flux in high vacuum. The compound crystallizes in a new structure type based on sheared double and single rutile chains of TiO6 octahedra. Formation of a new monoclinically distorted reduced Ti-hollandite, La1.33Ti8O16 (space group I2/m, a = 9.946(2) Å, b = 2.9668(4) Å, c = 10.280(2) Å, β = 91.09(2)°), was observed upon dissolving LaTiO3 in a molten La2O3 · 3B2O3 flux. The crystal structures of both compounds were determined by single crystal X-ray diffraction