V. Y. Lee

Organic glasses : A new class of photorefractive materials

The performance of amorphous organic photorefractive (PR) materials in applications such as optical data storage is generally limited by the concentration of active molecules (chromophores) that can be incorporated into the host without forming a crystalline material with poor optical quality. In polymeric PR systems described previously, performance has been limited by the necessity of devoting a large fraction of the material to inert polymer and plasticizing components in order to ensure compositional stability. A new class of organic PR materials composed of multifunctional glass-forming organic chromophores is described that have long-term stability and greatly improved PR properties.

New procedure for determination of [Cu-O]+p charge and oxygen content in high Tc copper oxides

Abstract The properties of High Tc Superconductors are critically dependent on the degree of oxidation, i.e., on the average [Cu-O]+p charge (or formal oxidation state Cu+2+p of copper). This fact makes it important to accurately characterize High Tc copper oxide samples in term of this key parameter. We describe here a simple and reliable procedure for determining the effective copper oxidation level and, as a result, the oxygen content. Using a modification of standard iodometric titration techniques, our procedure involves the ratio of the results of two titrations: one to determine the copper concentration in the sample and the second to measure the degree of oxidation beyond Cu+. Using this ratio, numerous errors and difficulties are minimized. In addition, the exact stoichiometry of the sample need not be known. Experimental results on YBa2Cu3Oy are discussed which demonstrate the accuracy, reproducibility and reliability of our procedure.

Quaternary phase relations near YBa2Cu3O6+x at 850°C in reduced oxygen pressures

Abstract The quatenary phase relations near YBa 2 Cu 3 O 6+ x at 850°C were determined by oxygen coulometric titration and microanalysis. The phase relations change markedly as the oxygen pressure decreases, due primarily to the formation of a liquid phase and a reduced oxide, BaCu 2 O 2 . Nine invariant reactions were discovered, including decomposition of YBa 2 Cu 3 O 6+ x into Y 2 BaCuO 5 , BaCu 2 O 2 and YBa 3 Cu 2 O 6+ y . The lower limit of oxygen partial pressure at which pure YBa 2 Cu 3 O 6+ x is stable is 4.0×10 −4 atm O 2 at 850°C; the lower limit increases in the presence of BaCuO 2 and Cu 2 O impurities. These results indicate that the temperatures used for single crystal growth of YBa 2 Cu 3 O 6+ x could be lowered by using reduced oxygen pressures or, alternatively, that constant temperature crystal growth could be induced by increasing the oxygen partial pressure.

Pore size distributions in nanoporous methyl silsesquioxane films as determined by small angle x-ray scattering

Small angle x-ray scattering (SAXS) measurements were performed on nanoporous methyl silsesquioxane films that were generated by the incorporation of a sacrificial polymeric component into the matrix and subsequently removed by thermolysis. The average pore radii ranged from 1 to 5 nm over a porosity range of ∼5–50%. The distribution in pore size was relatively broad and increases in breadth with porosity. The values and observations obtained by SAXS are in good agreement with field emission scanning electron microscopy.

The Many Faces of ET

Abstract A major feature of the ET molecule is its ability to form many different phases and structures with the same or different anions. Whilst this considerably complicates the growth and characterisation of these materials this does allow a systematic study of compounds differing only in charge state or crystal structure, but otherwise containing the same donor and acceptor species. We show that ET forms at least four different phases with the ReO4 −anion, allowing such a systematic study. We show that one of these, γ-ET3(ReO4)2, is the only currently known example of an ET charge transfer salt in which the anions are not ordered at room temperature. We find the anions become ordered at a lower temperature. We show that anions smaller than some certain value stabilise a single phase containing 3 ET molecules for every two anions and that larger anions form different sets of crystal structures.

Specific heat (1–330 K), Meissner effect and magnetic susceptibility of Tl2Ba2Ca2Cu3O10, Tl2Ba2CaCu2O8 and Tl2Ba2CuO6 ceramic samples

Abstract The specific heat (T⩽330 K), magnetic susceptibility (T⩽250 K) and Meissner effect of three samples of the series Tl2Ba2Can−1CunO4+2n+x with n=1, n=2 and n=3 are reported. The critical temperatures measured by AC susceptibility are Tc(onset)≈10 K for n=1, Tc(midpoint)=99.5 K for n=2 and Tc(midpoint)=121.4 K for n=3, with transition widths (10%–90%) smaller than 10 K. The Meissner field expulsion at 4 K in 20 G amounts to 8%, 51% and 24%, respectively. The constant component of the susceptibility increases with the number of Cu layers and with Tc. The linear term in the low-temperature specific heat is lower than 7 mJK-2mol-1 and scales with the impurity Curie term of the magnetic susceptibility. Small anomalies are observed in the specific heat at Tc; their shape and their amplitude are consistent with those of the Meissner transitions. The origin of smearing is discussed in relation with the small coherence length. Thermochemical data are tabulated.

POLYSILOXANE-BASED PHOTOREFRACTIVE POLYMERS FOR DIGITAL HOLOGRAPHIC DATA STORAGE

A photorefractive polymer system based on a substituted polysiloxane backbone is described. In addition to high holographic diffraction efficiency and reasonable writing speed, the material reported exhibits excellent optical clarity and low optical scattering characteristics. These latter optical properties are necessary in high density holographic digital data storage applications. The utility of the polysiloxane based photorefractive polymers for storage applications is demonstrated by recording digital data at a density of 0.52 Mbit/cm2 and reading it back without error.

Studies of superconducting oxides with a solid-state ionic technique

Abstract This paper describes the use of a solid-state ionic technique to prepare Y1Ba2Cu3Ox samples with precisely controlled oxygen contents and to determine the Y Ba Cu O quaternary phase diagram in the vicinity of Y1Ba2Cu3Ox.

Preparation and X-ray characterization of superconducting and related Tl2Ba2CuO6 phases

Abstract Polycrystalline oxides of the form Tl 2 Ba 2 CuO 6 have been synthesized by a solid-state reaction technique. Tl 2 Ba 2 CuO 6 displays a variety of properties, ranging from superconductivity to semiconductivity depending on the preparation conditions. X-ray diffraction analysis showed that a superconducting Tl 2 Ba 2 CuO 6 phase with Tc∼20K has a body-centered tetragonal structure similar to those of the Tl 2 Ca 1 Ba 2 Cu 2 O 8 and Tl 2 Ca 2 Ba 2 Cu 3 O 10 high Tc superconductors. The tetragonal unit cell dimensions refined from a high quality powder diffraction pattern are a =3.8632(2) A , c =23.160(1) A , and V =345.7(2) A 3 . The non-superconducting Tl 2 Ba 2 CuO 6 phase was found to have a face-centered orthorhombic structure with a =5.4451(3) A , b =5.4961(7) A , c =23.152(2) A , and V =692.9(1) A 3 .

High Tc YBa2Cu3O7−x superconducting thin films by rf magnetron sputtering

Thin films of YBa2Cu3O7−x were deposited by RF magnetron sputtering onto BaTiO3, Al2O3, MgO, and SrTiO3 substrates at ambient temperature to 500°C from a sintered powder target having the same nominal composition. Four point AC resistivity measurements indicate that these films (∼2μm thick) are superconducting after annealing in flowing oxygen at ∼900°C for several minutes. While the onset Tc remains fairly constant at ∼88–91 K, the ending Tc depends strongly on the deposition and annealing conditions, and substrates used. The films deposited at 300°C and annealed at 900°C for 1min., for example, have an ending Tc of 45–53, 70–83, and 80–86.5 K when deposited, respectively, on Al2O3, BaTiO3 and MgO, and SrTiO3. Electron microprobe analyses show that these films have a spatially homogeneous composition of typically ∼YBa2Cu3O8. X‐ray diffraction data also reveal that the annealed films have the characteristics orthorhombic structure with lattice constants practically identical to those of the bulk YBa2Cu3O7−x, although minor phases (e.g., BaCuO2) with concentration of up to 15% can also be detected. These effects of ion bombardment during deposition and of interdiffusion of film and substrate during annealing are also presented, with emphasis on the film composition and Tc, and stability in room ambient.Thin films of YBa2Cu3O7−x were deposited by RF magnetron sputtering onto BaTiO3, Al2O3, MgO, and SrTiO3 substrates at ambient temperature to 500°C from a sintered powder target having the same nominal composition. Four point AC resistivity measurements indicate that these films (∼2μm thick) are superconducting after annealing in flowing oxygen at ∼900°C for several minutes. While the onset Tc remains fairly constant at ∼88–91 K, the ending Tc depends strongly on the deposition and annealing conditions, and substrates used. The films deposited at 300°C and annealed at 900°C for 1min., for example, have an ending Tc of 45–53, 70–83, and 80–86.5 K when deposited, respectively, on Al2O3, BaTiO3 and MgO, and SrTiO3. Electron microprobe analyses show that these films have a spatially homogeneous composition of typically ∼YBa2Cu3O8. X‐ray diffraction data also reveal that the annealed films have the characteristics orthorhombic structure with lattice constants practically identical to those of the bulk YBa2Cu3O7...