William Y. Hsu

Ion transport and clustering in nafion perfluorinated membranes

Abstract A theory of ion-clustering is presented for Nation resins. An expression for the cluster diameter is derived; it correctly describes the experimental variations in cluster diameter with cation form, equivalent weight, and water content of the polymers. The theory further suggests that short channels connecting adjacent clusters are thermodynamically stable in support of the cluster-network model proposed by one of us earlier. The percolative aspects of ion transport and an ionic insulator-to-conductor transition in Nation resins are discussed.

Fabrication and characterization of optical waveguides in KTiOPO4

Planar and channel optical waveguides have been fabricated in KTiOPO4 (KTP) using ion exchange processes. The exchange kinetics are diffusion limited and depend strongly on composition and surface orientation. Increases in surface refractive index of up to 0.23 and waveguide thicknesses of up to 15 μm have been observed. A simple waveguide modulator has been fabricated which shows electro‐optic r coefficients in the waveguide region to be similar to bulk KTP.

Percolation phenomena in polymer/carbon composites

Etude systematique de la percolation dans les proprietes electriques et dielectriques de polyester, PVC, polycarbonate renfermant du noir de carbone

Band structure of metallic pyrochlore ruthenates Bi2Ru2O7 and Pb2Ru2O6.5

The band structure of Bi2Ru2O7 and Pb2Ru2O6.5 has been computed self‐consistently from first principles for the first time by the pseudofunction method. We discover that the 6s bands of Bi and Pb are very deep and unlikely to contribute to the metallic behavior as previously believed. The unoccupied 6p bands, however, are only several eV above the Fermi energy and are mixed with the Ru 4d band at the Fermi surface via the framework O atoms, leading to band conduction and delocalized magnetic moments. The predicted location of the 6s bands and the location and width of the O 2p band are confirmed by synchrotron radiation and ultraviolet electron spectroscopy of single crystals.

Electronic structure of pure and doped orthorhombic La2CuO4

Abstract The electronic structure of orthorhombic La2CuO4 has been investigated by first principles pseudofunction band calculations and group theoretical analysis. We find that pure as well as doped compounds remain metallic at all finite temperatures as a conseqence of the Cmca (D182h) space group symmetry. The experimentally observed rapid rise in resistivity below 30°K suggests a structural transition to a lower symmetry space group that could be driven electronically or magnetically. One possible candidate is monoclinic C2/m (C32h) which is a subgroup of Cmca and can be obtained by distorting the CuO bonds or rotating the CuO6 octahedra. Implications for superconductivity are discussed.

Electronic properties of polyacetylene, polyethylene, and polytetrafluoroethylene

The extended muffin tin orbitals technique was applied to study electronic properties of polyethylene (PE), polytetrafluoroethylene (PTFE), and polyacetylene (PA). The computed optical gap is 9 and 6.2 eV, respectively, for PE and PTFE whereas it is 1.2 to 1.7 eV for PA depending on the isomeric forms. Three dimensional charge density contours are also presented.

Urbach rule behavior in strongly absorbing fine particle solids

The intrinsic absorption spectrum α(E) of fine particles (<0.2 μm) of PbCrO4, BiVO4, CdS, and ZnS, respectively, was measured in the vicinity of the visible absorption edge from 20 to 300 K by diffuse reflectance. In PbCrO4 and in isoelectronic BiVO4, the edge is exponential and steepens, while remaining fixed in energy, as T is reduced. The edge fits the Urbach behavior: σ=dlnα/dE=(2σ0/ℏΩ0) tanh (ℏΩ 0/2 kBT), where σ0 and Ω0 are constants and σ≂80 eV−1 at 300 K. We find ℏΩ0=25×10−3 eV in PbCrO4, nearly coinciding with the 23×10−3 eV vibration observed in natural crocoite. For the exponential edge in single crystal CdS, σ≂80 eV−1 at 300 K and also increases at low T. However, the edge shifts to higher energy at low T. In fine particles of pure CdS, the energy shift was again found, but σ≂20 eV−1, independent of T. This suggests that in pigmentary CdS, σ is controlled by extrinsic factors such as defects. It behaves as a compensated semiconductor in which color arises from a transition between delocalized ...

Electronic structures of BaB2O4 and LiB3O5

The electronic bands of BaB2O4 (BBO) and LiB3O5 (LBO) have been computed from first principles using the ab initio pseudofunction method. Optical conductivities and densities of states in good agreement with optical measurements and x‐ray photoemission are obtained. The gap in BBO results from the borate group to the Ba. In LBO, the gap is from a fourfold coordinated borate to a threefold coordinated borate. This suggests that cluster calculations based on the borate groups alone is a reasonable approximation for LBO, but not for BBO.

Scanning tunneling microscopy of the cleaved surface of Bi2Sr2CaCu2O8

Abstract Scanning tunneling microscopy images of the cleaved surface of Bi 2 Sr 2 CaCu 2 O 8 at room temperature have been obtained. Comparison of the dependence of scan line characteristics on tunneling bias with calculations on the density of states of each elemental component around the Fermi energy suggests that tunneling occurs on the Bi face. High resolution images show striations ∼ 10 to 30 A wide. On samples oriented with Laue reflection, the striations are observed to run parallel to the a or b axis of the crystal. These striations are likely to be associated with the undulations along the b axis of the Bi layer observed in the bulk in recent transmission electron micrographs. The topographical and tunneling bias dependence results show that the B-rich surface of this new superconductor is essentially a termination of the bulk structure even though structural or chemical defects are common.

The quasi-1D nature of the superconducting instability in La2-xSrxCuO4

Abstract The effect of the orthorhombicity of La0.85Sr0.15CuO4 on its superconducting behaviour is discussed. Calculations of the superconducting transition temperature are carried out as a function of the doping x using a mechanism we recently proposed that exploits electron-hole fluctuation in one dimension. In a single consistent picture, we have been able to produce the increase of the superconducting transition temperature from zero as the doping concentration x is increased from zero.