Jack M. Williams

Surface relief structures on azo polymer films

All optical fabrication of Surface Relief Grating (SRG) on azobenzene functionalized polymer films is reviewed in this article. The uniqueness of the single step erasable photofabrication of large amplitude surface relief structures on azo polymer films is mainly due to the photoisomerization and photoanisotropic behavior of the azobenzene group. The design and synthesis of several classes of suitable azobenzene functionalized polymers for the SRG formation is presented. Experimental studies are carried out in a systematic manner to understand the chemical and physical intricacies involved in the SRG formation process. Structural requirements and their limitations for the efficient fabrication of SRGs on azo functionalized polymer films are also discussed. Understanding the fundamental mechanism of SRG formation on the azo functionalized polymer films enables us to make use of these structures in a variety of applications.

Organic Superconductors—New Benchmarks

Recent advances in the design and synthesis of organic synthetic metals have yielded materials that have the highest superconducting transition temperatures (Tc ≈ 13 kelvin) reported for these systems. These materials have crystal structures consisting of alternating layers of organic donor molecules and inorganic anions. Organic superconductors have various electronic and magnetic properties and crystal structures that are similar to those of the inorganic copper oxide superconductors (which have high Tc values); these similarities include highly anisotropic conductivities, critical fields, and short coherence lengths. The largest number of organic superconductors, including those with the highest Tc values, are charge-transfer salts derived from the electron donor molecule BEDT-TTF or ET [bis(ethylenedithio)-tetrathiafulvalene]. The synthesis and crystal structures of these salts are discussed; their electrical, magnetic, and band electronic structure properties and their many similarities to the copper oxide superconductors are treated as well.

Photoinduced surface deformations on azobenzene polymer films

Laser-induced surface deformation and surface relief grating formation at different intensity levels on azobenzene-functionalized polymer films were investigated. The surface deformation profile induced by a Gaussian laser beam at low intensities reveals strong polarization dependence and establishes that an optical-field component in the direction of the optical-field gradient is essential for this deformation process. A theoretical analysis based on the optical-field gradient force model is presented to account for the deformation behavior at low intensities. In the higher-intensity regime, however, a different mechanism is found to dominate the surface deformation process. Simultaneous formation of two different types of topographic gratings is observed in a two-beam interference experiment. The phase relationship between these surface relief gratings and the interference intensity pattern was deduced based on the results from the single-beam experiments. The erasing behaviors of the two distinct surfa...

Single laser beam-induced surface deformation on azobenzene polymer films

The surface deformation induced by a Gaussian laser beam on azobenzene polymer film reveals strong polarization dependence and establishes that an optical field component in the direction of the intensity gradient is essential for the deformation process. The experimental results allow us to infer the phase relationship between the surface relief gratings and the writing interference pattern.

Strain index, lattice softness and superconductivity of organic donor-molecule salts: Crystal and electronic structures of three isostructural salts k-(BEDT- TTF)2Cu[N(CN)2]X (X=Cl, Br, I)

Abstract The recently discovered k -phase salts k -(BEDT-TTF) 2 ]Cu[N(CN) 2 ]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c =11.6K, the Cl-salt becomes superconducting ( T c =12.8K) under a slight applied pressure of 0.3 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar. We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements. The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties. The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts. The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.

A new ambient-pressure organic superconductor: (BEDT-TTF)2(NH4)Hg(SCN)4

Abstract A new ambient-pressure organic superconducting charge-transfer salt, (BEDT-TTF) 2 (NH 4 )Hg(SCN) 4 , is reported. The unit cell parameters of this salt indicate that it is isostructural to (BEDT-TTF) 2 KHg(SCN) 4 , which possesses an α-type packing motif. The broad ESR line width of 56 to 84 G at 300 K as well as its temperature dependence further indicates that this salt has an α-type packing. Conductivity measurements show that (BEDT-TTF) 2 (NH 4 )Hg(SCN) 4 is metallic to low temperatures and inductive studies by RF penetration depth measurements demonstrate that it is superconducting with an onset at 1.15 K.

New κ-phase materials, κ-(ET)2Cu[N(CN)2]X.X=Cl, Br and I. The synthesis, structure and superconductivity above 11 K in the Cl (Tc = 12.8 K, 0.3 kbar) and Br(Tc = 11.6 K) salts

Abstract The syntheses, structures, selected physical properties, and band electronic structures of three copper (I) dicyanamide halide salts of bis(ethylenedithio)tetrathiafulvalene [κ-(ET) 2 Cu[N(CN) 2 ]X, where X = Cl, Br, and I] are discussed. X-ray crystallographic studies demonstrate that the three derivatives are isostructural. The bromide salt is an ambient pressure superconductor with an inductive onset at 11.6 K and a resistive onset at 12.5 K. κ-(ET) 2 Cu[N(CN) 2 ]Cl exhibits the highest reported superconducting transition temperature (T c = 12.8 K. 0.3 kbar) for an organic superconductor, once a semiconductor-semiconductor transition (42 K) is suppressed. The application of GE varnish or Apiezon N grease to crystals of κ-(ET) 2 Cu[N(CN) 2 ]Cl provides sufficient stress to induce superconductivity at “ambient pressure”. Crystals of the iodide remain metallic to ∼150 K, where they become weakly semiconductive. No sign of superconductivity was detected at pressures (hydrostatic and shearing) up to 5.2 kbar and at temperatures as low as 1.1 K. The band electronic structures of the three salts are essentially identical. The differences in superconducting properties are explained in terms of differences in lattice softness, which are strongly influenced by short C-H…donor and C-H…anion contacts.

Effect of structural disorder on organic superconductors: a neutron diffraction body of "high-Tc" .beta.*-(bis(ethylenedithio)tetrathiafulvalene)2 triiodide at 4.5 K and 1.5 kbar.

In this paper the authors report the existence of a new ordered structural phase, denoted as ..beta..*-(ET)/sub 2/I/sub 3/, at T = 4.5 K and P = 1.5 kbar. To our knowledge, this is the first structural study of an organic superconductor with the sample at a temperature below the onset temperature for superconductivity. The structure of ..beta..*-(ET)/sub 2/I/sub 3/ derived from the data set obtained with the sample below the superconducting transition temperature, at 4.5 K and 1.5 kbar, is completely ordered but is not isostructural with the ..beta..-phase structures of the I/sub 2/Br/sup -/, AuI/sub 2//sup -/, and IBr/sub 2//sup -/ salts.

Infrared properties of the ambient pressure organic superconductor (BEDT-TTF)2I3

Abstract The infrared principal axes of superconducting β-(BEDT-TTF)2I3 are found to be directed along the chain axis and perpendicular to the axis in the a-b plane. At low temperatures well-defined plasma edges appear in both directions. Adopting a simplified 2-D orthorhombic band structure model, effective transfer integrals are found to be  = 0.10 eV and t perp; = 0.13 eV . A cross-over from semiconductor-like optical properties at 300 K to metallic-like optical properties at 40 K is observed in both directions. This is the first such observation in any organic conductor.

New cation-anion interaction motifs, electronic band structure, and electrical behavior in .beta.-(ET)2X salts [ET = bis(ethylenedithio)tetrathiafulvalene; X = dichloroiodide, chlorobromoiodide]

Etude structurale des composes du titre. Determination des temperatures de transition metal-isolant