Ej Williams

Microstructure of (001), (110) and (103) oriented thin films of YBa2Cu3O7−x investigated with STM, SEM and HRTEM

Abstract Epitaxial films of (110) or (103) YBa 2 Cu 3 O 7− x grown on (110) SrTiO 3 substrates by laser ablation are compared to (001) films. Their microstructure is investigated with scanning tunneling microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. The growth of steps, terraces and spirals originating from screw dislocations commonly seen in c -axis films is inhibited in films grown on (110) SrTiO 3 . In the case of (103) oriented films, the growth of 90° twisted domains produces a tooth-like structure with preferred orientation. The influence of vicinal substrates (a few degrees of misorientation) on the film growth, on the nature of defects and on the resistive behavior is also investigated.

Electrochemical oxidation of La2CuO4 thin films grown by molecular beam epitaxy

Thin insulating and c-axis oriented films of La2CuO4 are grown using a molecular beam epitaxy technique. Subsequently, these films are oxidized electrochemically using a 1N KOH solution. This approach is used to induce superconductivity, leading to a maximum Tc0 of 31 K,, measured both resistively and inductively. The surface morphology, lattice constants and the resistivity before and after the electrochemical treatment are compared.

Local electrochemical oxidation/reduction: First step towards a new lithography?

Using c‐axis La2CuO4 thin films, we first demonstrate that the electrochemical oxidation mainly occurs along the c axis. Then we induce oxygen‐rich regions electrochemically into an otherwise oxygen‐deficient matrix. On a microscopic scale the extra oxygen introduced creates metallic and/or superconducting regions in the insulating matrix. Contrary to other lithographic techniques in which large amounts of material are either removed or deposited, this technique does not induce significant height differences.

High-Tc Superconductor-Insulator-Superconductor Heterostructures with Highly Resistive Insulator Layers

Trilayers in which a dielectric or ferroelectric film such as SrTiO3, BaTiO3 or Bax Sr1-x TiO3 separates two layers of a high-T c superconductor are required for many device applications of high-T c materials. To clarify the mechanism limiting the insulation properties of the insulating layer, the microstructure of such samples was analyzed using transmission electron microscopy. These studies suggest that diffusion along threading dislocations or along grain boundaries in the insulating layer lowers its resistivity. Substitution of the insulating monolayer with a bilayer increases the resistivity from 1011 to 3×1013 Ω cm (at 2×105 V/cm, below 100 K). To our knowledge, this is by far the highest resistivity achieved in such heterostructures.

Growth and electrochemical oxidation of MBE-grown c-axis La2CuO4 thin films on different substrates

Abstract The growth and subsequent electrochemical oxygen intercalation of c -axis La 2 CuO 4 + δ thin films on substrates with different lattice mismatch [SrTiO 1 (001) (+3.03%), NdGaO 1 (001) (+1.90%). LaAlO 3 (001) (−0.05%) and SrLaAlO 4 (001) (−0.95%) substrates] are compared. The films grown on the different substrates can all be oxidized electrochemically, but their structural and transport properties differ vastly. The results can roughly be divided into two categories, i.e., those of films grown with a compressive or a tensile lattice mismatch where the former have a larger c -axis latice parameter and better transport properties.

Characterization of MBE-grown ultrathin films in the La2-xSrxCuO4±δ system

Using a molecular beam epitaxy deposition technique, c-axis La2-xSrxCuO4+/- (delta ) ultrathin films have been prepared on (001) SrTiO3 substrates. Several superconductive properties such as the critical temperature Tc, the penetration depth (lambda) ab(0), the activation energy for flux flow (Delta) U and the Hall coefficient RH are reported for the same set of films. As the dopant content is increased, maximum values for Tc and (Delta) U are observed near the optimum doping while RH decreases continuously.

Superconducting La2-xBaxCuO4 thin films prepared by chemical beam epitaxy☆

Abstract A chemical beam epitaxy system comprising low temperature effusion cells and a source of atomic oxygen has been developed for the preparation of cuprate thin films. Superconducting, smooth and single phase c -axis La 2− x Ba x CuO 4 thin films have been grown using Cu ( piv ) 2 , La ( piv ) 3 and Ba ( thd ) 2 metallorganic precursors, demonstrating the viability of this inexpensive technique.

Visualization of columnar defects in superconductors

Abstract Columnar defects in single crystals of superconductors were investigated using scanning probe microscopy. We show that the observable topography strongly depends on the crystal structure as well as on the type of the interaction with the probe. In scanning tunneling microscopy studies, the low conductance of the amorphous tracks leads to tip-surface contact. Owing to this contact, the defects are imaged as hollows having a depth that primarily reflects the tunneling distance. For the high transition temperature materials, atomic force microscopy images the real defect structure as hillocks growing out of the surface. This outgrowth of amorphous material is time dependent and produced by the relaxation of irradiation-induced stress. The dynamic outgrowth of the columnar defects is discussed in terms of a so-called “tooth paste” model.

Transmission electron microscopy investigations of defects in molecular beam epitaxy-grown oxide films

Abstract With a view to increasing the superconductive transition temperature of thin films of La 2 CuO 4 grown by molecular beam epitaxy, films were grown on substrates of lattice parameters such that the film-substrate misfit became small and tensile, or compressive. The microstructure of these films was investigated using transmission electron microscopy and qualitative correlations with physical properties suggested.

3D-XY critical behavior in La2−xSrxCuO4 thin films probed by penetration depth measurement

We sketch the evidence for anisotropic 3D-XY critical behavior in La2−xSrxCuO4 thin films and explore the implications of this scenario, supplemented by the experimental phase transition line.