V.L. Svetchnikov

A reliable method of TEM cross section specimen preparation of YBCO films on various substrates

Abstract A high yield procedure of cross section preparation of high- T c superconducting YBCO thin films on single crystalline substrates (SrTiO 3 , MgO, YSZ, etc) is presented. The procedure can result in large thin areas (of the order of hundreds of μm) which are sufficiently thin for HREM studies. Some examples of large area electron transparency and high quality HREM images are given.

Dislocation model of superconducting transport properties of YBCO thin films and single crystals

Abstract Based on structural investigations and measurements of the superconducting electrical properties of YBa 2 Cu 3 O 7−x thin films and single crystals we suggest a dominant contribution for the vortex pinning ( H | C -axis) of low angle grain boundary edge dislocations with Burgers vector b = a [100] with a concentration of about 10 11 cm −2 . A theoretical model is developed that describes the depinning mechanism for individual vortices pinned on adjacent parallel linear defects. The vortex energy activation U ( J ) in this pinning mechanism is shown to depend significantly on the transport current magnitude even in the low current region. This brings about the non-linear behaviour of I - V curves in the TAFF regime.

HREM study of the YBCO/MgO interface on an atomic scale

Abstract The film-substrate interface of c oriented YBCO thin films grown by sputtering or laser ablation on (001) MgO substrate has been investigated with high-resolution electron microscopy. The first atomic plane of the YBCO lattice is a CuO chain layer. Two interface configurations occur: (1) the YBCO lattice and the MgO lattice continue up to the interface (this configuration is occasionally associated with some periodic strain in the MgO lattice; (2) the YBCO lattice and the MgO lattice are separated by an (almost) amorphous layer with a thickness of the order of two atomic layers. This amorphous layer is found to lead to the absence of strain. In some cases the surface roughness coincided with misoriented grains but most of the steps in the MgO substrate were accommodated by steps in the YBCO of one or more complete unit cells in height and some lattice bending in the YBCO film.

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg2Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ? 5?nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg2Si formation is fast relative to MgH2. The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg2Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg2Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg2Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg2Si.

An improvement of surface smoothness and lattice match of CeO2 buffer layers on R-sapphire processed by MOCVD

Abstract Epitaxial thin films of CeO 2 and Ce 1− x R x O 2− y (R=Y, Nd; x =0–0.32) on R-sapphire were prepared by band flash evaporation MOCVD. It was demonstrated that the smoothness of the films can be improved by a decrease of deposition rate and by post deposition annealing at 1000 °C. The lattice match of buffer layers with R-Al 2 O 3 and YBa 2 Cu 3 O 7−δ was improved by doping of ceria yttrium and neodymium correspondingly. A bilayer structure Ce 0.7 Nd 0.3 O 2− y /Ce 0.68 Y 0.32 O 2 − y /R-Al 2 O 3 is proposed as potential substrate material for YBa 2 Cu 3 O 7 − 6 deposition.

HREM characterisation of interfaces in YBa2Cu3O7−δ/CeO2/R-Al2O3 structures

Abstract Interfaces between a sapphire ( R -Al 2 O 3 ) substrate, a CeO 2 buffer layer, and a YBa 2 Cu 3 O 7− δ (YBCO) layer — both layers obtained by MOCVD — have been studied by high-resolution electron microscopy. Lattice parameter mismatch between the sapphire substrate and the CeO 2 layer was observed to be accommodated within the first one or two atomic planes of the CeO 2 layer. The density of misfit dislocations correlated well with the mismatch between the CeO 2 and sapphire unit cell parameters. Small imperfections in the sapphire surface were observed not to lead to a change in orientation of the overlying CeO 2 layer, nor to an increased roughness of the CeO 2 surface. The roughness of the CeO 2 surface was found to be considerably less than that of the sapphire surface. The YBCO/CeO 2 interface contained only a small number of dislocations, located in the YBCO layer. Steps in the CeO 2 surface were observed to be accommodated by antiphase boundaries, or planar defects close to the interface.

Effect of growth-induced linear defects on high frequency properties of pulse-laser deposited YBa2Cu3O7-δ films

Growth-induced linear defects are shown to strongly affect the microwave surface resistance, Rs, of highly biaxially oriented high temperature superconductor (HTS) YBa2Cu3O7−δ (YBCO) films. Measured Rs(77 K) turned out to be 4–5 times higher than in single crystals. The films were deposited by modified pulse-laser technique, Jc(77 K) = (3-6) × 106 A/cm2, onto LaAlO3 substrates. Rs(T) was measured at 134 GHz and 20–100 K. TEM/HREM study of YBCO films deposited at Ts = 750°C–780°C revealed a reduction of edge dislocation density with Ts increase (from 2 · 1011 to 1010 lines/cm2). YBCO films deposited at Ts = 780°C exhibited the smallest Rs(77 K, 134 GHz) < 120 mΩ and the lowest density of dislocations detected by HREM and X-ray analysis. A nature of the dislocation effect is discussed within a model of local anisotropic elastic deformation in a vicinity of dislocation cores, where Tc variation and an enhancement of normal quasiparticle density are significant.

Metalorganic chemical vapour deposition of high quality LuBa2Cu3O7 −δ thin films Peculiarities of growth and superconducting properties

Abstract Epitaxial LuBa 2 Cu 3 O 7−δ films were prepared by flash evaporation metalorganic chemical vapour deposition (MOCVD) on LaAlO 3 , SrTiO 3 and ZrO 2 (Y 2 O 3 ) substrates. The highest T c and j c (77 K, 100 Oe) values achieved were 89 K, 2.7·10 6 A cm −2 , 88 K, 1.5·10 6 A cm −2 and 87 K, 1.0·10 6 A cm −2 , respectively. An influence of p (O 2 ) (at the deposition temperature T =800 °C) on structural and superconducting characteristics of LuBa 2 Cu 3 O 7−δ films was found to be similar to that in the case of YBa 2 Cu 3 O 7−δ , in spite of the difference in morphology features and j c ( T ) dependencies. Occurrence of secondary phases inclusion in LuBa 2 Cu 3 O 7−δ films was analysed by XRD and TEM. The “fully oxygenated” LuBa 2 Cu 3 O 7 was found to be overdoped: an increase of T c value by 3 K was observed after an annealing of the films at reduced p (O 2 ).

Vortex pinning/dynamics in YBCO films with J{sub c} (77 K) {gt} 3{center{underscore}dot}10{sup 6} A/cm{sup 2} studied by direct transport measurements

Critical current density, Jc, is studied for highly biaxially-oriented YBCO thin films with Jc(77 K) > 3 MA/cm2in a range of magnetic fields, temperatures, angles between magnetic field vector and film c-axis. Jc(H, θ) is shown to be dominated by vortex interaction with high density (> 1011lines/cm2) of linear pins — edge dislocations. A model is developed to describe different vortex arrays behavior in the presence of two-dimensional square network of linear pins.

The structure of BaCu3O4 particles occurring on thin HoBa2Cu3O7 films prepared by MOCVD

Abstract The structure of BaCu 3 O 4 phase occurring as particles on the surface of (001) RBa 2 Cu 3 O 7 epitaxial films prepared by metalorganic chemical vapor deposition (MOCVD) has been investigated with quantitative electron diffraction and HREM. The orthorhombic unit cell is a =1.097(9) nm, b =0.554(3) nm, c =0.394(2) nm with space group Cmmm, the values being in agreement with X-ray diffraction (XRD) study. The structure consists of alternating Cu 3 O 4 and Ba layers along the c -axis. The compound is stabilised due to the formation of low-energy coherent boundaries with RBa 2 Cu 3 O 7 and/or perovskite substrate.