A. Brazdeikis

Effect of thallium oxide, Tl2O3 on the formation of superconducting HgBaCaCuO films

Abstract We show that a small doping of Tl 2 O 3 in the bulk material significantly promotes the formation of superconducting HgBaCaCuO films during encapsulated post-annealing. We also demonstrate that by using this method good quality superconducting HgBa 2 CaCu 2 O 6+δ films can directly be prepared using Hg-free thin film precursors. As-prepared films display a strong (00 l ) texture, layer-by-layer growth surface morphology and exhibit superconducting transition temperatures, T c,zero s as high as 117 K as determined from magnetic and electric transport measurements.

Molecular beam epitaxy growth and microstructure of thin superconducting Bi2Sr2CaCu2Ox films

Abstract The microstructure of molecular beam epitaxy (MBE) grown Bi 2 Sr 2 CaCu 2 O x films has been studied by reflection high-energy electron diffraction (RHEED), high-resolution electron microscopy (HREM) and X-ray diffraction (XRD). In situ recorded RHEED images of as-grown films show two-dimensional growth, 90° oriented domains (twist domains) and an incommensurate superstructure. For the first time, the presence of both 2201 and 2223 stacking faults in Bi 2 Sr 2 CaCu 2 O x thin films is reported. These local structural defects are not easily observed by standard XRD techniques. To examine the structure of the films quantitatively, a general one-dimensional XRD model was fitted to the experimental XRD data. The model considered changes in peak intensities, positions and line-widths, and thus allowed a quantitative determination of the structural properties of the high- T c superconducting thin films.

Layer-by-layer growth of Bi2Sr2Can-1CunOx films with n≥3 by molecular beam epitaxy

Abstract Thin films of Bi 2 Sr 2 Ca n-1 Cu n O x were grown using a layer-by-layer molecular beam epitaxy technique. The results demonstrate that nearly single-phase highly metastable films with n =4 to 11 can be synthesized both on SrTiO 3 and MgO substrates and their physical properties studied. X-ray diffraction data of the films with n =8,9, 10 and 11 are presented. The observed Laue oscillations indicate that the films exhibit high crystal quality and nearly perfect surfaces and interfaces. Resistivity measurements show semimetal-semiconductor transitions to occur at temperatures below 100 K, a result of insufficient doping by holes in the CuO 2 planes as n increases.

Synthesis of 2212 structure Bi-Sr-Ca-Cu-O thin films superconducting in the 90 K range

Abstract Thin superconducting Bi-Sr-Ca-Cu-O films with dominating 2212 structure have been synthesized on MgO substrates by simultaneous evaporation of metallic Bi, Sr, Ca and Cu metals from four independent Knudsen evaporation sources and subsequent postannealing of the prepared metal alloy films in air. The thin film zero resistance temperature, T c,zero , dependence on cooling rate from the annealing temperature and on composition changes was investigated. Rapid cooling of the films from 820°C increased T c,zero up to 94 K. Thin films with T c,zero above 90 K were enriched in Bi and Cu with respect to the nominal 2212 composition. Electrical resistivity, X-ray diffraction and optical reflectivity measurements have been performed. Possible factors which can influence T c variation in the films are discussed.

Cation disorder in superconducting HgBa2CaCu2O6+δ films prepared on SrTiO3 substrates

Abstract Hg–Ba–Ca–Cu–O thin films prepared on SrTiO3 (100) substrates generally exhibit shorter c-axis unit cell lengths and lower zero-resistance superconducting transition temperatures as compared to bulk. Microstructural studies of HgBa2CaCu2O6+δ films based on X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) revealed shorter BaO–CuO2 interplanar distances and longer CuO2–CuO2 average distances as compared to those reported for bulk compounds. It is suggested that the observed cationic disorder causes smaller c-axis lattice parameter and is probably induced by the lattice mismatch between film and substrate. The observed atomic rearrangements are expected to influence superconducting properties of the Hg-based films.