R. Savoy

Crystallography and microstructure of Y1Ba2Cu3O9−x, a perovskite‐based superconducting oxide

We have investigated the crystallography and microstructure of Y1Ba2Cu3O9−x with transmission electron microscopy and x‐ray diffraction. Y1Ba2Cu3O9−x is a distorted, oxygen‐defect perovskite with ordering of the yttrium and barium ions. Its unit cell is orthorhombic with space group Pmm2 and lattice parameters a=3.893 A, b=11.688 A, and c=3.820 A. The structure is heavily twinned on {101} type planes, possibly due to a tetragonal‐to‐orthorhombic transition above room temperature.

Superconducting Tl-Ca-Ba-Cu-O thin films with zero resistance at temperatures of up to 120 K

We have prepared superconducting Tl‐Ca‐Ba‐Cu‐O thin films on a variety of substrates with transition temperatures as high as ≂120 K, confirmed by sharp onsets of substantial Meissner and shielding signals at the same temperatures. The properties of the films are found to depend sensitively on the post‐annealing conditions. Highly textured c‐axis‐oriented films comprised mostly of Tl2Ca1Ba2Cu2Ox, Tl1Ca2Ba2Cu3Ox, and Tl2Ca2Ba2Cu3Ox were synthesized by varying the annealing procedure with corresponding maximum superconducting transition temperatures of ≂100, 110, and 120 K, respectively.

Crystallography and microstructure of Tl-Ca-Ba-Cu-O superconducting oxides

We report on the crystallography and microstructure of six oxides in the Tl‐Ca‐Ba‐Cu‐O system with nominal compositions Tl1Ba2Cu1O5, Tl1Ca1Ba2Cu2O7, Tl1Ca2Ba2Cu3O9, Tl2Ba2Cu1O6, Tl2Ca1Ba2Cu2O8, and Tl2Ca2Ba2Cu3O10. The structures consist of one, two, or three Cu perovskite‐like units sandwiched between Tl‐O monolayers or bilayers. The predominant defects in the crystals with bilayer and trilayer Cu perovskite‐like units are stacking faults that produce local intergrowths of related structures. The density of stacking defects in these oxides correlates with changes in the superconducting transition temperatures.

High Tc YBa2Cu3O7−x thin films on Si substrates by dc magnetron sputtering from a stoichiometric oxide target

Thin films of YBa2Cu3O7−x were deposited on Si substrates at 600–700 °C by dc magnetron sputtering from a stoichiometric oxide target. Resistivity measurement results indicate that these films are superconducting with a zero resistance Tc as high as 76 K, without further high‐temperature post‐annealing treatments. These films give both core and valence‐band x‐ray photoemission, and x‐ray diffraction spectra similar to those for superconducting films prepared with a high‐temperature post‐annealing step. No significant diffusion of Si from the substrate into the film was detected for the films deposited at 650 °C or lower, according to depth profiles obtained using secondary ion mass spectrometry.

Phase stability limits of Bi2Sr2Ca1Cu2O8+δ and Bi2Sr2Ca2Cu3O10+δ

We determined the phase stability limits of Bi2Sr2Ca1Cu2O8+δ and Bi2Sr2Ca2Cu3O10+δ in the temperature range 650–880 °C using a solid‐state electrochemical technique. These phases decompose by incongruent melting above ∼790 °C, whereas they decompose by a solid‐state reaction at lower temperatures. The solid‐state decomposition reaction is reversible for Bi2Sr2Ca1Cu2O8+δ, but not for Bi2Sr2Ca2Cu3O10+δ.

Annealing treatment effects on structure and superconductivity in Y1Ba2Cu3O9−x

We report the effects of heat treatment and ambient on the structure and superconducting properties of Y1Ba2Cu3O9−x. The structure undergoes an orthorhombic‐to‐tetragonal transition on heating at about 700 °C, caused by oxygen loss and disordering of oxygen vacancies on the copper plane between the barium layers. Heat treatments that promote maximum ordering of the oxygen vacancies result in superior superconducting properties.

Low‐temperature formation of epitaxial Tl2Ca2Ba2Cu3O10 thin films in reduced O2 pressure

Epitaxial Tl2Ca2Ba2Cu3O10 films on (100) LaAlO3 are prepared by post‐annealing 2Tl:2Ca:2Ba:3Cu precursor films at 830–860 °C in ≂0.03–0.15 atm of O2. These films (0.2–1.1 μm thickness) are smooth and shiny to the eye, and have a sharp zero resistance and onset diamagnetic transition temperature of 117–121 K. Transport critical current densities of 1.6×106 A/cm2 at 77 K and ≂105 A/cm2 at 100 K are obtained for a 0.38‐μm‐thick film in magnetic fields up to 100 Oe. Strong flux pinning at low temperatures is inferred from the weak‐field dependence of the critical current density calculated from magnetic hysteresis loops. At 5 K, the best film has a magnetic critical current density of 9×106 A/cm2 in zero field, decreasing gradually to 1.5×106 A/cm2 in a 5‐T field.

Low-temperature formation of epitaxial Tl sub 2 Ca sub 2 Ba sub 2 Cu sub 3 O sub 10 thin films in reduced O sub 2 pressure

Epitaxial Tl2Ca2Ba2Cu3O10 films on (100) LaAlO3 are prepared by post‐annealing 2Tl:2Ca:2Ba:3Cu precursor films at 830–860 °C in ≂0.03–0.15 atm of O2. These films (0.2–1.1 μm thickness) are smooth and shiny to the eye, and have a sharp zero resistance and onset diamagnetic transition temperature of 117–121 K. Transport critical current densities of 1.6×106 A/cm2 at 77 K and ≂105 A/cm2 at 100 K are obtained for a 0.38‐μm‐thick film in magnetic fields up to 100 Oe. Strong flux pinning at low temperatures is inferred from the weak‐field dependence of the critical current density calculated from magnetic hysteresis loops. At 5 K, the best film has a magnetic critical current density of 9×106 A/cm2 in zero field, decreasing gradually to 1.5×106 A/cm2 in a 5‐T field.

Phase stability limits and solid-state decomposition of Bi2Sr2CaCu2O8+δ and Bi2Sr2Ca2Cu3O10+δ in reduced oxygen pressures

Abstract We measured the phase stability of the Bi 2 Sr 2 CaCu 2 O 8+δ and Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductors as a function of temperature and oxygen pressure. Below the solidus temperature of ∼790°C, Bi 2 Sr 2 CaCu 2 O 8+δ is stable at oxygen pressures one fourth of those where CuO becomes metastable, while Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ is stable at the same pressures where CuO is stable. Above the solidus temperature, the pressure required to stabilize either phase increases rapidly with temperature. The major solid-state decomposition products of Bi 2 Sr 2 CaCu 2 O 8+δ are Bi 2 Sr 3− x Ca x O 6+δ and Cu 2 O, and also Bi 2 Sr 2− x Ca x O 5 for bismuth-rich samples. For Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ the major decomposition products are Bi 2 Sr 2 CaCu 2 O 8+δ , Ca 2− x Sr x CuO 3 , and Cu 2 O. The solid-state decomposition reaction is reversible for the Bi-2212 phase, but not for the Bi-2223 phase.

Effects of deposition conditions on the superconducting properties of r.f. and d.c. magnetron sputter-deposited YBa2Cu3O7−x films

Abstract Thin films of YBa 2 Cu 3 O 7− x were r.f. and d.c. magnetron sputter deposited onto silicon, Al 2 O 3 , MgO, BaTiO 3 and SrTiO 3 substrates at up to 700°C from a stoichiometric oxide target in an argon and O 2 mixture. Stoichiometric films were obtained using a substrate-to-target configuration so that bombardment of the growing film by energetic particles is minimized. These films show an ending T c of about 87 K when deposited on SrTiO 3 (100) at 300°C and after annealing in flowing O 2 at 900°C for about 1 min, or an ending T c of 76 K when deposited on Si(100) at 650°C without further high temperature treatments. The effects of substrate, deposition temperature, and substrate bias on the superconducting properties of these films are presented.