The available data on the microwave surface impedance Z/sub s/ of high-T/sub c/ superconductors from more than 30 laboratories around the world are reviewed to show the frequency, temperature, and field dependence of the surface resistance R/sub s/ and the temperature dependence of the field penetration depth lambda . The data are discussed on the basis of a two-fluid model and are compared with results from the classical superconductors Nb and Nb/sub 3/Sn. At temperatures above 0.8 T/sub c/, Z/sub s/ values of classical and high-T/sub c/ superconductors show strong similarities and are in close agreement with the Mattis-Bardeen theory. The comparatively very high R/sub s/ of the cuprates at 4.2 K, however, is indicative of a high density of unpaired charge carriers. The lowest values for R/sub s/ and lambda obtained until now with epitaxially grown YBa/sub 2/Cu/sub 3/O/sub 7- delta / films are 16 mu Omega at 4.2 K and 10 GHz and 140 nm, respectively. Such films are required for high RF field applications. Polycrystalline samples show a strong field dependence of R/sub s/. The highest magnetic surface field without R/sub s/ deterioration achieved so far on single crystals of YBa/sub 2/Cu/sub 3/O/sub 7- delta / is about 100 G.

The microwave surface impedance of high-T/sub c/ superconductors

Substitutions for copper in YBa 2 (Cu 1− x M x ) 3 O 7− δ (1:2:3) were investigated. It was found that indirect methods to determine solubility into 1:2:3 typically used in the study of intermetallic compounds (observed variation with composition of superconducting transition temperature and of lattice parameters) are not entirely reliable. Optical micrography was used to asses the morphology of the sintered polycrystalline specimens. Differential thermal analysis provides melting point data signaling the presence of impurities which melt at lower temperatures. Electron microprobe studies were undertaken to determine actual M content of the superconducting phase and to check for the presence of impurity phases. Estimates of solubility limits are made. Transmission electron microscope studies were used to determine the twin structure and investigate the occurrence of a tweed pattern. Superconducting transition temperature as a function of x was determined below the solubility limits. In several cases T c 's observed in this work differ from some previous reports. Possible sources of these discrepancies are discussed. Variation of lattice parameters as a function of x is reported. Also described are electrical resistivity and DC susceptibility measurements for selected samples.

Substitutions for Cu in YBa2(Cu1−xMx)3O7−δ (M = Fe, Co, Al, Cr, Ni, and Zn)

Nb and Ta substitutions in YBa 2 Cu 3 O 7 and related phases have been examined in an attempt to synthesise phases of formula LBa 2 Cu 2 MO 8 (L=rare earth, M=Nb, Ta), in which the square planar Cu1 sites of the YBa 2 Cu 3 O 7 structure are occupied by M, and complete oxygen occupancy in the basal plane of the unit cell provides octahedral coordination for M. Single phase materials of composition La 1.1 Ba 1.9 Cu 2.1 M 0.9 O 8 have been successfully synthesised and their structures examined by powder X-ray and neutron diffraction. The location of M atoms solely in the Cu1 sites with octahedral symmetry is confirmed. The square planar CuO 4 units, which link the layers of square pyramidal CuO 5 units in YBa 2 Cu 3 O 7 , are replaced by MO 6 octahedra in the new phases. This new link provides interesting scope for the synthesis of new phases with potential as superconductors, but to date no superconducting properties have been confirmed, possibly due to insufficient oxidation within the layers of CuO 5 square pyramids.

Nb and Ta substitutions in YBa2Cu3O7−x and related phases: Structural characterisation of La1.1Ba1.9Cu2.1M0.9O8 (M=Nb, Ta)

The structure of cobalt substituted YBa 2 Cu 3− x Co x O 7± z compounds with x =0.05, 0.1, 0.2, 0.3 and 0.65 was determined by high-resolution neutron powder diffraction at 5 and 300 K. The samples are superconducting up to x =0.3. YBa 2 Cu 2.95 Co 0.05 O 7 has orthorhombic structure with space group Pmmm; all compounds with x ⩾0.1 have tetragonal structure with space group P4/mmm. The Rietveld refinements show clearly that the Co-atoms occupy only the Cu(1)-sites at (0, 0, 0), i.e. copper chain sites in the orthorhombic structure, for the whole composition range. The characteristic structural changes as a function of cobalt concentration and temperature are discussed. Along the c -axis the Cu(1) to O(2) distance is decreasing while the Cu(2) to O(2) distance is increasing as in the Co-free, oxygen deficient compounds. A charge transfer from the Cu/Co chains to the Cu planes is discussed. For the nonsuperconducting YBa 2 Cu 2.35 Co 0.65 O 7.23 some magnetic peaks are observed.

Structural changes of the superconductor YBa2Cu3O7 by cobalt substitution. A high-resolution neutron powder diffraction study

Twelve compounds with the formula RBa2Cu3O7-δ where δ≍0.1 and R=Y or a lanthanide element except Ce, Pm, or Tb, crystallize in the same layered orthorhombic perovskite-like structure (see the article by I.K. Schuller and J.D. Jorgensen in this issue). All such compounds are superconducting with superconducting transition temperatures Tc≍92-94 K, except for R=Pr. The compound PrBa2Cu3O7-δ appears to be a special case, since it does not exhibit metallic behavior and is not superconducting. Historically, the prototype YBa2Cu3O7-δ compound is singularly important since it was the first superconducting material with a Tc greater than 77 K, the boiling point of liquid nitrogen. In the meantime, yet higher Tc's have been obtained in YBa2Cu3O7-δ at very high pressure (Tc˜107 K at 150 kbar)5 and in new layered compounds in the Bi-Sr-Ca-Cu-O (maximum Tc≍110 K)6 and Tl-Ba-Ca-Cu-O (maximum Tc≍125 K)7 systems (see the article by A.W. Sleight, M.A. Subramanian, and C.C. Torardi in this issue).The origin and nature of the high Tc superconductivity of the RBa2Cu3O7-δ compounds, and the other layered copper oxide compounds as well, are presently not understood and constitute a formidable challenge to experimentalists and theorists alike. One of the most intriguing possibilities is that a magnetic mechanism, rather than the electronphonon interaction, is responsible for the formation of the superconducting electron pairs in the high Tc copper oxides. The primary evidence for a magnetic pairing mechanism is the proximity of antiferromagnetism and superconductivity as the concentration of holes in the conducting CuO2 planes is varied,8 as discussed below for the RBa2Cu3O7-δ compounds.

Magnetism, Superconductivity, and Chemical Substitutions in YBa2Cu3O7-δ

Y-Ba-Cu-O superconducting tapes over 10 m long were prepared by an atmospheric plasma spraying method. The tapes were annealed at a maximum temperature of 900°C in an oxygen flow. The tapes consisted of a Y-Ba-Cu-O layer and a Ni alloy substrate. After annealing at 900°C, Tc onset and Tc offset of the tape were 94 K and 88 K, respectively. The critical current density at liquid nitrogen temperature was about 6 A/cm2 in the length range of 7.8 m. The superconductivity of the tapes was observed in the liquid nitrogen until it was bent to a curvature radius of 60 mm.

Preparation of Y-Ba-Cu-O superconducting tape by atmospheric plasma spraying

Thick films composed of Y-Ba-Cu-O (e.g. 70 µm thick) were prepared on Ni alloy substrates at various temperatures by the atmospheric plasma powder spraying method. By increasing the substrate temperature during the spraying, the amount of unknown structure in the film sprayed was lowered and the structure of the film sprayed on 600°C substrate was tetragonal in phase. After the postannealing at 600°C, this film revealed superconducting temperatures for onset and zero resistance of 92 K and 86 K, respectively.

Superconductivity in YBa2Cu3O7-δ Thick Films by the Atmospheric Plasma Powder Spraying

X-ray powder diffraction, thermal analysis and resistivity measurements have been carried out for the YBa2(Cu1-x, Cox)3O7-δ system with 0<x<0.333. With increasing Co content, weight loss in the thermogravimetric (TG) curve steadily decreases and the structure transforms from orthorhombic to tetragonal. Around this phase transition at x=0.02, Tc starts to decrease. This phenomenon is considered to be ascribed to the disordering of oxygen sandwiched between Ba planes.

Orthorhombic-to-Tetragonal Phase Transition in the Y–Ba–(Cu, Co)–O System

Orthorhombic-to-Tetragonal Phase Transition in the Y-Ba-(Cu,Co)-O system : Electrical Properties of Condensed Matter

The magnetic field dependence of microwave surface resistance of both single crystals and sintered polycrystals of YB/sub a2/Cu/sub 3/O/sub 7-delta/ (YBCO) has been measured at 24 GHz. The resistance decreases below superconducting transition temperature and reaches constant residual resistance. The value is much lower for single crystals than for polycrystals. Furthermore polycrystals showed magnetoresistance at low field and one with hysteresis at high field which resembles the magnetization behavior of typical type II superconductor. The single crystal, however, showed smaller magnetoresistance. The magnetoresistance observed is proved to be an inherent phenomena to sintered polycrystal which is composed of grains and contains many voids.

Tsuneo Konaka

Papers

Preparation of Y-Ba-Cu-O superconducting tape by atmospheric plasma spraying

Superconductivity in YBa2Cu3O7-δ Thick Films by the Atmospheric Plasma Powder Spraying

Orthorhombic-to-Tetragonal Phase Transition in the Y–Ba–(Cu, Co)–O System

Orthorhombic-to-Tetragonal Phase Transition in the Y-Ba-(Cu,Co)-O system : Electrical Properties of Condensed Matter

Microwave surface resistance of YB/sub a2/Cu/sub 3/O/sub 7-delta/