We report a study of anisotropic upper critical field of single crystal samples of orthorhombic YBa2Cu3Ox. The critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor, and is highest when the field is oriented perpendicular to the c-axis. For this field direction, the zero resistance state persists up to 86 K in a magnetic field of 90 kOe, which is lower by only 5 K than the zero resistance temperature in the absence of magnetic field. The anisotropy Hc2⊥/Hc2// is about 2 at 90 K, 5 at 86.5 K and tends to increase with decreasing temperature.

The Anisotropic Upper Critical Field of Single Crystal YBa2Cu3Ox

Raman and IR-measurements on the high- T c material series La 2 CuO 4 and YBa 2 Cu 3 O 7 are reviewed. This summary focuses on lattice vibrations in these superconducting systems, on impurity phases, the influence of oxygen stoichiometry and element substitution. The determination of the superconducting gap, the phonon renormalization in the superconducting state, and the isotope effect with its implications on the theory of superconductivity in these materials are discussed. In addition, the measured lattice vibrations are compared with the results of lattice dynamics calculations.

Lattice vibrations in high-Tc superconductors: Optical spectroscopy and lattice dynamics☆

Hydroxyls of silica powders

From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y1Ba2Cu3O7−x
 is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0≦x≦0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu2+ and Cu3+ in YBa2Cu3O7−x
 is constructed.

The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x

Publisher Summary This chapter discusses the structural studies performed on superconducting oxides. It focuses primarily on Y 1 Ba 2 Cu 3 O 7–δ , nicknamed the 123 structure, because this structure has been the most extensively studied to date. The chapter describes the basic atomic arrangements in Y 1 Ba 2 Cu 3 O 7–δ and the way these arrangements change as a function of processing. The defects that are commonly found in the 123 structure are discussed, some of which play a dominant role in controlling superconducting properties. The studies of elemental substitutions for Y, Ba, Cu and/or O in 123 are summarized, and 123 with the other families of superconducting oxides is compared. These studies have both theoretical and practical interest because they provide clues to the roles that various structural features play in the mechanism of high-temperature superconductivity, which, in turn, may aid in the discovery of even higher temperature superconductors. It is evident from the variety of behaviors exhibited by 123 after different processing conditions that the structure has considerable flexibility. The oxygen stoichiometry can be varied over a wide range, and different cations can be substituted at least partially into every site.

The Structure of Y1Ba2Cu3O7-δ and its Derivatives

The high-Tc superconductor Ba2Y1Cu3O7 with the oxygen-deficient perovskite structure was prepared almost in a single-phase with Tco of 125 K, Tc of 95 and Tce of 93 K. The XPS valence-band spectrum showed a relative large N(EF), density of states at the Fermi level. The XPS Cu 2p3/2 peak is divided into three peaks which are attributed to monovalent, bivalent and trivalent Cu ions with the concentration ratio of 1:2:1. These results and the peculiar lattice structure lead to the Tc enhancement mechanism from the plasmon or exciton origin due to the Cu d-hole creation and annihilation resonating with vibration modes of the CuO5 and CuO4 clusters. The Tc value was calculated.

Electronic Structures and Superconducting Mechanism of Ba2Y1Cu3O7

The maximum yield for each solute is obtained at different pH values. The pH values of 2.3 and 4 are optimum for the preparation of Ba-La-Cu-O, Sr-La-Cu-O and Ba-Y-Cu-O, respectively. The composition ratio of the high Tc superconducting materials was 0.54:0.33:1 (Ba:Y:Cu).

https://iopscience.iop.org/article/10.1143/JJAP.26.L734/pdf

On the Coprecipitation Method for the Preparation of High T c M-X-Cu-O (M=Ba, Sr, X=La, Y) System

Cu valency and local structure in the orthorhombic and tetragonal phases of high Tc superconductor Ba2YCu3O7-y (0.03<y<0.64) have been studied by the near-edge and extended X-ray absorption fine structure (EXAFS). Oxygen vacancies on the z = 1/2 plane modify both Cu valency and local structure. The near-edge spectra show that Cu ions at Cu1 sites in tetragonal Ba2YCu3O7-y are monovalent. From the temperature dependence of the Cu-O distance and its mean relative displacement, it is suggested that the breathing mode Cu-O stretching vibration is slightly suppressed in the tetragonal phase, which may degrade superconducting properties.

Local Structure in Orthorhombic and Tetragonal Ba 2 YCu 3 O 7-y : The Role of Oxygen Vacancies for High T c Superconductivity

We have succeeded in obtaining a new single phase of oxygen-deficient perovskite ABO3-x type. The sample of nominal composition Y1/3Ba2/3CuO3-x exhibits the superconductivity at 125 K and a zero resistance state at 83 K. X-ray diffraction patterns of this compound show the oxygen deficient tetragonal perovskite to be of a nearly single phase of the formula Y1Ba2Cu3O7. This structure is a layered perovkite which is associated with the high-Tc superconductivity.

Structure and Superconductivity in a New Type of Oxygen Deficient Perovskites Y1Ba2Cu3O7

Cu K near-edge and extended X-ray absorption fine structure (EXAFS) have been measured for 90 K superconductor BaxY1-xCuO3-y with x=0.6. The Fourier analysis of EXAFS shows that Cu ions are coordinated with basal plane oxygens having the average Cu-O distance of 1.97 A. Determined average oxygen coordination number of ca. 4.8 suggests the presence of axial oxygens with a similar Cu-O distance. The near-edge spectra provide an evidence for the mixed valence character of Cu ions in BaxY1-xCuO3-y between predominantly divalent states and trivalent states. The strong electron-phonon interaction may be further enhanced by a charge-coupled motion of oxygens induced by the Cu valence fluctuation.

Kiyoshi Senzaki

Papers

Electronic Structures and Superconducting Mechanism of Ba2Y1Cu3O7

On the Coprecipitation Method for the Preparation of High T c M-X-Cu-O (M=Ba, Sr, X=La, Y) System

Local Structure in Orthorhombic and Tetragonal Ba 2 YCu 3 O 7-y : The Role of Oxygen Vacancies for High T c Superconductivity

Structure and Superconductivity in a New Type of Oxygen Deficient Perovskites Y1Ba2Cu3O7

Valence and Coordination of Cu Ions in High T c Superconductor Ba x Y 1− x CuO 3− y