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

The use of synchrotron radiation techniques in the characterization of strained semiconductor heterostructures and thin films

Changes in the copper K-edge x-ray absorption spectrum of YBa{sub 2}Cu{sub 3}O{sub 7} across the critical temperature indicate that, accompanying the superconducting transition, the mean square relative displacement of some fraction of the Cu2-O4 bonds becomes smaller or more harmonic (or both), that there may be a slight increase in the associated Cu1-O4 distance, and that electronic states involving these atom pairs become more atomic-like. If there is an association between the superconductivity and this lattice instability, then the bridging axial oxygen is of central importance in determining the high transition temperature of YBa{sub 2}Cu{sub 3}O{sub 7}. Because this structural perturbation will affect the dynamic polarizability of the copper oxygen sublattice, it is consistent with an excitonic pairing mechanism in these materials. 36 refs., 4 figs.

https://science.sciencemag.org/content/sci/243/4896/1340.full.pdf

The Axial Oxygen Atom and Superconductivity in YBa2Cu3O7.

Evidence of 3d9-ligand hole states in the superconductor La1.85Sr0.15CuO4 from L3 X-ray absorption spectroscopy

The valency of Cu ions in the orthorhombic and tetragonal phases of high-Tc superconductor Ba2YCu3Oy (6.22<y<6.96) has been studied by near-edge X-ray absorption spectroscopy. The results indicate that the Cu valency at Cu1 sites on the z=1/2 plane is strongly correlated with the high-Tc (90 K) superconductivity, while the Cu valency of Cu2 sites on the basal planes affects the low-Tc (50-60 K) superconductivity of the orthorhombic phase.

https://iopscience.iop.org/article/10.1143/JJAP.26.L1561/pdf

Valence Study of Orthorhombic and Tetragonal Ba2YCu3Oy: The Role of Oxygen Vacancies in High-Tc Superconductivity

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

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.

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

X-Ray Absorption Spectra of High T_c Superconducting Sr_xLa_ CuO_

Cu K-edge extended X-ray absorption fine structure (EXAFS) and near-edge structure have been measured in high Tc superconducting SrxLa2-xCuO4-y. The Fourir transform analysis shows the axial elongation of CuO6 octahedron along the c-axis with four short (1.89 A) and two long (2.41 A) Cu-O bonds, which are slightly shorter than those in CuO and La2CuO4 indicating the presence of enhanced Cu(3d)-O(2p) interaction. The near-edge spectrum of SrxLa2-xCuO4-y is similar to that of Cu2O rather than CuO, which is interpreted as an evidence for the monovalent character of Cu ions.

Takafumi Yao

Papers

Valence Study of Orthorhombic and Tetragonal Ba2YCu3Oy: The Role of Oxygen Vacancies in High-Tc Superconductivity

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

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

X-Ray Absorption Spectra of High T_c Superconducting Sr_xLa_ CuO_

X-ray absorption spectra of high Tc superconducting SrxLa2-xCuO4-y