Hole Distribution and Magnetic-Field Irreversibility in Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z

Abstract

Mutual correlations of (1) the irreversibility field (H irr ), (2) changes in the local atomic positions and (3) the concentration and distribution of holes over the crystal of the Cu-1212:P (or “123”) phase were investigated. A series of samples, Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z, with a variety of doping levels (0.1 ≤ x ≤ 0.35 , 0.55 ≤ z ≤ 0.96) was employed. The hole distributions among the different oxygen sites were calculated from the O K-edge XANES results previously obtained for the same samples. Moreover, the gradual changes of the local crystallographic structure and in the BVS’s of the Cu and O atoms in the CuO2 plane were followed utilizing the neutron-diffraction data previously measured for the samples with increasing Ca substitution and/or O doping levels. It was concluded that the H irr characteristics of Cu-1212:P is controlled by (i) the hole concentration in the infinite-layer block, (ii) the thickness of the charge-reservoir block, and (iii) the hole concentration in the charge-reservoir block.