EPR and NMR measurements on high-temperature superconductors

Some EPR and NMR experiments at the Cu sites in the mixed-phase materials Basub(0.6)Ysub(0.4)CuOsub(y) and Basub(0.2)Ysub(0.8)CuOsub(y), and the pure-phase compounds Yba2Cu3Osub(y) (1:2:3), BaCuO2(0:1:1), Y2Cu2O5 (2:0:2) Y2BaCuO8 (2:1:1), YBa3Cu2Osub(y) (1:3:2) and CuO are presented and discussed. In particular, it is shown that any EPR signal in nominally pure 1:2:3 superconducting material is probably due to a small amount of impurity 2:1: 1 (green-phase) compound. For the 2:1:1 and 1:3:2 compounds we find gsub(parallel) approx. = 2.23 and gsub(perpendicular) approx. = 2.09 with a linewidth of approx. 0.025 T, whereas for the 2:0:2 compound g = 2.12(2) with a linewidth of 0.047(3)T. Some suggestions are also made concerning the absence of both NMR and EPR signals in the 1: 2:3 compound, in terms of (i) Anderson's resonating bond model and (ii) recent FLAPW band-structure calculations. In particular, it is argued that the delocalised nature of the Cu d electrons probably gives rise to exceptional spin-spin dipolar broadening of both the NMR and EPR transitions. (author)