EPR and NMR measurements on high-temperature superconductors

Abstract

Some EPR and NMR experiments at the Cu sites in the mixed-phase materials Ba_0.6Y_0.4CuO_y and Ba_0.2Y_0.8CuO_y, and the pure-phase compounds YBa₂Cu₃O_y (1:2:3), BaCuO₂ (0:1:1), Y₂Cu₂O₅ (2:0:2), Y₂BaCuO₈ (2:1:1), YBa₃Cu₂O_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 (greenphase) compound. For the 2:1:1 and 1:3:2 compounds the authors find g/sub /// approximately=2.23 and g_{perpendicular to} approximately=2.09 with a linewidth of approximately=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.