Fluctuation effects in underdoped YBCO from susceptibility and nuclear relaxation

The effects of superconducting fluctuations (SF) on the diamagnetic suscept ibility and on the dynamical spin susceptibility in oriented powders of und erdoped YBCO123 (U) are investigated and compared with the ones in the opti mally doped compound (O). In U the (CU)-C-63 relaxation rate W, and then th e spin susceptibility, around T-c(+) is only slightly affected by a magneti c field, in contrast with the case of O compound. The temperature dependenc e of W and the sizeable enhancement of the diamagnetic fluctuations in U ev idence the breakdown of the conventional theory for the SF, possibly relate d to fluctuating pairs of polaronic character, in the presence of charge in homogeneities. (C) 1999 Elsevier Science B.V. All rights reserved.