NMR EVIDENCE FOR THE INFLUENCE OF CUO CHAINS ORDERING ON THE OPENING OF THE SPIN-GAP IN YBA2CU3O6+X

Cu-63 NMR relaxation measurements in oriented powders of YBa2Cu3O6+x a re presented. Despite of the different oxygen contents these samples s how very similar T-c and Y-89 NMR paramagnetic shifts, indicating a ve ry similar hole doping of the Cu(2)O-2 planes. However, the amplitude of the gap in the spin excitations derived from Cu-63(2) nuclear spin- lattice relaxation rate 1/T-1, varies up to more than 60% from sample to sample. This fact evidences that there is not a biunique relation b etween the hole concentration and the amplitude of the spin-gap at the boundary of the Brillouin zone in YBa2Cu3O6+x. The strong variation i n the amplitude of the gap is associated to the different ordering of the CuO chains in these samples. The role of the chain ordering in the opening of the spin-gap is briefly discussed in the frame of differen t theoretical models and a simplified phenomenological description of the experimental results, based on the effects of the pinning potentia l created by the CuO chains on stripes of itinerant holes, is presente d. (C) 1997 Elsevier Science B.V.