Effect of CuO chains ordering on the opening of the spin-gap in YBa2Cu3O6+x

The authors report on Cu-63 NMR relaxation measurements in YBa2Cu3O6+x samp les having similar T-c's but a different structural ordering of the Cu(1)O chains. It is found that the gap in the spin excitations estimated from Cu- 63(2) nuclear spin-lattice relaxation rate 1/T-1, varies from 110 K up to 1 90 K depending on the sample. This observation suggests that there is not a one to one relationship between the hole concentration and the amplitude o f the spin-gap in the normal state of YBa2Cu3O6+x. The sizeable variation i n the gap, related to the different ordering of the Cu(1)O chains, is tenta tively explained in the framework of a phase separation scenario where Cu(1 )O chains act as pinning wells for the stripes.