Staircase superconducting structure and oxygen intercalation inYBa2Cu3O7−δsingle crystals

Ac susceptibility studies of single crystals of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ have been carried out with an ac field parallel or perpendicular to the surface of these crystals. The analysis of the results gives evidence for a staircase structure in which oxygen deficiencies increase towards the sample core. As a consequence, the oxygen mobility appears to be extremely anisotropic, favoring the $c$ axis.