Surface-sensitive paramagnetic Meissner effect in YBa2Cu3Ox single crystals

It is demonstrated that the paramagnetic Meissner effect observed in some of our YBa2Cu3Ox single crystals is associated with an anomalous surface layer. This surface layer, which has a superconducting transition temperature slightly higher than the bulk, develops when the crystals are annealed below 400 degree C. When the layer is mechanically removed, the paramagnetic Meissner effect disappears and the crystals behave as normal. Our results demonstrate that the Meissner effect can be masked by unusual properties of a thin surface layer. Thus the measured field-cooled magnetization may not represent the bulk Meissner effect