Reentry-like features in YBa²Cu³O⁷and GdBa²Cu³O⁷

The diamagnetic field-cooled susceptibility of YBa²Cu³O⁷and GdBa²Cu³O⁷curves upward at low temperature and crosses over to a positive value at temperature T* which increases with increasing field. We demonstrate that the low-temperature phase is superconducting, and the reentry-like features are a result of a paramagnetic background inherent in this class of materials. The relevance of this behavior to the pairing mechamism is discussed.