Anomalous Temperature Dependence of the Lower Critical Fields in High-Temperature Superconductors

We present an interpretation of the anomalous temperature dependence of the lower critical field H c1 recently observed in high-temperature superconductors. In the presence of the two ( s - and d -wave) attractive forces, the superconducting phase first realized at the transition temperature T c can become unstable and undergo another superconducting transition at a temperature T c * (< T c ); this leads to an abrupt increase of H c1 . The properties of the phase realized below T c * are briefly discussed.

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