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

抄録

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.