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 Hc1 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 Tc can become unstable and undergo another superconducting transition at a temperature Tc* (<Tc); this leads to an abrupt increase of Hc1. The properties of the phase realized below Tc* are briefly discussed. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.