Ginzburg-Landau-type equations are derived describing the model of tetragonal high-temperature superconducting cuprates based on the dominant role of extended saddle-point singularities in the electron spectrum and the assumption that the interaction between electrons consists of a strong long-range phonon-mediated attraction and a weak short-range repulsion. The connection between layers is assumed to be established by resonant tunneling. As an example, the temperature dependence of the upper critical field along the axis is calculated, which appears to have a positive curvature, as observed in many experiments. This is explained by the fact that with departure from the connection between different singular points becomes increasingly less important, and the electrons become more one-dimensional. Other explanations are briefly discussed.
- Received 30 October 1996
- Revised 24 February 1997
- Published in the issue dated 1 July 1997
© 1997 The American Physical Society