New symmetry breaking in high Tc superconductors

We study a nearly square 2D Fermi surface with strong nesting properties and a Van Hove singularity at the Fermi level, as a model for high To superconductors, such as Bi2Sr2CaCu2O8 (Bi2212). Such a model is consistent with recent photoemission data. We assume an effective (phenomenological) attractive coupling between electrons as well as an exchange coupling. The Ln(kT/Ef)2 behaviour of the superconducting response favours a d-wave superconducting instability. When a magnetic field is applied perpendicularly to the electron plane, an orbital effect restores a 1D behaviour for the magnetic response, with an effective exchange constant rapidly increasing with the field. Therefore, we expect, above a critical field, a coexistence of superconductivity and spin density wave (SDW). This new symmetry breaking should open a gap on the whole Fermi surface. We propose to ascribe to this affect the anomalies observed in the thermal conductivity of Bi2212 and the apparent disappearance of the quasi-particles in an applied field.