Phase boundary and magnetization in field-induced spin-density-wave systems

We suggest a very general thermodynamic description of a system with many gaps such as the field-induced spin-density-wave phase in a quasi-one-dimensional conductor. We show how BCS-like relations hold between the ground-state properties and the phase boundary. In particular, we find that the puzzling reentrant transitions seen in low field by specific-heat measurements and at high field by transport measurements are directly connected with the equally puzzling diamagnetic magnetization. This description gives a coherent framework for further investigation.