Thermodynamic investigations of the field-induced spin-density wave: Significant departure from Bardeen–Cooper–Schrieffer behavior at high field

We have investigated the field-induced spin-density wave (FISDW) state of the quasi-one-dimensional organic conductor (TMTSF)2ClO4 by means of calorimetric experiments. Specific heat measurements were performed at low temperature and in fixed magnetic fields up to 10 T. The specific heat jump at the metal-FISDW transition is found to exhibit an oscillatory behavior as the field is varied: First, the jump goes through a series of discontinuities at the reentrance points of the phase diagram, corresponding to the transitions between two magnetic subphases. Next, around 10 T, a second-order phase transition occurs 1 K above the first-order line deduced from the magnetization data. Finally, in the same field range, the jump value deviates significantly from the Bardeen–Cooper–Schrieffer (BCS) prediction and rises up to about four times the BCS value at 10 T. Such a behavior appears to be in a strong departure from our present understanding of the field-induced magnetic ordering and seems to invalidate the weak-coupling limit in use up to now for the description of the Bechgaard salts.