CASCADE OF 1ST-ORDER PHASE-TRANSITIONS IN FIELD-INDUCED SPIN-DENSITY WAVE STATES

An anisotropic Hubbard model in quasi-two dimension under perpendicular fields is treated theoretically to explain cascade of first phase transitions in field-induced spin density wave states (FISDW) observed in the Bechgaard salts; (TMTSF)2X (X = ClO4, PF6, ReO4). We show that under a fixed field (H) and temperature (T) there exist abundant local minima of the free energy, each of which corresponds to the FISDW subphase characterized by a distribution of many order parameters. As T varies, the free energy curves for these subphases cross each other, giving rise to a first order transition between the integer subphases. In addition to this sub-subphase transition, the sub-subphases themselves split further via a first order, followed by sub-sub-subphase transition. This process proceeds iteratively to infinity. We discuss the observed recursive phase diagram in the T vs H in terms of this theory.