SIMULTANEOUS THERMODYNAMIC AND TRANSPORT MEASUREMENTS OF THE FIELD-INDUCED SPIN-DENSITY-WAVE TRANSITIONS IN (TMTSF)(2)CLO4

The standard model for the field-induced spin-density-wave (FISDW) transitions in the Bechgaard salts (TMTSF)(2)X, where TMTSF is tetramethyltetraselenafulvalene, explains a cascade of phase transitions with each phase associated with the quantum Hall effect. The ClO4 salt is sufficiently different that it has inspired a series of theoretical modifications from the standard model. To test these models we have performed simultaneous rho(xx), rho(xy), specific-heat, magnetocaloric effect, and magnetization measurements in the held range from 0-9 T. We find that all of the transport transitions, specifically the Hall resistance jumps, are associated with thermodynamic transitions. We observe the emergence of a new FISDW state characterized by a distinct Hall plateau. It arises from what was originally believed to be a tetracritical point in the phase diagram. We find no evidence for an arborescent phase diagram, but rather the signature of a single pairwise splitting of the phase boundaries. The higher-held transitions are decidedly first order, hysteretic, and ''lossy.'' Anion disorder decreases the number of observed phases and shifts the transition fields.