Experimental reinvestigation of the thermodynamics of the field induced SDW in (TMTSF)₂CIO₄

The unique nature of the spin density wave transitions induced by a moderate magnetic field in the Bechgaard salts (TMTSF)₂X (where X=PF₆ ClO₄, etc) result from competition between the length scales set by the magnetic field and the SDW wavevector and possibly the lattice periodicity. The complex competition has led to experimental and theoretical suggestions of a phase diagram with some novel features including reentrance, arborescence and multicritical points. We have performed high resolution heat capacity and magnetocaloric measurements to reinvestigate these questions. We find no evidence for arborescence, but clear evidence of low field reentrance and a tetracritical point (as in previous work by Pesty et al) indicating coexistence of order parameters for several FISDW phases.