CALORIMETRIC STUDIES OF THE MULTICRITICAL BEHAVIOR OF THE SPIN-DENSITY-WAVE INDUCED BY THE MAGNETIC-FIELD IN (TMTSF)2CLO4

Specific heat and magnetocaloric investigations have been performed at low temperature in the molecular conductor (TMTSF)2ClO4. We have stud ied the effect of anion disorder on the spin-density wave (SDW) induce d by the magnetic field. This communication is focused on the tetracri tical behavior that we observe experimentally along the phase transiti on line separating the normal metal from the quantized SDWs. It is fou nd that this behavior evolves from a bicritical to a tetracritical beh avior (following Griffiths' terminology), as the amount of disorder is decreased. This change corresponds to the appearance of a treelike tr ansition line in the diagram. We suggest that the mean free path plays a key role in the electronic phase transitions, contrary to what is e xpected for a conventional density wave.