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 studied the effect of anion disorder on the spin-density wave (SDW) induced by the magnetic field. This communication is focused on the tetracritical behavior that we observe experimentally along the phase transition line separating the normal metal from the quantized SDWs. It is found that this behavior evolves from a bicritical to a tetracritical behavior (following Griffiths' terminology), as the amount of disorder is decreased. This change corresponds to the appearance of a treelike transition line in the diagram. We suggest that the mean free path plays a key role in the electronic phase transitions, contrary to what is expected for a conventional density wave.