The phase diagram of the quantised Spin Density Wave phases induced by the magnetic field in (TMTSF)₂ClO₄ still possess rich aspects! Our simultaneous specific heat and magnetocaloric effect measurements also enabled us to discover a tetracritical point. It is characterised by the following behaviours:

-1- The specific heat demonstrates that the system exhibits a return to the normal metal after it enters first a FISDW sub-phase (Fig. 1).

FIG. 1. Cp vs B for two temperatures (T=0.6 and 0.7 K). The lower curve, at high temperature, exhibits the reentrance of the metallic phase, associated with the tetracritical point. The specific heat indeed returns to the normal metal value, in a short magnetic field range [publi 18].

-2- The critical temperature exhibits a non monotonic variation.

-3- The transitions between sub-phases are characterised by negative entropy variations (endothermal, in the case of an increasing field), however the hatched area of Figure 2 indicates a locally positive variation. This exothermal variation indicates a locally negative variation of the critical temperature as a function of the magnetic field, while the overall variation corresponds to a rise in the critical temperature (the magnetic field favours the FISDW appearance). This means that the high field state is more and more ordered. During a fixed temperature experiment in a decreasing field, the anomalies become positive (exothermal phenomena), which results from the return toward quantised phases less and less ordered, until the normal metal phase is recovered, a situation where the system is completely disordered (the magnetic order parameter is equal to zero). The endothermal/exothermal symmetry is the signature of second order (reversible) phase transitions.

FIG. 2. Magnetocaloric effect associated with the specific heat curve measured at T=0.6 K of Fig. 1 (simultaneous measurements) [publi 18].

-4- The reentrance is the meeting point of four critical lines (Fig. 3): two second order lines (the borders with the metal), and two first order lines, which surround an "intermediate" phase. We have called such a critical point that separates four different phases a "tetracritical" point (according to Griffiths' terminology[1]) [publis 20 and 22]. The existence of this tetracritical point, together with the splitting of the FISDW transition lines, the partial reentrance of the normal metal phase and the high-field strong irreversibility effects, have been confirmed by similar measurements performed in the Princeton group [2]. To explain these behaviours, it is necessary to go beyond the "standard" model of the FISDW phases [publi 21].

FIG. 3. Phase diagram for two anion cooling states. The tetracritical point, indicated by the vertical arrow, disappears as the (TMTSF)2ClO4 sample is quenched (closed symbols). The lines are moving (oblique arrows) [publis 20 and 22].

-5- The effect of anion disorder is spectacular on several accounts: it causes the vanishing of the tetracritical point, it suppresses the line splitting and the arborescence (Fig. 3). Moreover, the following section shows that it also yields a non conventional pair breaking in the Spin Density Wave phases induced by the magnetic field.

[1] R.B. Griffiths, Proposal of notation at tricritical points, Phys. Rev. B 7, 545 (1973).
[2] U. Scheven, W. Kang et P.M. Chaikin, An experimental reinvestigation of the thermodynamics of the field induced SDW in (TMTSF)₂ClO₄, J. Phys. IV (Paris) Colloq. 3, C2-287 (1993).