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).
-2- The critical temperature exhibits a non monotonic
6-b- Evolution of the phase diagram with the cooling rate
-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)
[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 .
To explain these behaviours, it is necessary to go beyond the "standard"
model of the FISDW phases [publi 21].
-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.
Organic conductors (6/8)