One of our most spectacular results is the discovery of the arborescent (treelike) phase diagram of the slowly cooled (TMTSF)₂ClO₄. Probably this is the one that initiated the most theoretical works [1-5]. It has been evidenced thanks to an experimental set-up we have developed in our laboratory, enabling simultaneous measurements of both heat capacity, C_p, and isofield magnetisation coefficient (magnetocaloric effect), (dM/dT)_B. Note that the latter is equal to the isothermal entropy coefficient, (dS/dB)_T, in accordance with the Maxwell-Weiss relation. We were able to associate the magnetocaloric anomalies with the specific heat jumps at the transitions between FISDW sub-phases (Fig. 1).

FIG. 1. Simultaneous measurements of the specific heat, Cp, and magnetocaloric effect, dM/dT. Vertical arrows indicate the transitions between successive quantised FISDW sub-phases. The area below the dM/dT peaks is homogeneous to an entropy. During a sweep in the direction of an increasing magnetic field, such as this example, the effect is endothermal. Except for a few exothermal regions (above the dashed line marking the zero line), which correspond to reentrance regions (see the following). Anomalies appear in the lower curve that do not yield any event in the upper curve [F. Tsobnang's thesis].

So new transition lines are revealed thanks to magnetocaloric effect measurements. Figure 2 displays a two level iterative process in the case of a particular transition line (anomaly labelled "1"). This phenomenon only occurs for very slowly cooled samples (here: 1.3 K/h)

FIG. 2. dM/dT anomalies as functions of the magnetic field reveal a two level splitting of transition lines (arrows), as temperature is decreased, from T=625 mK (-a-) to 545 (-b-), 470 (-c-) and 425 (-d-) [publi 12]. The splitting of main transition lines is particularly clear at the lowest temperature (425 mK, in -d-).

[1] M. Héritier, Field-induced quantized magnetic ordering in quasi-one-dimensional conductors, in "Low Dimensional Conductors and Superconductors", edited by D. Jérome and L.G. Caron (NATO-ASI, Plenum Press) 155, 243 (1986) ; also see publi 17.
[2] V.M. Yakovenko, Theory of the quantum Hall effect in quasi-one-dimensional conductors, Synth. Metals 43, 3389 (1991) ; and Phys.Rev. B 43, 11353 (1991).
[3] K. Machida and M. Nakano, Infinite cascades of field-induced spin density wave states in anisotropic two-dimensional conductors, J. Phys. Soc. Jpn. 59, 4223 (1990).
[4] A.G. Lebed', New phases in organic superconductors, JETP Lett. 51, 663 (1990) [Pis'ma v Zh.Eksp.Teor.Fiz. 51, 583 (1990)].
[5] D. Poilblanc and P. Lederer, Collective modes in an "ultraquantum crystal": Field-induced spin-density-waves. II Coupling between longitudinal and transverse fluctuations, Phys. Rev. B 37, 9672 (1988).

These calorimetric investigations have led us to propose an arborescent phase diagram for the quantised FISDW phases (Fig. 3).

FIG. 3. Arborescent phase diagram of the FISDW phases in slowly cooled (TMTSF)2ClO4 (dT/dt=1.3 K/h), as determined by our low temperature high magnetic field nanocalorimetry experiments [publi 16].

The transition lines splitting has been confirmed by Paul Chaikin's experimentalist team, in Princeton [6], at least the first level (the Princeton experimental set-up did not allow temperatures as low as ours).
Our calorimetry experiments have then been developped in the framework of F. Tsobnang's thesis. They enabled us to reveal new unexpected phenomena, which will be presented in the following section.

[6] U. Scheven, W. Kang and 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).