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,
Cp, 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.
||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
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-).
 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.
 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).
 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).
 A.G. Lebed', New phases in organic superconductors,
JETP Lett. 51, 663 (1990) [Pis'ma v Zh.Eksp.Teor.Fiz. 51,
 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).
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
The transition lines splitting has been confirmed by
Paul Chaikin's experimentalist team, in Princeton ,
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.
 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).