Through ac and thermal relaxation calorimetry techniques, we have
performed a thermodynamic investigation of the spin-Peierls systems,
(DMe-DCNQI)2Ag and (DMe-DCNQI)2Li. The fully
gapped nature characteristic of
the spin-Peierls (sP) ground state was confirmed by the absence of gamma
(T-linear) term in the low-temperature heat capacity for a
single
crystal. At higher temperatures, two distinct peaks were observed at 71
K
and 86 K in heat capacity of (DMe-DCNQI)2Ag, while only a
single
peak was observed at 52 K for (DMe-DCNQI)2Li. The two-step
structure
in the Ag salt suggests that the sP transition of this material
involves
an intermediate state which is probably attributable to the degrees of
freedom
inside the dimers. For compacted pellets of numerous pieces of tiny
crystals,
however, the thermal anomaly around the sP transition disappears, but
instead
a gamma term probably associated with the 1D antiferromagnetic
(AF)
spin excitations appears. This implies that the present sP state is
quite
fragile against disorder or stress. These findings can be regarded as
novel
features of the sP transition in the quarter-filled band systems.
(Phys. Rev. B, 68,
085112 (2003))
Heat capacity measurements of a charge-ordered organic conductor (DI-DCNQI) 2Ag have been performed in a temperature range
between 0.3 and
14 K. We found no thermal anomaly at the Neel temperature (T
N = 5.5 K) but instead a T-linear term suggestive of the
spin excitations of one-dimensional character in the charge-ordered
insulating state. The analysis of the T-linear term and
the excess entropy indicates that the charge fluctuations in the
charge-ordered state influence the growth of spin excitations at
elevated temperatures, which seems to
be a peculiar aspect of a 1D charge-ordered system.
(Phys. Rev. Lett., 88, 076402 (2002))