Spin-Peierls Transition of Quasi-One-Dimensional Electronic System (DMe-DCNQI)2M (M = Li, Ag) Probed by Heat Capacity

Yasuhiro NAKAZAWA, Akane SATO, Mitsuro SEKI, Kazuya SAITO, Ko-ichi HIRAKI, Toshihiro TAKAHASHI, Kazushi KANODA and Michio SORAI

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))

Thermodynamic Investigation of the Charge-Ordered Insulating State of the Quasi-One-Dimensional Organic System (DI-DCNQI)2Ag

Yasuhiro NAKAZAWA, Mitsuro SEKI, Kazuya SAITO, Ko-ichi HIRAKI, Toshihiro TAKAHASHI, Kazushi KANODA and Michio SORAI

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))

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