Spin-Peierls transition of the quasi-one-dimensional electronic system(DMe−DCNQI)2M(M=Li,Ag)probed by heat capacity

Abstract

Through ac and thermal relaxation calorimetry techniques, we have performed a thermodynamic investigation of the spin-Peierls systems, $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Ag}$ and $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Li},$ where DMe-DCNQI is 2,5-dimethyl-${N,N}^{\ensuremath{'}}$-dicyanoquinonediimine. The fully gapped nature characteristic of the spin-Peierls (sP) ground state was confirmed by the absence of \ensuremath{\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 $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Ag},$ while only a single peak was observed at 52 K for $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Li}.$ 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 \ensuremath{\gamma} term probably associated with the one-dimensional antiferromagnetic 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.