Fig.1. Temperature dependence of ac heat capacities of κ-HgCl and κ-HgBr.
Organic radical salts (BEDT-TTF)2X provide variety of electron states due to difference of molecular packing. The compounds possessing κ-, β'-, λ-type structure have dimerized donor arrangements and tend to have strong correlated metallic states or Mott insulating states. κ-(BEDT-TTF)4Hg2.78Cl8 (κ-HgCl) and κ-(BEDT-TTF)4Hg2.89Br8 (κ-HgBr) are recognized as hole-doped organic dimer Mott systems. The mercury ions form a one-dimensional chain structure which periodicity is incommensurate with that of donors. In these compounds, strong antiferromagnetic fluctuations in two-dimensional donor layers were observed and the γ value was twice larger than that of other organic conductors due to strong spin fluctuation. In order to clarify the nature of the low temperature electron states, we have performed heat capacity measurements for κ-HgCl under magnetic fields and compared the result with κ-HgBr. We observed the large γ value as 42 mJK−2mol−1 in κ-HgCl consistent with present measurement. We also observed that the low temperature heat capacity under magnetic fields shows large proton contribution explained by two-level Schottky models which is located on the edge of BEDT-TTF molecules. Similar behavior was observed in κ-HgBr. We suggests that the strong spin fluctuations in hole-doped organic conductor is coupled with relaxation process of nuclei.
Fig.1. Temperature dependence of ac heat capacities of κ-HgCl and κ-HgBr.
Fig.2. Low temperature heat capacity of κ-HgCl. The blue line is a fitting line of Cp = γT − αTlnT+ βT 3+ βT 5 with fitting parameter of γ = 41.9 mJK−2mol−1, α = 34.2 mJK−2mol−1, β = 30.9 mJK−4mol−1, δ = 4.41~10−2 mJK−6mol−1. The dash line is a extrapolation of higher temperature data down to 0 K.
Fig.3. Magnetic fields dependence of low temperature heat capacity in κ-HgCl. The solid lines show their fitting result by Schottky model.