Heat capacities of antiferromagnetic dimer-Mott insulators in organic charge-transfer complexes

Heat capacity measurements of quasi-two-dimensional Mott insulating compounds consisting of BEDT-TTF (bisethylendithiotetrathiafulubalene) donor molecules and counter anions were performed by the thermal relaxation calorimetry technique for single crystal samples. No distinct thermal anomalies at the predicted antiferromagnetic transition temperatures in κ-(BEDT-TTF)2Cu[N(CN)2]Cl (TN = 27 K) and β′-(BEDT-TTF)2ICl2 (TN = 22 K) were observed. These results demonstrate that the Mott insulating state of the organic salts which are dominated by the strong two-dimensional intra-layer antiferromagnetic interactions between neighboring S = 1/2 spins shows somewhat different features from the simple quasi-two-dimensional Heisenberg model with S = 1/2. The strong quantum fluctuations produced by the electron correlations suppress the long-range character of the spin correlations, which seems to be an important aspect of this kind of Mott insulating materials.