Spin Liquid State
on Molecular Triangular System

The molecular compounds κ-(BEDT-TTF)2 Cu2(CN)3 and EtMe3Sb [Pd(dmit)2]2 have ideal triangular lattices based on the BEDT-TTF or Pd(dmit)2 dimers in the two dimensional structure. In these compounds, a quantum spin liquid ground state is expected as their good states. We have performed a calorimetry for these materials using the relaxation technique at low temperature. We have observed a distinct T-linear term in heat capacity of which electric heat capacity the coefficient γ is 12.9 mJ K−2 mol−1 for κ-(BEDT-TTF)2 Cu2(CN)3. That of EtMe3Sb [Pd(dmit)2]2 was found to be about 20 – 30 mJ K−2 mol−1. The existence of T-linear contribution is considered as an evidence of spin liquid ground state. Furthermore, we have observed a broadened hump structure around 3 K in EtMe3Sb [Pd(dmit)2]2 as well as 5.7 K in κ-(BEDT-TTF)2 Cu2(CN)3. Probably these anomalies are related to the crossover between the spin-liquid ground state with strong quantum characters and the classical Heisenberg-state in the 2D triangular structure. However, the γ-value and the hump temperature are different in each compounds. These discrepancies are discussed in terms of the difference of crystal structures.

(by S. Yamashita & Y. Nakazawa)

	Fig. 1. Low temperature heat capacities of κ-(BEDT-TTF)2 Cu2(CN)3 and typical Mott insulator based on BEDT-TTF dimers. A distinct T-linear contribution with the coefficient γ = 12.9 mJ K−2 mol−1 even in the insulating state is observed. The heat capacities of κ-(BEDT-TTF)2 Cu2(CN)3 show no magnetic field dependence.
	Fig. 2. Low temperature heat capacities of EtMe3Sb [Pd(dmit)2]2 under different magnetic fields. A distinct T-linear contribution with the coefficient γ = 20 – 30 mJ K−2 mol−1 in the insulating state is observed. The heat capacities EtMe3Sb [Pd(dmit)2]2 show weak magnetic field dependence.