Gapped Ground State in the Spin-\(\frac{{\bf 1}}{{\bf 2}}\) Trimer Chain System Cu 3Cl 6(H 2O) 2·2H 8C 4SO 2

The magnetic susceptibility and magnetization curves of an \(S=\frac{1}{2}\) trimer spin chain system Cu 3 Cl 6 (H 2 O) 2 ·2H 8 C 4 SO 2 have been measured using single crystals and polycrystals. It was found that the ground state at zero-field is a spin singlet with an excitation gap. This result suggests that the translational symmetry of the Hamiltonian is spontaneously broken in the ground state. A model for the ground state is proposed. The magnetization curve measured at 0.08 K for polycrystals shows that the gap closes at H c ≈3.9 T, from which the magnitude of the gap is evaluated as Δ/ k B ≈5.2 K.