Gapped Ground State in the Spin-½ Trimer Chain System Cu₃Cl₆(H₂O)₂·2H₈C₄SO₂

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The magnetic susceptibility and magnetization curves of an S=½ trimer spin chain system Cu₃Cl₆(H₂O)₂·2H₈C₄SO₂ 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.