¹H-NMR spin-lattice relaxation rate of the quantum spin system (CH ₃) ₂CHNH ₃Cu(Cl _xBr _1-x) ₃ with x = 0 and 0.35

The spin-lattice relaxation rate T ₁ ^-1 of ¹H-NMR has been measured in (CH ₃ ) ₂CHNH ₃Cu(Cl _xBr _l-x) ₃ with x - 0 and 0.35, in order to investigate their microscopic magnetism. Previous macroscopic magnetization and specific heat measurements suggested that these two exist in a singlet-dimer phase. The temperature dependence of T ₁ ^-1 in an x - 0 system decreased exponentially toward zero, providing microscopic evidence of a gapped singlet ground state, which is consistent with results of macroscopic experiments. At the same time, in the x = 0.35 system, T ₁ ^-1 showed a sharp peak structure at approximately 7.5 K but no splitting of ¹H-NMR spectra, indicative of magnetic ordering. We discuss the observed sharp peak structure in the x - 0.35 system with the soft mode toward the exotic magnetic ground state suggested by recent μSR experiments.