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.
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