1H-NMR Spin–Lattice Relaxation Rate of the Quantum Spin System (CH3)2CHNH3Cu(ClxBr1-x)3 with x=0 and 0.35

The spin–lattice relaxation rate T 1 -1 of 1 H-NMR has been measured in (CH 3 ) 2 CHNH 3 Cu(Cl x Br 1- 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 1 H-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.