$^1$H-NMR Spin-Lattice Relaxation Rate of IPACu(Cl$_{x}$Br$_{1-x}$)$_3$ 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 the microscopic magnetism of systems. 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, indicating microscopic evidence of the gapped singlet ground state, which is consistent with the macroscopic experiments. At the same time, in the $x=0.35$ system, $T_1^{-1}$ showed a sharp peak structure at around 7.5 K though no splitting of $^1$H-NMR spectra indicative of the magnetic ordering was observed. 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 the recent $mu$SR experiments.

$^1$H-NMR Study of the Random Bond Effect in the Quantum Spin System (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$

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 $x=0.88$, which has been reported to be gapped system with singlet ground state from the previous macroscopic magnetization and specific heat measurements, in order to investigate the bond randomness effect microscopically in the gapped composite Haldane system (CH$_3$)$_2$CHNH$_3$CuCl$_3$. It was found that the spin-lattice relaxation rate $T_1^{-1}$ in the present system includes both fast and slow relaxation parts indicative of the gapless magnetic ground state and the gapped singlet ground state, respectively. We discuss the obtained results with the previous macroscopic magnetization and specific heat measurements together with the microscopic $mu$SR experiments.