${}^1$H-NMR Spin-Lattice Relaxation Rate of IPACu(Cl${}_x$Br${}_{1-x}$)${}_3$ with $x=0$ and 0.35

Abstract

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.

submitted to JPSJ (accepted and will appear in the issue of No. 5)