¹H-NMR Spin–Lattice Relaxation Rate of the Quantum Spin System (CH₃)₂CHNH₃Cu(Cl_xBr_1−x)₃ with x=0 and 0.35

Abstract

The spin–lattice relaxation rate T₁⁻¹ of ¹H-NMR has been measured in (CH₃)₂CHNH₃Cu(Cl_xBr_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 T₁⁻¹ 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, T₁⁻¹ 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.