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

TL;DR

This study investigates the microscopic magnetic properties of IPACu(Cl$_x$Br$_{1-x}$)$_3$ with $x=0$ and 0.35 using $^1$H-NMR spin-lattice relaxation rates, revealing a gapped singlet ground state at $x=0$ and magnetic fluctuations at $x=0.35$.

Contribution

The paper provides the first microscopic NMR evidence of the singlet ground state and magnetic fluctuations in IPACu(Cl$_x$Br$_{1-x}$)$_3$ with different compositions.

Findings

Exponential decrease of $T_1^{-1}$ in $x=0$ indicating a gapped singlet ground state.

Sharp peak in $T_1^{-1}$ at 7.5 K for $x=0.35$, suggesting magnetic fluctuations.

No magnetic ordering observed in NMR spectra for $x=0.35$.

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.