Observation of a random singlet state in a diluted Kitaev honeycomb material

TL;DR

This study uses $^{35}$Cl NMR to reveal a random singlet state in diluted $ ext{Ru}_{1-x} ext{Ir}_x ext{Cl}_3$, showing how non-magnetic doping induces magnetic inhomogeneity and gapless excitations.

Contribution

It provides experimental evidence for a random singlet phase in a diluted Kitaev honeycomb material, highlighting the effects of doping and frustration.

Findings

Large magnetic inhomogeneity below the Kitaev energy scale

Critical doping level at $x_c \,\approx 0.22$ suppresses spin gap and magnetic order

Emergence of gapless low-energy spin excitations

Abstract

We report a $^{35}$Cl nuclear magnetic resonance (NMR) study of the diluted Kitaev material $\alpha$-Ru$_{1-x}$Ir$_x$Cl$_3$ ($x=0.1$ and $0.2$) where non-magnetic Ir$^{3+}$ dopants substitute Ru$^{3+}$ ions. Upon dilution, the $^{35}$Cl spectra exhibit unusual large magnetic inhomogeneity, which sets in at temperatures below the Kitaev exchange energy scale. At the same time, the $^{35}$Cl spin-lattice relaxation rate $T_1^{-1}$ as a function of dilution and magnetic field unravels a critical doping of $x_c\approx 0.22$, towards which both the field-induced spin gap and the zero-field magnetic ordering are simultaneously suppressed, while novel gapless low-energy spin excitations dominate the relaxation process. These NMR findings point to the stabilization of a random singlet phase in $\alpha$-Ru$_{1-x}$Ir$_x$Cl$_3$, arising from the interplay of dilution and exchange frustration in the quantum limit.