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

Abstract

We report a $^{35}\mathrm{Cl}$ nuclear magnetic resonance (NMR) study of the diluted Kitaev material $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{Cl}}_{3}$ ($x=0.1$ and 0.2) where nonmagnetic ${\mathrm{Ir}}^{3+}$ dopants substitute ${\mathrm{Ru}}^{3+}$ ions. Upon dilution, the $^{35}\mathrm{Cl}$ spectra exhibit unusual large magnetic inhomogeneity, which sets in at temperatures below the Kitaev exchange energy scale. At the same time, the $^{35}\mathrm{Cl}$ spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ as a function of dilution and magnetic field unravels a critical doping of ${x}_{c}\ensuremath{\approx}0.22$, 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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{Cl}}_{3}$, arising from the interplay of dilution and exchange frustration in the quantum limit.