We report a ${}^{35}\mathrm{Cl}$ nuclear magnetic resonance (NMR) study of the diluted Kitaev material $\alpha \text{−}{\mathrm{Ru}}_{1-x}{\mathrm{Ir}}_x{\mathrm{Cl}}_3$ ($x=0.1$ 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 $T_1^{-1}$ 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 $\alpha \text{−}{\mathrm{Ru}}_{1-x}{\mathrm{Ir}}_x{\mathrm{Cl}}_3$, arising from the interplay of dilution and exchange frustration in the quantum limit.

• Received 24 May 2020
• Revised 5 July 2020
• Accepted 25 August 2020

DOI:https://doi.org/10.1103/PhysRevB.102.094407