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

We report a Cl-35 nuclear magnetic resonance (NMR) study of the diluted Kitaev material alpha-Ru1-xIrxCl3 (x = 0.1 and 0.2) where nonmagnetic Ir-3(+) dopants substitute Ru3+ ions. Upon dilution, the Cl-35 spectra exhibit unusual large magnetic inhomogeneity, which sets in at temperatures below the Kitaev exchange energy scale. At the same time, the Cl-35 spin-lattice relaxation rate T-1(-1) as a function of dilution and magnetic field unravels a critical doping of x(c) approximate to 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-Ru1-xIrxCl3, arising from the interplay of dilution and exchange frustration in the quantum limit.