Spin dynamics in Na_4−xIr₃O₈ (x  =  0.3 and 0.7) investigated by ²³Na NMR and μSR

We report ²³Na nuclear magnetic resonance (NMR) and zero-field (ZF) and longitudinal-field (LF) muon spin relaxation (μSR) measurements of the depleted hyperkagome compounds Na_4−xIr₃O₈ (x  =  0.3 and 0.7), which undergo an insulator-semimetal transition as a function of x. The ²³Na spin-lattice relaxation rates, $T_1^{-1}$, follow a T ^2.5 power law behavior at accessible temperatures of T  =  120–350 K. A substantial temperature dependence of $T_1^{-1}$ indicates the presence of gapped excitations at elevated temperatures through the transition to a semimetallic phase. ZF-μSR results reveal that hole-doping leads to a melting of quasi-static order to a dynamically fluctuating state. The very slow muon depolarization rate which varies hardly with temperature indicates that spins are close to an itinerant limit in the largest doping x  =  0.7. The dynamic relaxation rates extracted from the LF-μSR spectra show a three-dimensional diffusive transport. Our combined NMR and μSR results suggest the occurrence of intriguing spin and charge excitations across the insulator-semimetal transition. (paper)