Spin dynamics in Na4-xIr3O8 (x=0.3 and 0.7) investigated by Na-23 NMR and mu SR

Abstract

We report Na-23 nuclear magnetic resonance (NMR) and zero-field (ZF) and longitudinal-field (LF) muon spin relaxation (mu SR) measurements of the depleted hyperkagome compounds Na4-xIr3O8 (x = 0.3 and 0.7), which undergo an insulator-semimetal transition as a function of x. The Na-23 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-mu SR 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- aeSR spectra show a three-dimensional diffusive transport. Our combined NMR and ae SR results suggest the occurrence of intriguing spin and charge excitations across the insulator-semimetal transition.