$^{23}$Na and $^{17}$O NMR studies of hyperkagome Na$_4$Ir$_3$O$_8$

Na$_4$Ir$_3$O$_8$ is a unique case of a 3D corner sharing triangular lattice which can be decorated with quantum spins. It has spurred a lot of theoretical interest as a spin liquid candidate of a new kind where the Hamiltonian might not be thought in terms of a simple Heisenberg case because of spin orbit coupling on the Ir 5d element. We present a comprehensive set of NMR data taken on both the $^{23}$Na and $^{17}$O sites. We have found that magnetic freezing of all Ir sites sets in below $T_f\sim$ 7.5K $\sim 0.019J$ with a clear hyperfine field transferred from Ir moments and a drastic decrease of $1/T_1$ . Above $T_f$, physical properties are expected to be a landmark of frustration in this exotic geometry. We will discuss our shift and relaxation data in the temperature range of 300K to 7.5 K in the light of published thermodynamic measurements (Y. Okamotoa et al, PRL 99 137207, 2007 and Y. Singh et al, PRB 88 220413(R), 2013) and comment on their implications for the already existing large body of theoretical work.