Spin liquid close to a quantum critical point in Na4Ir3O8

Abstract

Na4Ir3O8 is a candidate material for a three-dimensional quantum spin liquid on the hyperkagome lattice. We present thermodynamic measurements of heat capacity C and thermal conductivity kappa on high-quality polycrystalline samples of Na4Ir3O8 down to T = 500 and 75 mK, respectively. Absence of long-range magnetic order down to T = 75 mK strongly supports claims of a spin-liquid ground state. The constant magnetic susceptibility chi below T approximate to 25 K and the presence of a small but finite linear-T term in C(T) suggest the presence of gapless spin excitations. Additionally, the magnetic Gruneisen ratio shows a divergence as T -> 0 K and a scaling behavior, which clearly demonstrates that Na4Ir3O8 is situated close to a zero-field QCP.