Spin liquid close to a quantum critical point in Na 4 Ir 3 O 8

Abstract

Na 4 Ir 3 O 8 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 κ on high-quality polycrystalline samples of Na 4 Ir 3 O 8 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 χ below T ≈ 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 Gr ¨ u neisen ratio shows a divergence as T → 0 K and a scaling behavior, which clearly demonstrates that Na 4 Ir 3 O 8 is situated close to a zero-field QCP.