Spin-Liquid State in the S = 1/2 Hyperkagome Antiferromagnet Na4Ir3O8

A spinel related oxide, Na4Ir3O8, was found to have a three dimensional network of corner shared Ir4+ (t2g^5) triangles. This gives rise to an antiferromagnetically coupled S = 1/2 spin system formed on a geometrically frustrated hyperkagome lattice. Magnetization M and magnetic specific heat Cm data showed the absence of long range magnetic ordering at least down to 2 K. The large Cm at low temperatures is independent of applied magnetic field up to 12 T, in striking parallel to the behavior seen in triangular and kagome antiferromagnets reported to have a spin-liquid ground state. These results strongly suggest that the ground state of Na4Ir3O8 is a three dimensional manifestation of a spin liquid.