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+ (t(2g)(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 C-m data showed the absence of long range magnetic ordering at least down to 2 K. The large C-m 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.