Thermal conductivity across the metal-insulator transition in the single-crystalline hyperkagome antiferromagnet Na3+xIr3O8

The hyperkagome antiferromagnet Na4Ir3O8 represents the first genuine candidate for the realization of a three-dimensional quantum spin liquid. It can also be doped towards a metallic state, thus offering a rare opportunity to explore the nature of the metal-insulator transition in correlated, frustrated magnets. Here, we report thermodynamic and transport measurements in both metallic and weakly insulating single crystals down to 150 mK. While in the metallic sample the phonon thermal conductivity (κph) is almost in the boundary scattering regime, in the insulating sample, we find a large reduction κph over a very wide temperature range. This result can be ascribed to the scattering of phonons off the gapless magnetic excitations that are seen in the low-temperature specific heat. This works highlights the peculiarity of the metal-insulator transition in Na3+xIr3O8 and demonstrates the importance of the coupling between lattice and spin degrees of freedom in the presence of strong spin-orbit coupling.