Metallic spin-liquid behavior of the geometrically frustrated Kondo lattice Pr2Ir2O7

Strongly frustrated magnetism of the metallic pyrochlore oxide Pr2Ir2O7 has been revealed by single crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of parallel to T-*parallel to=20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges and leads to a partial screening of the moments below parallel to T-*parallel to. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.