Effect of Pb doping on metallic state of cubic pyrochlore Cd2Ru2O7

Abstract

Many exotic phenomena in strongly correlated electron systems, such as unconventional superconductivity, metal-insulator transition, and quantum criticality, take place in the intermediate regime between localized and itinerant electronic state. To understand the electronic behaviors near the localized-to-itinerant crossover remains a challenging problem in condensed matter physics. The Ru5+ cubic pyrochlores A2Ru2O7 (A=Cd, Cd, Hg) constitute such a system that the Ru-4d electrons acquire characters of both itinerancy and localization. In addition, the magnetic Ru5+ ions that are situated on the vertices of corner-shared tetrahedral lattice are expected to experience strong geometrical frustration given an antiferromagnetic (AF) arrangement. In this work, we investigate the cubic pyrochlore Cd2Ru2O7, which develops a peculiar metallic state below the AF transition. We synthesize a series of Pb-doped Cd2-xPbxRu2O7 (0 x 2) polycrystalline samples under high-pressure condition, and study the effects of Pb doping on their crystal structure and physical properties. Although the Pb2Ru2O7 pyrochlore is a Pauli paramagnetic metal, we find that the substitution of 10% Pb2+ for Cd2+ in Cd1.8Pb0.2Ru2O7 converts the metallic state of Cd2Ru2O7 into an insulating ground state, in a manner similar to the consequence of exerting hydrostatic pressure or substituting 10% Ca2+ for Cd2+ ions as we found recently. We propose that the electronic state of Cd2Ru2O7 be located at the itinerancy to localization crossover, and the introduction of chemical disorder via Pb2+ substitution may enhance the localized character and induce the metal-to-insulator transition. Our results further demonstrate that the cubic Ru5+ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.