Anomalous transport behavior in geometrically frustrated system R_2−xBi_xRu₂O₇ (R = rare earth, Y)

We have studied transport and magnetic properties of ruthenium pyrochlore oxides R_2−xBi_xRu₂O₇ (R = rare earth, Y), which exhibit the Mott insulator to metal transition with increasing x. The spin-glass transition can be observed basically in the insulating x-region, which may be due to the freezing of Ru spins. Transport quantities, the electrical resistivity ρ, the Hall coefficient R_H and the thermoelectric power S are found to be strongly affected by the spin-glass transition. Furthermore, near the metal-insulator phase boundary, R_H increases rather rapidly with decreasing temperature and the Hall angle θ_H obeys the relation cot θ_H = a+bT² in the paramagnetic state. These results suggest that there exist strong spin fluctuations in the metallic state of R_2-xBi_xRu₂O₇, which is suppressed as the spin freezing proceeds.