Metal-insulator transition of R2-xCaxRu2O7 (R = Y, Sm)

We have studied physical properties of pyrochlore ruthenates R2-xCaxRu2O7 ( R = Y,Sm). Upon increasing Ca concentration x, or upon increasing holes int roduced into the Mott insulating state, the system exhibits a transition to a metallic state (Mott transition). For the R atom with larger ionic radiu s, the critical concentration x(c) of the transition is smaller. The suppre ssion of the spin-glass transition has also been found with increasing Ca c oncentration. In the proximity region of the metal to insulator phase bound ary, the Hall coefficient is meaningfully enhanced with decreasing temperat ure and exhibits a pronounced peak at around T-G, where the zero-field cool ing susceptibility shows a maximum. Overall features of transport and magne tic properties of R2-xCaxRu2O7 are quite similar to those of other Mott M-I system R2-xBixRu2O7, in which the M-I transition is caused by a change of the transfer integral. (C) 2000 Elsevier Science Ltd. All rights reserved.