Oxygen-isotope effects on La0.65Ca0.35MnO3 under pressure

We have measured the O-isotope effects on the magnetic and electrical prope l-ties of La0.65Ca0.35MnO3 under hydrostatic pressure up to 1.9 GPa. The pr essure was found to reversibly enhance the metal-insulator transition tempe rature T-C and suppress the resistivity rho of both isotopic samples, but a t different rates, depending on the O-isotopic mass of the samples. It was also found that pressure suppresses the isotope effect linearly, while the O-18-->O-16 exchanee drastically enhances rho. The greater d In T-C/dP and \d In rho/dP\ observed in the O-18 sample can he understood qualitatively i n terms of the double exchange interaction between Mn ions and the band red uction effect of polarons, but the d In T-C/dP difference is much greater t han predicted. The discrepancy suggests that modifications may be needed fo r the model. [S0163-1829(99)50146-X].