We have measured the oxygen isotope effect on the electrical properties of La0.65Ca0.35MnO3 under hydrostatic pressure up to 1.9 GPa. Special attention is paid to the conductivity activation energy, Ea, above the insulator-metal transition temperature, Tc. Ea is a universal linear fiinction of Tc. The change of Ea by pressure and isotopic mass can be scaled to the change of Tc. These results can be qualitatively understood in terms of the double exchange model and the adiabatic hopping of Jahn Teller polarons.
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