High pressure and isotope effects on polaron hopping in La_0.65Ca_0.35MnO₃

We have measured the oxygen isotope effect on the metal-insulator transition and the electrical properties of La_0.65Ca_0.35MnO₃ under hydrostatic pressure up to 1.7 GPa. The conductivity activation energy E_A in the insulating phase is a linear function of T_c and coincides for both isotopes ¹⁶O and ¹⁸O if compared at different pressures but for the same T_c. The pressure and isotope effects can be qualitatively understood in terms of the double exchange model and the adiabatic hopping of Jahn-Teller polarons. Higher order corrections to the Holstein model in the intermediate coupling regime explain the unusually large isotope effect on the relative pressure coefficient of T_c.