High pressure study on isotope effects in lanthanum calcium manganese oxide: The double exchange and polaron effects

High pressure study has been carried out on isotopic La1-x CaxMnO3. The pressure was found to reversibly enhance the metal-insulator transition temperature Tc of both isotopic samples, but at different rates, depending on the doping level, pressure and oxygen-isotopic mass. For the optimally doped (x = 0.35) sample, the Tc was observed to increase linearly with pressure up to ∼1 GPa at a greater rate of dlnTc/dP = 0.048/GPa for the 18O sample than that of 0.038/GPa for the 16O sample. The greater dlnT c/dP observed in the 18O sample can be understood qualitatively in terms of the double exchange model with the Jahn-Teller polaron effect included. The relative difference of dlnTc/dP between the two isotopes shows the electron-phonon interaction is in the intermediate coupling region. The electrical resistivity rho was suppressed rapidly by pressure for both isotopes and is well described by the hopping of adiabatic Jahn-Teller polarons at high temperature. The pressure as well as the isotope dependence of the activation energy scales well with the metal-insulator translation temperature, Tc. The Jahn-Teller energy, EJT, is only weakly dependent on pressure.;The FM - CO transition temperature Tco was found to decrease with pressure for both over doped isotopic samples (x = 0.43). The shrinkage of the hysteresis loop under pressure in rho(T) and chiac(T) curves indicates the suppression of the 1st order transition. The pressure and isotope dependence of Tco can be qualitatively explained in terms of the double exchange model with the Jahn Teller polaron effect included. The result is consistent with the internal pressure and magnetic field dependence of Tco.;We have found the similarities between the curves of a ∼ x, dalpha/dP ∼ x, dlnTc/dP ∼ x and d(dlnTc/dP)/dlnM ∼ x. The results can be understood qualitatively in terms of the double exchange (DE) interaction between Mn ions and the band reduction effect of polarons, but not quantitatively.