Oxygen Isotope Effects in the Manganates and Cuprates Studied by Electron Paramagnetic Resonance
We present the results of the oxygen isotope effect study in colossal magnetoresistive manganate La1−x Ca x MnO3+y, using electron paramagnetic resonance (EPR). We observed strong isotope effects on EPR intensity and linewidth which can be explained by a model where a bottlenecked spin relaxation takes place from the exchange-coupled constituent Mn4+ ions via the Mn3+ Jahn-Teller ions to the lattice. For x = 0.2 the ferromagnetic exchange energy J exhibits a16O/18O oxygen isotope effect of ~ -10%. The observed isotope effects suggest the presence of Jahn-Teller polarons in these materials.
We also report the results of the similar study in cuprate superconductors La2-x SrxCuO4. Experiments showed large oxygen isotope effect on EPR linewidth. It was found that isotope effect is strong in samples with small Sr doping and decreases with Sr concentration increase. These results provide the first microscopic evidence for the polaronic charge carriers in the cuprate superconductors.
KeywordsOxygen Isotope Cuprate Superconductor Double Exchange Model Oxygen Isotope Effect Strong Isotope Effect
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