Polarons in Manganites; Now You See Them Now You Don’t

Summary

We have reviewed the evidence in local structural measurements for the existence of polarons in the phase diagrams of La_{1- x}(Ca,Sr)_x MnO₃ with greater emphasis being given to our own work on the Ca system. The existence or absence of localized carriers seems to be closely associated with the observation of a local Jahn-Teller distortion which essentially disappears in the metallic phase when charges are delocalized. Localized carriers coexist with delocalized carriers throughout most of the ferromagnetic metallic phase as two fluids, presumably in a microscopically phase separated form. As a function of temperature carriers progressively delocalize on cooling into the FM phase. When the IM transition is crossed as a function of doping the response of the local structure is less apparent and the change in the macroscopic resistivity is presumably coming about because the microscopic regions delocalized carriers percolate the sample.

The x=0.5 sample tries to delocalize in a similar way as the lower doped materials. However, charge ordering of the localized carrier component sets in and stabilizes the localized carriers with respect to the delocalized carriers. This suppresses the delocalization process. The delocalized carriers are gradually relocalized as the temperature is lowered until all the carriers are localized and the sample becomes insulating and antiferromagnetic at low temperature.