Effect of Ga substitution on the optical properties of La-Sr manganites

In a metallic manganite, the substitution of Mn+3 by Ga+3 locally destroys both the ferromagnetic order and the Jahn-Teller distortion, without heavily affecting the crystal structure. One can thus observe an unusual metal-to-insulator transition, which is induced neither by the temperature nor by changes in the divalent-ion doping. This transition is studied here by comparing the infrared reflectivity of five samples of La2/3Sr1/3Mn1-xGaxO3, with x increasing from 0 to 0.30. The dramatic effect of Ga on the transport properties is monitored through the decrease in the number of free carriers and the increase in their effective mass, until a good metal such as La2/3Sr1/3MnO3 is turned into an insulator at all temperatures. A simple model which links the measured optical parameters to the magnetization M(x,T) well describes the behavior of the plasma frequency, the scattering rate, and the midinfrared absorption, throughout the Ga-induced metal-to-insulator transition.