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

In a metallic manganite, the substitution of ${\mathrm{Mn}}^{+3}$ by ${\mathrm{Ga}}^{+3}$ 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 ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}$, 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 ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ 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.