Light-induced Resistance Changes in Epitaxial Thin Films of Hole Doped Rare Earth Manganites

We observe light induced resistance changes in oxygen deficient La$_{0.67}$Ba$_{0.33}$MnO$_{3-y}$(LBMO) thin films and compressively strained La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) thin films grown by Pulsed Laser Deposition. LBMO thin films grown under low oxygen pressures are insulating down to 140 K where we observe a hump in resistivity reminiscent of transition to a ferromagnetic metallic state, immediately followed by an upturn in resistivity. Illumination of these films by continuous Argon laser causes a pronounced decrease in resistivity over a broad temperature range. In compressively strained LCMO thin films which are insulating, we observe a similar suppression of resistivity under illumination at temperatures below 250 K (the temperature range of the metal insulator transition in the unstrained films). Both lattice mismatch strain and oxygen deficiency are known to suppress the insulator-metal transition and enhance charge and orbital ordering in manganite materials. Radiation over a broad range of wavelength is known to destroy charge/orbital ordering. We will discuss the origin of the observed photoconductivity and the possible role of charge/orbital ordering .