Nonthermal Photoresponse in Epitaxial Thin Films of (La,Pr).67Ca.33MnO4: Correlation with Non-ohmic Electrical Transport and Magnetoresistance

We have recently observed a non-thermal component of light induced resistance change in the vicinity of the insulator-metal transition, in epitaxial thin films of the CMR manganite material (La,Pr).67Ca.33MnO4 (LPCMO). LPCMO is known to have the co-existence of insulating and metallic regions. On cooling, the metallic regions grow at the expense of the insulating regions, giving rise to a percolative insulator-metal transition. Our results indicate that light may cause electronic changes in the insulating regions thus decreasing the electrical resistance. We will present our studies of the photoresponse and the correlation of the observed non-thermal photoresponse with magnetoresistance as well as current-voltage (I-V) characteristics. I-V measurements show that there is a current induced change in resistance which is not due to the joule heating effects. This effect is only seen in the metal-insulator transition range of the samples, similar to the nonthermal photoresponse, suggesting a common origin for these two phenomena. The effects of magnetic field however are seen to be distinct. A large magnetoresistance is seen at lower temperatures where the light and current induced effects are absent, thus suggesting a very different physical origin for the magnetoresistance.