Persistent X-ray photoconductivity and percolation of metallic clusters in charge-ordered manganites

– Charge-ordered manganites of composition Pr1−x(Ca1−ySry)xMnO3 exhibit persistent photoconductivity upon exposure to X-rays. This is not always accompanied by a significant increase in the number of conduction electrons as predicted by conventional models of persistent photoconductivity. An analysis of the X-ray diffraction patterns and current-voltage characteristics shows that X-ray illumination results in a microscopically phase-separated state in which charge-ordered insulating regions provide barriers against charge transport between metallic clusters. The dominant effect of X-ray illumination is to enhance the electron mobility by lowering or removing these barriers. A mechanism based on magnetic degrees of freedom is proposed. The manganite perovskite Pr0.7Ca0.3MnO3 has recently been shown to undergo an unusual insulator-metal transition when it is exposed to an X-ray beam [1]. Without X-rays, the material is semiconducting (with an average valence of +3.3 distributed uniformly over the Mn sites) at high temperatures, and charge-ordered insulating (with a static superlattice of Mn and Mn ions) below ∼ 200 K. Below ∼ 40 K, X-rays convert the insulating state to a metallic state which persists when the X-ray beam is switched off, but is removed on thermal cycling [1]. Interestingly, an equilibrium insulator-metal transition can also be induced by an external magnetic field [2], but not by irradiation with visible light. The material is, however, susceptible to dielectric breakdown at comparatively low electric fields [3] that can be triggered by laser light [4]. A microscopic explanation for this unusual behavior has thus far not been given. In this paper, we take the first steps in this direction by proposing a new mechanism () Present address: Department of Physics, 13-2154, MIT, Cambridge, MA 02139, USA.