Irreversible photoinduced insulator-metal transition in the Na-doped manganite Pr_0.75Na_0.25MnO₃

Persistent photoconductivity has been observed in a perovskite manganite doped with an alkali metal element, Pr_0.75Na_0.25MnO₃. Considering the shallow penetration depth (∼0.1 μm) of visible light, the increase of the conductance is estimated to be of the order of 10⁴ to 10⁵, which implies an irreversible photoinduced insulator-metal transition. Analysis of the dc- and ac-susceptibility data shows that the system is in a glassy state below 45 K. The system is therefore viewed as being in a highly nonuniform and metastable state, in which ferromagnetic metallic clusters exist in insulating matrix. The photoexcitation helps the clusters to dislodge from a local energy minimum and to seek more stable configuration. The percolative conduction can well explain the observed large increase of the conductivity. We argue that metastability is due to the large charge mismatch between the monovalent Na⁺ ion and Pr³⁺ or the incommensurability of the Mn valency from 3.5+.