Photoinduced nanodots in Bi₂Sr₂CaCu₂O _8+d

We report a fabrication of high-density nanodots by photodoping in overdoped Bi₂Sr₂CaCu₂O_8+d thin film (T_c = 80 K). A scanning near-field optical microscope probe is used to locally excite carrier, and photodoped region is associated with lower T _c phase (T_c = 75 K) via overdoping. Nanoscale characterizations with optical reflectivity reveal that nanodots (30-nm diameter) are regularly distributed in 50-nm step. The resultant films with photoinduced nanodots enhance J_c, a situation being similar to strong pinning effects observed in films modified by either ion irradiation or sputtered nanoparticles. These results suggest that photoinduced nanodots with lower T_c act as effective pinning centers.