Magnetic-field-dependent optical studies of Pr_0.69Ca_0.31MnO₃

We investigated magnetic-field-dependent optical conductivity spectra σ(ω) of Pr_0.69Ca_0.31MnO₃ at 4.2 K, which was in a charge-ordered insulating state at 0 T. With increasing the magnetic field up to 12 T, we observed an insulator-metal transition around 4 T and large spectral weight changes up to 4 eV. Especially, we found that the spectral weight above 0.5 eV moved to a lower energy region and that a Drude-like peak appeared clearly at 12 T. By analyzing dielectric response changes, we suggested that the field-dependent insulator-metal transition might occur through a percolation process of ferromagnetic metal domains. On the other hand, with decreasing the magnetic field from 12 to 0 T, σ(ω) showed little changes, which showed that the ferromagnetic metallic state did not return to the insulating state.