Melting of charge/orbital ordered states in Nd1/2Sr1/2MnO3:: Temperature and magnetic-field-dependent optical studies

We investigated the temperature (T = 15 similar to 290 K) and the magnetic-field-dependent (H=0 similar to 17 T) optical conductivity spectra of a charge-orbital-ordered manganite Nd1/2Sr1/2MnO3. With variation of T and H, large spectral weight changes were observed up to 4.0 eV. These spectral weight changes could be explained using the polaron picture. Interestingly, our results suggested that some local ordered state might remain above the charge ordering temperature, and that the charge/arbital melted state at a high magnetic field (i.e., at H = 17T and T=4.2 K) should be a three-dimensional ferromagnetic metal. We also investigated the first order phase transition from the charge-orbital- ordered state to ferromagnetic metallic state using the T- and H-dependent dielectric constants epsilon. Through the analysis of epsilon using an effective medium approximation, we found that the melting of charge-orbital-ordered states should occur through the percolation of ferromagnetic metal domains.