Melting of charge/orbital ordered states in Nd 1 / 2 Sr 1 / 2 MnO 3 : Temperature and magnetic-field-dependent optical studies

We investigated the temperature $(T=15\ensuremath{\sim}290\mathrm{K})$ and the magnetic-field-dependent $(H=0\ensuremath{\sim}17\mathrm{T})$ optical conductivity spectra of a charge-orbital-ordered manganite ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}.$ 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/orbital melted state at a high magnetic field (i.e., at $H=17\mathrm{T}$ and $T=4.2\mathrm{K})$ 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 $\ensuremath{\varepsilon}.$ Through the analysis of $\ensuremath{\varepsilon}$ using an effective medium approximation, we found that the melting of charge-orbital-ordered states should occur through the percolation of ferromagnetic metal domains.