Optical evidence of mixed-phase behavior in manganite films

Synchrotron infrared measurements were conducted on a self-doped ${\text{La}}_x{\text{MnO}}_{3-\delta }$ $\left(x\sim 0.8\right)$ film. From these measurements we determined the conductivity and the temperature dependence of the effective number of carriers. While the metal-insulator transition temperature $\left(T_{\text{MI}}\right)$ and the magnetic ordering temperature $\left(T_C\right)$ approximately coincide, the onset of the change in the free carrier density occurs at a significantly lower temperature ($\sim 45\text{K}$ below). This suggests that local distortions exist below $T_{\text{MI}}$ and $T_C$ which trap the $e_g$ conduction electrons. These regions with local distortions constitute an insulating phase which persists for temperatures significantly below $T_{\text{MI}}$ and $T_C$.

Figure 1

(Color online) (a) ${\text{La}}_{0.8}{\text{MnO}}_{3-\delta }/\text{LAO}$ reflectivity spectra at 10, 20, 50, 80, 100, 125, 150 (purple dotted lines), 200 (green dashed lines), 225, 245, 255, 265, 275 (dash dotted lines), 282 (dash dotted lines), and 304 (dash dotted lines) K on a logarithmic energy scale. The vertical dashed lines indicate phonon positions. The $y$ scale corresponds to the 304 K spectrum, and the others are shifted up by 0.1 relative to the previous temperatures. (b) LAO substrate reflectivity spectra. The energy is given on a logarithmic scale in (a) and (b). (c) Inset is an expansion of curves in (a) over the range from 100 to $550{\text{cm}}^{-1}$ on a linear energy scale.

Figure 2

(Color online) (a) An example fit of ${\text{La}}_{0.8}{\text{MnO}}_{3-\delta }/\text{LAO}$ reflectivity spectra at 10 K. (b) Real part of the optical conductivity spectra of ${\text{La}}_{0.8}{\text{MnO}}_{3-\delta }$ at different temperatures. (c) Real part of the optical conductivity spectra of LAO substrate.

Figure 3

(Color online) (a) Magnetization and calculated and measured dc resistivities of ${\text{La}}_{0.8}{\text{MnO}}_{3-\delta }/\text{LAO}$ film. Inset is the comparison for ${\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3$ (symbols) and ${\text{La}}_{0.8}{\text{MnO}}_{3-\delta }$ (line). (b) Effective carrier numbers and Drude weight (free carriers).