Low-energy excitations in ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$ investigated by ellipsometry

The low-energy excitations of the bilayered manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$ have been explored by spectral ellipsometry from two faces of a single crystal over the range from 0.006 to 0.6 eV. This compound is a paramagnetic insulator at ambient temperature, with a transition to a ferromagnetic metal below a Curie temperature $\left(T_{\mathrm{c}}\right)$ of 125 K. Both the $ab$-plane and $c$-axis temperature-dependent conductivities have been determined. Essentially no temperature-dependent behavior is observed above $T_{\mathrm{c}}$ although below $T_{\mathrm{c}}$ both the phonon and electronic contributions are strongly temperature sensitive. The highest-frequency phonons, especially those involving Mn-O bond stretching, split and show frequency changes consistent with structural results in the literature, and furthermore there is clear evidence of an increase in electron-phonon coupling at and below $T_{\mathrm{c}}$. We interpret the temperature-dependent electronic spectral contribution in the light of recent calculations that indicate that a mixed phase exists in the doped manganites below $T_{\mathrm{c}}$, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase.

Figure 1

Energy-dependant $ab$-plane and $c$-axis conductivities. Data at only a selection of temperatures are displayed.

Figure 2

Spectral conductivity in the low energy region with both phonon and charge carrier contributions. (a) $ab$-plane and (b) $c$-axis conductivity. The $ab$-plane dc conductivities are indicated along the zero-energy axis, and connected to the corresponding spectral data with arrows. Data for only a selection of temperatures are shown.

Figure 3

Comparison of the spectral strengths of the two low-energy $ab$-plane broad peaks. The erect triangles and the right-hand axis represent the strength of the feature centerd at 250 meV, integrated from 0.09 to 0.5 eV (MIR), while inverted triangles and the left-hand scale present the integrated strength of the 37 meV peak between 0.01 and 0.09 eV (FIR). Note that the integrations have omitted both the phonon lines and the rising background from the small-polaron tail represented by the 250 K data.