Existence of orbital polarons in ferromagnetic insulating La 1 − x Sr x Mn O 3 ( 0.11 ⩽ x ⩽ 0.14 ) revealed by giant phonon softening

We present an inelastic light scattering study of single crystalline ${({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y})}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14$, $y=0$ and $x=1∕8$, $0\ensuremath{\leqslant}y\ensuremath{\leqslant}0.5$). A giant softening up to $20\char21{}30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ of the $\mathrm{Mn}\ensuremath{-}\mathrm{O}$ breathing mode has been observed only for the ferromagnetic insulating (FMI) samples $(0.11\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14)$ upon cooling below the Curie temperature $({T}_{C})$. With increasing Pr-doping the giant softening is strongly suppressed. This is attributed to a coupling of the breathing mode to orbital polarons, which are present in the FMI state. Moreover, the softening that persists to lowest temperature without saturation suggests a continuous, temperature-dependent evolution of orbital polarons below ${T}_{C}$.