Optical studies of a layered manganiteLa1.2Sr1.8Mn2O7:Polaron correlation effect

Abstract

Optical conductivity spectra of a cleaved ab plane of ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ exhibit a small polaron absorption band in the midinfrared region over all temperatures. With decreasing temperature $(T)$ up to Curie temperature ${(T}_{C}),$ the small polaron band moves to a higher frequency, resulting in a finite gaplike feature. However, it shifts to a lower frequency below ${T}_{C}.$ With decreasing T, a stretching phonon mode hardens above ${T}_{C}$ and softens below ${T}_{C}.$ These concurring changes of lattice and electronic structure indicate that short-range polaron correlation exists above ${T}_{C}$ but disappears with the magnetic ordering.