Optical studies of a layered manganite La1.2Sr1.8Mn2O7:: Polaron correlation effect

Optical conductivity spectra of a cleaved ab plane of La1.2Sr1.8Mn2O7 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.