Optical study on the doping and temperature dependence of the anisotropic electronic structure in bilayered manganites: La_2-2xSr_1+2xMn₂O₇ (0.3<~x<~0.5)

The electronic structure of bilayered manganite (La_2-2xSr_1+2xMn₂O₇) has been investigated by optical measurements. In the paramagnetic phase, the in-plane and out-of-plane optical spectra show a doping dependence reflecting the change in the orbital character of conduction electrons. A drastic electronic structural change has been observed with the evolution of in-plane magnetic ordering irrespective of the ferromagnetic (e.g., x=0.4) or antiferromagnetic (x=0.3) interbilayer magnetic coupling. The optical conductivity spectra of the ferromagnetic state show a strongly incoherent feature, such as a minimal Drude component followed by a dominant broad peak centered around 0.4 eV. Anomalies in the phonon spectra are found in the antiferromagnetic state of the x=0.45 and 0.5 compounds, indicating a different structural change accompanied by the A-type antiferromagnetic (and perhaps x²-y²-type orbital) ordering. The relevant charge ordering and lattice distortion may be responsible for the nonmetallic nature of the A-type antiferromagnetic phase.