Optical study on the doping and temperature dependence of the anisotropic electronic structure in bilayered manganites: La2-2xSr1+2xMn2O7 (0.3 <= x <= 0.5)

The electronic structure of bilayered manganite (La2-2xSr1+2xMn2O7) has bee n investigated by optical measurements. In the paramagnetic phase, the in-p lane and out-of-plane optical spectra show a doping dependence reflecting t he change in the orbital character of conduction electrons. A drastic elect ronic structural change has been observed with the evolution of in-plane ma gnetic ordering irrespective of the ferromagnetic (e.g., x = 0.4) or antife rromagnetic (x = 0.3) interbilayer magnetic coupling. The optical conductiv ity spec tra of the ferromagnetic state show a strongly incoherent feature, such as a minimal Dr de component followed by a dominant broad peak center ed around 0.4 eV. Anomalies in the phonon spectra are found in the antiferr omagnetic state of the x=0.45 and 0.5 compounds, indicating a different str uctural change accompanied by the A-type antiferromagnetic (and perhaps x(2 )-y(2)-type orbital) ordering. The relevant charge ordering and lattice dis tortion may be responsible for the nonmetallic nature of the A-type antifer romagnetic phase.