Anisotropic optical spectra of doped manganites with pseudocubic perovskite structure

To investigate the effect of charge-spin-orbital correlations on electronic structures, anisotropic optical conductivity spectra have been investigated for twin-free crystals of various doped manganites R_1-xAₓMnO₃ (R=Pr and Nd, A=Ca and Sr; $0.4<~x<~0.7)$ with pseudo-cubic perovskite structure. In doped manganites having an antiferromagnetic ground state, a remarkable anisotropy has been observed in their optical spectra, reflecting the respective ferroic/antiferroic orders of eg-orbital on Mn sites. Such orbital-order patterns give rise to characteristic optical spectra that show a strong spectral weight below ~2eV with light polarization parallel to the direction of orbital lobes. Moreover, even in the ferromagnetic-metallic phases of Pr1/2Sr1/2MnO₃ and Nd1/2Sr1/2MnO₃, we observed substantial anisotropy in optical spectra. The results indicate the existence of orbital-polarized ferromagnetic-metallic phase in doped manganites although the ferromagnetic-metallic state attributed to the double-exchange interaction has been normally believed to accompany the orbital disordering.