Anisotropic optical spectra of doped manganites with pseudocubic perovskite structure

Abstract

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 ${R}_{1\ensuremath{-}x}{A}_{x}{\mathrm{MnO}}_{3}$ $(R=\mathrm{Pr}$ and Nd, $A=\mathrm{Ca}$ 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 ${e}_{g}$-orbital on Mn sites. Such orbital-order patterns give rise to characteristic optical spectra that show a strong spectral weight below $\ensuremath{\sim}2\mathrm{eV}$ with light polarization parallel to the direction of orbital lobes. Moreover, even in the ferromagnetic-metallic phases of ${\mathrm{Pr}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$ and ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3},$ 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.