Anisotropic optical signatures of orbital and charge ordering inBi1−xCaxMnO3

We present a spectroscopic ellipsometry study of the optical anisotropies at energies above and below a crossing point at $\approx 2$ eV in the optical conductivity of ${\mathrm{Bi}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ crystals. Below $T_{\mathrm{co}},$ we observe a sudden increase of the low-frequency dielectric function, reflecting charge ordering. Moreover, we find a gradual development in the high-frequency dielectric function, primarily influenced by orbital effects, which suggest that fluctuations associated with short-range orbital ordering precede charge ordering.