Optical study ofPr1−xCaxMnO3(x=0.4)in a magnetic field: Variation of electronic structure with charge ordering and disordering phase transitions

The temperature (10–290 K) and magnetic-field (0–7 T) dependencies of optical spectra and their anisotropy have been investigated for a single crystal of ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3\mathrm{}(x=0.4\mathrm{}),$ which undergoes a charge and orbital ordering transition at $T_{\mathrm{co}}=235\mathrm{}\hspace{0.5em}\mathrm{K}.$ A clear anisotropic feature has been observed between the b- and c-axis polarized optical conductivity spectra $\left[\sigma \right(\omega \left)\right]$ at 10 K, reflecting a pattern of spatial charge and orbital ordering. The gap value of the charge-ordered (CO) state is estimated to be $\approx 0.18\hspace{0.5em}\mathrm{eV}$ in the ground state. $\sigma \left(\omega \right)$ is drastically transformed from an anisotropic gaplike shape into an isotropic metallic band with a conspicuous spectral weight transfer over a wide photon energy region (0.05–3 eV) by application of a magnetic field of 7 T. As the magnetic field is decreased from 7 T, such metallic $\sigma \left(\omega \right)$ suddenly becomes gaplike again around 4.5 T, yet the anisotropy between both polarizations is decreased perhaps owing to orbital disordering. The temperature dependence of the b-axis polarized $\sigma \left(\omega \right)$ was investigated in a constant magnetic field of 7 T. As the temperature decreases, the onset energy of $\sigma \left(\omega \right)$ shows a blueshift from $T_{\mathrm{co}}$ down to the antiferromagnetic spin ordering temperature $T_N,$ but shows a redshift once the spin ordering takes place below $T_N.$ This result indicates that the charge gap value as the order parameter of the CO state couples with the spin ordering.