Optical study of Pr_1-xCa_xMnO₃(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 Pr_1-xCa_xMnO₃(x = 0.4), which undergoes a charge and orbital ordering transition at T_co = 235 K. A clear anisotropic feature has been observed between the b- and c-axis polarized optical conductivity spectra [σ(ω)] 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 ≈ 0.18eV in the ground state. σ(ω) 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 σ(ω) 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 σ(ω) was investigated in a constant magnetic field of 7 T. As the temperature decreases, the onset energy of σ(ω) shows a blueshift from T_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.