Optical study of Pr1-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 optic al spectra and their anisotropy have been investigated for a single crystal of Pr1-xCaxMnO3 (x=0.4), which undergoes a charge and orbital ordering tra nsition at T-co=235 K. A clear anisotropic feature has been observed betwee n the b- and c-axis polarized optical conductivity spectra [sigma(omega)] a t la K, reflecting a pattern of spatial charge and orbital ordering. The ga p value of the charge-ordered (CO) state is estimated to be approximate to 0.18 eV in the ground state. sigma(omega) is drastically transformed from a n anisotropic gaplike shape into an isotropic metallic band with a conspicu ous spectral weight transfer over a wide photon energy region (0.05-3 eV) b y application of a magnetic field of 7 T. As the magnetic field is decrease d from 7 T, such metallic sigma(omega) suddenly becomes gaplike again aroun d 4.5 T, yet the anisotropy between both polarizations is decreased perhaps owing to orbital disordering. The temperature dependence of the b-axis pol arized sigma(omega) was investigated in a constant magnetic field of 7 T. A s the temperature decreases, the onset energy of sigma(omega) shows a blues hift 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 re sult indicates that the charge gap value as the order parameter of the CO s tate couples with the spin ordering. [S0163-1829(99)01312-0].