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

Abstract

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 ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}(x=0.4),$ which undergoes a charge and orbital ordering transition at ${T}_{\mathrm{co}}=235 \mathrm{K}.$ A clear anisotropic feature has been observed between the b- and c-axis polarized optical conductivity spectra $[\ensuremath{\sigma}(\ensuremath{\omega})]$ 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 $\ensuremath{\approx}0.18 \mathrm{eV}$ in the ground state. $\ensuremath{\sigma}(\ensuremath{\omega})$ 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 $\ensuremath{\sigma}(\ensuremath{\omega})$ 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 $\ensuremath{\sigma}(\ensuremath{\omega})$ was investigated in a constant magnetic field of 7 T. As the temperature decreases, the onset energy of $\ensuremath{\sigma}(\ensuremath{\omega})$ 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.