Ultraviolet photoelectron spectroscopy study of colossal magnetoresistive La_0.7-xPr_xCa_0.3MnO₃

We have investigated the electronic structure of colossal magnetoresistive perovskite manganites La_0.7-xPr_xCa_0.3MnO₃ using ultraviolet photoelectron spectroscopy as a function of temperature and Pr concentration x (x = 0.13, 0.3, and 0.4). Unusual temperature-dependent and x-dependent spectral changes are observed in the entire valence band region. Since Pr doping does not introduce carriers into the system but brings about strong lattice distortion effects which exert significant influence on the p-d hybridization, the degree of mixing between Mn 3d and O2p levels is considered to be the major factor responsible for the spectral weight transfers with temperature and Pr doping level. The observed changes are not consistent with the results of local-density-approximation (LDA) band calculation, but can be understood by the density-of-states from the LDA+U calculation or configuration-interaction cluster calculations. This implies that a strong correlation effect between d electrons plays an important role in these manganites. High resolution spectra near the Fermi level shows a clear metallic edge at low temperature for low Pr concentration sample (x = 0.13), but the metallic Fermi edge disappears at higher Pr doping levels (x = 0.3 and 0.4) even at the temperature below T_c where the bulk metal-insulator transition occurs. This may be due to the low effective carrier density resulting from the reduced ferromagnetic ordering and the lattice distortion, or the pseudo-gap phenomena and the surface effect.