Crossover in the nature of the metallic phases in the perovskite-type RNiO₃

We have measured the photoemission spectra of Nd_1-xSm_xNiO₃, where the metal-insulator transition and the Néel ordering occur at the same temperature for x≲0.4 and the metal-insulator transition temperature (T_MI) is higher than the Néel temperature for x≳0.4. For x≤0.4, the spectral intensity at the Fermi level is high in the metallic phase above T_MI and gradually decreases with cooling in the insulating phase below T_MI while for x>0.4 it shows a pseudogaplike behavior above T_MI and further diminishes below T_MI. The results clearly establish that there is a change in the nature of the electronic correlations in the middle (x∼0.4) of the metallic phase of the RNiO₃ system.