Temperature-dependent electronic structure of Nd_1-xSm_xNiO₃

RNiO₃ (R: rare earth) is one of the typical bandwidth-control metal-insulator transition systems. Its metal-insulator transition temperature (T_MI) is the same as the antiferromagnetic transition temperature (T_N) for R = Pr and Nd, while T_MI is higher than T_N for R = Sm and Eu. The boundary between the two types of phase transitions is located between NdNiO₃ and SmNiO₃. We have measured photoemission spectra of Nd_1-xSm_xNiO₃ at various temperatures above and below T_MI. The spectra show quite different temperature dependences between x ≤ 0.4 and x ≥ 0.6. For x ≤ 0.4, the spectral intensity near the Fermi level (E_F) is temperature-dependent even well below T_MI, while for x ≥ 0.6 it shows a significant temperature dependence only near T_MI. Also, the spectra near E_F above T_MI show a pseudo-gap like behavior for x ≥ 0.6. From these results, we conclude that the metallic phase has different natures between x ≤ 0.4 and x ≥ 0.6.