Differences in optical conductivity between one- and two-dimensional doped nickelates

We study the optical conductivity in doped nickelates, and find the dramatic difference of the spectrum in the gap (ω≲4 eV) between one- (1D) and two-dimensional (2D) nickelates. The difference is shown to be caused by the dependence of hopping integral on dimensionality. The theoretical results explain consistently the experimental data in 1D and 2D nickelates, Y_2-xCa_xBaNiO₅ and La_2-xSr_xNiO₄, respectively. The relation between the spectrum in the x-ray aborption experiments and the optical conductivity in La_2-xSr_xNiO₄ is discussed.