Commensurability effect on the charge ordering of La_2-xSr_xNiO₄

Measurements of Hall coefficients, thermoelectric power, and optical conductivity spectra were done for single crystals of La_2-xSr_xNiO₄ (x=0.27-0.50) showing prototypical charge ordering. We found that the character of the carriers in the charge-ordered state changes from electronlike to holelike when the hole concentration p crosses 1/3, and that the correlation gap in the optical spectrum for p=1/3 is collapsed when p is away from 1/3. These results indicate that the deviation of p from 1/3 can be regarded as electron or hole doping into the commensurate charge-ordered state at p=1/3.