Commensurability effect on the charge ordering ofLa2−xSrxNiO4

Measurements of Hall coefficients, thermoelectric power, and optical conductivity spectra were done for single crystals of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_4$ $(x=0.27\mathrm{}-0.50)$ 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 $p=1/3\mathrm{}$ 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 $p=1/3.\mathrm{}$