Charge ordering and chemical potential shift in La${}_{2-x}$Sr${}_x$NiO${}_4$ studied by photoemission spectroscopy

We have studied the chemical potential shift in La${}_{2-x}$Sr${}_x$NiO${}_4$ and the charge ordering transition in La${}_{1.67}$Sr${}_{0.33}$NiO${}_4$ by photoemission spectroscopy. The result shows a large ($\sim$ 1 eV/hole) downward shift of the chemical potential with hole doping in the high-doping regime ($\delta \gtrsim$ 0.33) while the shift is suppressed in the low-doping regime ($\delta \lesssim$ 0.33). This suppression is attributed to a segregation of doped holes on a microscopic scale when the hole concentration is lower than $\delta \simeq 1\mathrm{/}3$. In the $\delta =1\mathrm{/}3$ sample, the photoemission intensity at the chemical potential vanishes below the charge ordering transition temperature $T_{\mathrm{C}\mathrm{O}}=$ 240 K.