Chemical Potential Shift in Overdoped and UnderdopedLa2−xSrxCuO4

The downward shift of the electron chemical potential $\mu$ with hole doping in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ has been deduced from the shifts of photoemission and inverse-photoemission spectra. While the shift is large ( $\sim$1.5 eV/hole) in overdoped samples, it is suppressed ( $<0.2\mathrm{}$ eV/hole) in underdoped samples, implying a divergent charge susceptibility near the metal-insulator transition. In the overdoped regime, the $\mu$ and the electronic specific heat coefficient $\gamma$ are consistently explained within Fermi-liquid theory, whereas the same analysis gives unphysical results in the underdoped regime, indicating the breakdown of the Fermi-liquid picture in the underdoped regime.