Chemical Potential Shift in Overdoped and UnderdopedLa2−xSrxCuO4
The downward shift of the electron chemical potential $\ensuremath{\mu}$ with hole doping in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ has been deduced from the shifts of photoemission and inverse-photoemission spectra. While the shift is large ( $\ensuremath{\sim}$1.5 eV/hole) in overdoped samples, it is suppressed ( $<0.2$ eV/hole) in underdoped samples, implying a divergent charge susceptibility near the metal-insulator transition. In the overdoped regime, the $\ensuremath{\mu}$ and the electronic specific heat coefficient $\ensuremath{\gamma}$ 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.