Chemical Potential Shift in Overdoped and Underdoped La_2-xSr_xCuO₄

The downward shift of the electron chemical potential μ with hole doping in La_2-xSr_xCuO₄ has been deduced from the shifts of photoemission and inverse-photoemission spectra. While the shift is large ( ∼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 μ and the electronic specific heat coefficient γ 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.