Doping-dependent evolution of the electronic structure of La_2-xSr_xCuO₄ in the superconducting and metallic phases

The electronic structure of the La_2-xSr_xCuO₄ (LSCO) system has been studied by angle-resolved photo-emission spectroscopy (ARPES). We report on the evolution of the Fermi surface, the superconducting gap, and the band dispersion around the extended saddle point k = (π,0) with hole doping in the superconducting and metallic phases. As hole concentration x decreases, the flat band at (π,0) moves from above the Fermi level (E_F) for x>0.2 to below E_F for x<0.2, and is further lowered down to x = 0.05. From the leading-edge shift of ARPES spectra, the magnitude of the superconducting gap around (π,0) is found to monotonically increase as x decreases from x = 0.30 down to x = 0.05 even though T_c decreases in the underdoped region, and the superconducting gap appears to smoothly evolve into the normal-state gap at x = 0.05. It is shown that the energy scales characterizing these low-energy structures have similar doping dependences. For the heavily overdoped sample (x = 0.30), the band dispersion and the ARPES spectral line shape are analyzed using a simple phenomenological self-energy form, and the electronic effective mass enhancement factor m*/m_b≃2 has been found. As the hole concentration decreases, an incoherent component that cannot be described within the simple self-energy analysis grows intense in the high-energy tail of the ARPES peak. Some unusual features of the electronic structure observed for the underdoped region (x≤0.10) are consistent with numerical works on the stripe model.