Two doped holes with different distributions in Sr2CuO4-δ-La2CuO4 superlattices

X-ray absorption in Sr₂CuO_4-δ-La₂CuO₄ (SCO-LCO) superlattices shows a variable occupation with doping of two hole states. In addition to the holes doped for x<x_optimal in bulk La_2-xSr_xCuO₄, a variation with doping of a second hole state is observed at higher energy. Measurements suggest that this hole is located on apical oxygen atoms and polarized in the a-b plane. Considering the surface reflectivity gives a good qualitative description of the line shapes of resonant soft X-ray scattering. The interference between the superlattice and surface reflections was used to distinguish between scatterers in the SCO and the LCO layers, with the two hole states maximized in different layers of the superlattice. This suggests that the states emptied preferentially in the SCO layers are the additional states in the extension of the Zhang-Rice singlet-upper Hubbard band model to higher doping levels.