Oxygen K-edge fine structure of La_2-xM_xCuO_4-y (M=Sr, Ba and Ca) studied by electron energy loss spectroscopy

Fine structures of the oxygen K-edge in La_2-xSr_xCuO_4-y (x=0, 0.15, 0.3 and 0.4), La_1.85Ca_0.15CuO_4-y and La_1.85Ba_0.15CuO_4-y were investigated by electron energy loss spectroscopy. The height of a peak at around 528 eV well corresponds to the hole content of the samples. In La_2-xSr_xCuO_4-y, the peak height increases monotonically with the increase of Sr content (0≤ × ≤0.4), although T_c has a maximum at x = 0.15. The peak in La_1.85Ca_0.15CuO_4-y is smaller than those of La_1.85Sr_0.15CuO_4-y and La_1.85Ba_0.15CuO_4-y. It is considered that the smaller peak corresponds to the smaller hole content and attributes to the lower T_c in La_1.85Ca_0.15CuO_4-y than those of the other two superconductors.