¹⁷O NMR study of La_2-xSr_xCuO₄ in the lightly-and heavily-doped regions

A ¹⁷O NMR study has been carried out for La_2-xSr_xCuO₄ in the lightly- (x=0.075) and heavily-doped (x=0.24) regions in the normal state. Together with the data previously reported for x=0.15, it was found that the nuclear quadrupole frequency ν_Q in the apical oxygen (O(1)) does not change with x, suggesting that the doped hole does not go into the LaO layer at all. On the contrary, ν_Q in the planar oxygen (O(2)) increases appreciably with Sr doping. By analyzing ν_Q of Cu and O(2), it was shown that the increase of holes on the planar O 2p_σ orbit is more appreciable than that in the Cu 3d orbit. From the ratio of the hole density in Cu to that in oxygen, and the spin fluctuation probed by ⁶³(T₁T), it is suggested that the depression of T_c in the "heavily-doped" region is neither due to the over-doping nor to the collapse of the two-dimensionality, but to other causes. The temperature dependences of the Knight shift ¹⁷K in two samples are in sharp contrast with each other. In the lightly-doped region, ¹⁷K decreases with decreasing temperature. In the heavily-doped region, however, ¹⁷K increases slightly with decreasing temperature, becoming almost T-independent below 100 K. An increase of the anisotropic part of spin Knight shift ¹⁷K_s,ani with x also indicates that the hole density at the O 2p_σ orbit increases with Sr doping.