Hole states of hot-pressed samples of La_2-xSr_xCuO_4-δ X-ray absorption spectroscopy studies

A series of samples of La_2-xSr_xCuO₄ with x = 0, 0.10, 0.15, 0.20, and 0.30, had been prepared in the ambient pressure (AP). The AP samples were annealed under high pressure (2.0 GPa), with and without adding oxidant, to obtain hot-pressed (HP) samples. The lattice structures of the HP samples were observed to be similar to the AP samples, while T_c was found to decrease but the one with x = 0.30 became superconducting. The O-1s and the Cu-2p_3/2 X-ray near-edge-absorption spectra (XNEAS) are measured for all as prepared samples, by using synchrotron radiation. Both spectra indicate that the number of holes is depleted after the hot-press treatment. The depletion of holes is found mainly due to the depletion of oxygen. The correlation of T_c with the number of holes, as derived from the O-1s XNEAS, is found not universal. It can be grouped according to the Sr content, x, a new phenomenon that is not seen before.