Low Temperature Anharmonicity and Superconductivity in Cuprates

Temperature-dependent X-ray absorption spectra of the hole-doped La_2?x Sr _x CuO₄ and electron-doped Nd_2?x Ce _x CuO_4?δ high-temperature superconductors were investigated above the Cu K absorption edge. We observed strong anharmonicity in the superconductive CuO₂ plane. For x=0.15 it was shown that part of oxygen ions oscillate in a double-well potential and their vibrations are correlated with the local electron (hole) pair transfer. We suppose that at a low temperature the phase coherence of the local pair movement is determined by the peculiarities of the perovskite-like structure which includes the stiff CuO _n (n=4,6) complexes combined by collective rotational and breathing modes.