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Evidence for anharmonicity in axial Cu-O bonds in YBa_2Cu_3O_7 using XAFS

We present polarized XAFS measurements carried out in oriented samples of YBa_2Cu_3Q0_7 as a function of temperature in the region between 10 and 105K. The contribution from the Cu(1)-O(4) bond, isolated using usual Fourier transform techniques, indicates the presence of 2 different, Cu-O distances for temperatures T<~40K and T>~90K. Non linear least square fittings to single shell XAFS calculations using a backscattering amplitude and phase obtained from La_2CuO_4 and a radial distribution function derived from a \psi^4 potential V(u) = u^2 + u^4 but were performed. These fits support the evidence for 2 Cu-O distances that change with temperature, while the average Cu-O distance remains constant, and show that the Cu-O bond exhibits strong anharmonicity. Based on the implied couplings of this ionic motion to superconductivity, a scenario for high temperature superconductivity in which both phonon and electronic channels are synergistically involved is discussed.

anharmonicity, XAFS

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