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Correlation between axial oxygen anharmonicity and T_c in high temperature superconductors

The oxygen contributions to Cu K-edge polarized extended x-ray absorption fine structure (EXAFS) in La_{;2-x};Ba_xCuO_4, YBa_2Cu_2.8Co_0.2O_{;7+delta};, YBa_2Cu_3O_6.5, YBa_2Cu_3O_7, and TlBa_2Ca_3Cu_4O_11 at T=10K are analyzed, using radial distribution functions obtained from model potentials, to describe the effect of the Cu—O relative motion in the EXAFS signal. The analysis shows that, while the motion of the plane and chain oxygens is harmonic, the axial oxygen ater moves in a double well potential. The Cu—O pair potential becomes more anharmonic in systems with higher T_c, leading to a correlation between the interwell tunneling frequency and T_c. The results of this analysis are compared with infrared absorption and heat capacity measurements in these materials.

axial oxygen anharmonicity, superconductivity

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