Lattice effects observed by the isotope-difference pair density function of the YBa263/65Cu3O6.92 superconductor

The local lattice effects in the YBa2Cu3O6.92 superconductor were characterized by the neutron differential pair density function (DPDF) analysis. The DPDF provides an atom specific PDF, enabling the distinction between overlapping atom pair correlations in complicated systems. It utilizes the contrast in the scattering amplitude enhanced by the different neutron scattering lengths for Cu-63 and Cu-65 in two isotopically pure samples with identical compositions. Significant structural distortions are observed in the superconducting planes, in association with the plane copper (Cu2). A bifurcation of the Cu2 to apical oxygen (O4) pair correlations is evident which is temperature dependent. This effect originates from distortions along the c axis of the crystal of the Cu2 ions and not of the O4, as no distortions are observed in association with the connecting pair, the Cu1 (chain)-O4. Secondary lattice anharmonicities in the chains are also seen, arising mostly from defects in the vicinity of oxygen vacancies. [S0163-1829(99)12833-9].