Relevance of electron–lattice coupling in cuprate superconductors

We study the oxygen isotope (<sup>16</sup>O, <sup>18</sup>O) and finite size effects in Y<sub>1-x</sub>Pr<sub>x</sub>Ba<sub>2</sub>Cu<sub>3</sub>O <sub>7-δ</sub> by in-plane penetration depth (λ<sub>ab</sub>) measurements. A significant change of the length <em>L<sub>c</sub></em> of the superconducting domains along the c-axis and λ<sub>ab</sub><sup>2</sup> is deduced, yielding the relative isotope shift δ<em>L<sub>C</sub></em>/<em>L<sub>C</sub></em> ≈ δλ<sub>ab</sub><sup>2</sup>/λ<sub>ab</sub><sup>2</sup> ≈ 0.14 for x = 0, 0.2 and 0.3. This uncovers the existence and relevance of the coupling between the superfluid, lattice distortions and anharmonic phonons which involve the oxygen lattice degrees of freedom.

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