Fluctuations Indicate Strong Interlayer Coupling in Cuprate Superconductors

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From study of the Kosterlitz-Thouless-Berezinskii (KTB) transition in the superfluid density, nₛ(T), of ultrathin c-axis oriented YBa₂Cu₃O₇₋δ (YBCO) films, we find that interlayer coupling is unexpectedly strong. The KTB transition occurs at a high temperature, as if the films were isotropic rather than quasi-two-dimensional. This result agrees with a comparison of the superfluid density of YBCO with Bi₂Sr₂CaCu₂O₈ and with numerical simulations of Josephson junction arrays, and challenges the thermal phase fluctuation interpretation of critical behavior near Tc in YBCO.