Phonon anomalies in a-axis-oriented (YBa2Cu3O7)m(PrBa2Cu3O7)n superconducting superlattices: Evidence for an anisotropic gap function

The temperature dependence of phonon self-energies in a-axis-oriented superconducting superlattice films of (YBa2Cu3O7)m(PrBa2Cu3O7)n reveals several modes associated with the YBa2Cu3O7 layer exhibiting anomalies in their frequencies and linewidths below Tc. The ability to modify the superconducting properties through choice of m and n while not altering important phonon frequencies or the unit-cell atomic composition of the individual layers enable a critical examination of key aspects of the microscopic theory by Zeyher and Zwicknagl [Z. Phys. B 78, 175 (1990)] that is often used to account for such renormalizations in bulk samples. While our data cannot be satisfactorily accounted for by this theory when including isotropic s-wave pairing, the results are in qualitative agreement with a recent extension that considers an anisotropic gap parameter. Further, despite a relatively large value (20) for the superconducting coherence length in the CuO2 plane in bulk YBa2Cu3O7, vibrations confined primarily to the Pr layer show no anomalies below Tc indicating suppression of the order parameter within PrBa2Cu3O7 in these a-axis-oriented superconducting superlattices.