Tunneling studies of a metallic superlattice

Tunneling studies of artificially prepared Nb-Cu superlattices have been performed. Individual layers were in the range 8-5000 Å with total sample thicknesses ∼ 1 μm. The coupling strength $\frac{2\Delta }{k{T}_c}$ shows a continuous decrease from values of ∼ 3.8 (thick layers) appropriate for a strong-coupled superconductor toward the weak-coupled value of ∼ 3.5 (thin layers). Contrary to earlier theoretical predictions and reports of indirect experimental evidence, we find no evidence for electronic states within the superconducting energy gap. The energies of the Nb longitudinal and transverse acoustic (LA and TA) phonons are found to be unaltered from their bulk values down to layer thicknesses of 32 Å. At ∼ 10 Å the TA peak remains unaltered in energy but significantly broadens while the LA peak becomes too broad to measure. These results, in conjunction with earlier Brillouin scattering measurements, imply large changes in the phonon dispersion as a function of layer thickness.