Electronic transport properties of hafnium/zirconium multilayers

We have investigated the electrical resistivity, superconducting transition temperature, and upper critical field as a function of layer thickness in hafnium/zirconium ( ) metallic superlattices. These films have equal Hf and Zr layer thicknesses (d_h and d_z, respectively). We have studied a series of samples with modulation wavelength λ = d_h + d_z ranging from 20 to 250 Å. All films show a metallic type of resistivity, with remarkably little difference in both room temperature and liquid helium resistivities. All structures undergo a transition to a superconducting state, with the transition temperature remaining nearly constant across the series. This indicates that the interface region of these structures is of rather high quality. Upper critical fields both parallel and perpendicular to the sample plane were determined. Because of the relatively large superconducting coherence length, these films behave essentially three-dimenstionally throughout the range of λ studied. However, we also observe a somewhat anomalous behavior in the ratio of the parallel to perpendicular critical fields near the transition temperature, the origin of which is not yet known.