Transport properties of YBa2Cu3O7-δ/PrBa2Cu3O7-δ superlattices

A series of c-axis oriented high temperature superconductor superlattices YBa2Cu3O7-delta/PrBa2Cu3O7-delta were synthesized using de magnetron sputtering. We varied the thickness of intervening PrBa2Cu3O7-delta a layers as two, four, and eight unit cells with each individual YBa2Cu3O7-delta layer fixed to eight-unit-cell thick. X-ray diffraction (XRD) analysis indicates that these specimens were grown epitaxially having a rather satisfactory superlattice modulation along the c-axis. Furthermore, Atomic Force Microscope (AFM) images show that surfaces are very smooth. Shaped into a hall bar geometry, we have investigated its transport properties, including resistivity, current-voltage characteristics, and noise. In the superconducting transition region, the electrical conduction of these specimens was found to be influenced by dissociated free vortices. This was evident from the resistive transition and current-voltage characteristics, which are well explained by Bardeen-Stephen (BS) model and Kosterlitz-Thouless (KT) transition theory, respectively. KT transition temperatures determined from the resistance fittings and current-voltage characteristics were in good agreement. Somewhat complex features were observed in the noise measurement with the variation of the transport current and the external magnetic field. There appeared a single or multiple peaks near the resistive transition tail. While noise peaks were suppressed monotonically as the magnetic field increased, noise peaks were enhanced at first and then suppressed as the bias current increased. We explain these behaviors using the flux flow model.