EVIDENCE OF KOSTERLITZ-THOULESS TRANSITION IN YBA2CU3O7/PRBA2CU3O7 SUPERLATTICE FILMS

Experimental measurements of four-terminal (van der Pauw) transport-current resistivity and of dynamic AC magnetic response of certain superlattice films show broad resistive transitions and frequency-dependent magnetic transitions into the super-conductive state. Such observations indicate that the superconductive critical temperature is not described by mean-field theory. Epitaxial, c perpendicular-to superlattice films of YBa2Cu3O7/PrBa2Cu3O7 were grown in situ by laser ablation. Layer thicknesses of each compound ranged from two to four atomic cells. While material inhomogeneity can produce such a broad resistive transition, it would prevent our observation of a relatively sharp step-like transition in the inductive impedance. Such response must arise due to vibrations of pinned and bound supercurrent vortices. Observation of the Kosterlitz-Thouless (K-T) transition of bound vortices is expected to occur in a two-dimensional system. We see apparent K-T transitions only in films of thin (two or three atomic-lattice cells) Y-compound layers and thick Pr-compound layers. This systematically implies that extreme anisotropy of Cu-O superconductors can be tied to observations of two-dimensional physical phenomena.