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Dissipation and anisotropy in ultrathin YBa2Cu3O7/PrBa2Cu3O7 superlattices. J.E. Villegas, E.M. Gonzalez and J.L Vicent. Departamento de Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, SPAIN. Z. Sefrioui, M. Varela1 and J. Santamaría. GFMC, Departamento de Física Aplicada III, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, SPAIN 1 Departamento de Física, Universidad Carlos III de Madrid, 28911 Leganés, SPAIN ABSTRACT Ultrathin superlattices of 123 superconducting cuprates have been fabricated by high oxygen pressure sputtering technique. The superlattices show high quality, with negligible step disorder, roughness and interdiffusion. The insulating PrBa2Cu3O7 (PBCO) layer thickness has been kept constant at 5 unit cells (u.c.) and the superconducting YBa2Cu3O7 (YBCO) layer thickness has been changed from 8 u.c. to 1 u.c. In these uncoupled multilayers the critical temperature (Tc0) changes from 44 (1 u.c. YBCO) to 80 K (8 u.c. YBCO). In the vortex liquid regime, at constant temperature, the angular dependence of the magnetoresistance scales down with the applied magnetic field values in all the temperature range. This behavior allows us to study the anisotropy and dimensionality behavior of the mixed state in ultrathin superlattices. A dimensional crossover 3D-2D is found in the anisotropy with the 1 u.c. YBCO superlattices following a pure 2D behavior. INTRODUCTION The vortex phase diagram in the mixed state of high temperature superconductors (HTS’s) is strongly affected by the presence and interplay between anisotropy and different kinds of disorder [1]. Much work has been devoted to study the effect of anisotropy (ratio between the upper critical field parallel to the Cu-O planes and the upper critical field perpendicular to the Cu-O planes) γ = H ca2, b H cc2 on the dimensionality of vortex matter . Although YBCO is a moderately anisotropic superconductor, γ=6-10 [2, 3], nonsuperconductor/superconductor superlattices can be used to artificially tailor the anisotropy by varying the thickness of the non superconducting spacer. In recent years, the YBCO/PBCO system has been extensively investigated [4], and a 3D to 2D crossover has been reported in superlattices with 1 unit cell thick YBCO layers, when the PBCO thickness is increased between 1 and 3 unit cells [5, 6]. However, recent reports have shown that significant coupling between YBCO layers persists for PBCO layer thickness smaller than 5 unit cells [7,8,9]. Remaining coupling between YBCO layers in samples with thin PBCO spacers is likely to obscure effects related to the reduced dimensionality in YBCO. In this paper we examine the dimensionality crossover in c-axis oriented YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PBCO) superlattices with fixed PBCO spacer thickness of 5 unit cells, and varying the YBCO thickness between 1 and 8 unit cells. We explore the dimensionality of the vortex system in the liquid state, above the irreversibility line, from the angular dependence of magnetorresi