Vortex matter in clean single crystals of the intermediate-anisotropy superconductor Hg1201*

The rich vortex physics in oxide high T_c superconductors (HTS) arises from the strong influence of thermal fluctuations, which produce vortex liquid phases and fast dynamics (large creep rates, S) in the solid phases. The fluctuations effects increase with increasing anisotropy (γ). In moderately anisotropic 3D materials, the strength of the thermal fluctuations is measured by a combination of parameters, the Ginzburg number (Gi). In very anisotropic systems, 2D pancake vortices are formed and fluctuation effects are determined by a different number, Gi^2D. Vortex physics in the paradigmatic 3D and 2D HTS— YBCO (γ~5-7) and Bi-based compounds (γ>100), respectively— has been extensively explored. Here we investigate vortex matter in clean Hg1201 single crystals, an ideal intermediate-γ HTS to learn about the interplay of 3D and 2D effects. From the equilibrium magnetization in the liquid phase we calculate Gi and Gi^2D. In the solid phase the magnetization loops show a fishtail shape. Pinning at low fields (H) is very low, allowing us to study the single vortex regime in the dilute disorder limit. S is large at low H and decreases as vortex interactions strengthen with increasing H.