Shear viscosity measurements at the vortex melting transition in confined geometry in optimally doped Bi₂Sr₂CaCu₂O ₈

In order to probe the vortex shear viscosity in the vortex liquid phase, we have introduced two types of vortex-confining structures in optimally doped Bi₂Sr₂CaCu₂O₈ single crystals. First, walls of strong vortex pinning separated by weakly pinning channels are fashioned by heavy ion irradiation through 25 μm-thick Ni masks. Second, a low density of homogeneously distributed amorphous columnar defects is known to impose a polycrystalline structure to the vortex lattice. Resistivity measurements show that the inclusion of confining structures impede vortex flow in the liquid. The resistivity is remarkably well described by the Halperin-Nelson theory for the viscosity due to free two-dimensional vortex lattice dislocations.