Direct Shear Probe of Vortex Lattice Melting in B${\mathrm{i}}_2$S${\mathrm{r}}_2$CaC${\mathrm{u}}_2$${\mathrm{O}}_8$ Single Crystals

We report results of transport measurements on single crystals of the superconductor B${\mathrm{i}}_2$S${\mathrm{r}}_2$CaC${\mathrm{u}}_2$ ${\mathrm{O}}_8$ where weak pinning channels in a strong pinning environment were artificially introduced. The experimental data show that in this material the shear viscosity of the vortex liquid phase suddenly increases upon decreasing the temperature supporting the melting scenario. We also found that below the phase transition the data are in good agreement with plastic flow of a vortex solid through the channels.