Vortex Shear Modulus, Vortex Melting, and Peak Effect in Twin Boundary Channels of YBa_2Cu_3O_7-δ

Melting of the vortex lattice is probed directly by investigating the flow of vortices in channels created by twin boundaries. The angular dependencies of the resistivity at fixed temperatures ρ(θ, T) show a clear transition from the vortex liquid state (maximum at ρ(θ^circ, T>T_m)) to a vortex solid state (minimum at ρ(θ^circ, T<T_m)) via the development of a shear modulus at the freezing temperature T_m, which impedes the vortex flow within the channels formed between two twin planes. A peak effect in the critical current T_m is accompanied by a sharp increase in the twin boundary pinning accommodation angle. We calculate the effective twin boundary pinning energy by relating the accommodation angle to a thermal depinning angle.