Pinning in High-Tc Superconductors

Magnetic flux can penetrate a type-II superconductor in the form of flux-lines or Abrikosov vortices,each of which carry a quantum of flux and arrange in a more or less regular triangular lattice. Underthe action of an electric current, these flux lines move and dissipate energy unless they are pinned bymaterial inhomogeneities. In conventional superconductors, depinning occurs when a critical currentdensity Jc is exceeded. In high-Tc superconductors (HTSC), thermally activated depinning causes a finiteresistivity t9 even at current densities J ≪ Jc. At sufficiently large temperature T, linear (ohmic) resistivityis observed down to J → 0. This indicates that the flux lines are in a "liquid state" with no shear stiffnessand with small depinning energy. At lower T, ρ(J) is highly nonlinear, since the pinning energy increaseswith decreasing J. In highly anisotropic Bi- and Tl-based HTSC, thermal depinning occurs at ratherlow T, since short vortex segments ("pancake vortices" in the CuO layers) can depin individually withvery small activation energy