Higher H c and J c values depend upon two important parameters which influence energy minimization, penetration depth and coherence length. Penetration depth is the characteristic length of the fall off of a magnetic field due to surface currents. Coherence length is a measure of the shortest distance over which superconductivity may be established. The ratio of penetration depth to coherence length is known as the Ginzburg-Landau parameter. If this value is greater than 0.7, complete flux exclusion is no longer favorable and flux is allowed to penetrate the superconductor through cores known as vortices.
Currents swirling around the normal cores generate magnetic fields parallel to the applied field.
These tiny magnetic moments repel each other and move to arrange themselves in an orderly array known as a fluxon lattice .This mixed phase helps to preserve superconductivity between H c1 to H c2 . It is very important that these vortices do not move in response to magnetic fields if superconductors are to carry large currents. Vortex movement results in resistivity. Vortex movement can be effectively pinned at sites of atomic defects, such as inclusions, impurities, and grain boundaries. Pinning sites can be intentionally introduced into superconducting material by the addition of impurities or through radiation damage.