The magnetic behavior of a type II superconductor

The negative interfacial tension between the normal and superconducting phases is responsible for the unique magnetic behavior of type II superconductors. When ß < 0 the material is driven to create as much interface as possible, and can do so by spontaneously subdividing into a two-phase mixture. The two-phase mixture persists over a range of conditions (in our case, a range of H to either side of Hc) since the subvolumes of the high-energy phase can be made so small that the interfacial tension dominates the free energy. There are, however, limits on how small the regions of normal metal can be. These set upper and lower bounds on the magnetic field that produces the mixed state.