5. Irreversibility field

The phase diagram of a superconductor is sometimes more complex than that shown in figure 7 (Meissner phase and mixed state). In fact, vortices can behave in two main ways, separated by the irreversibility lineH *(T ) ( H * : irreversibility field) :

• the vortices form a network, even a glass, which can anchor on defects only below the irreversibility line;

• above, the vortexes behave like a liquid, with no anchoring possible and therefore free to move reversibly ( J _c = 0).

Beyond the irreversibility field, the transport current is dissipative....