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What does critical current density measure in a superconducting material?

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Figure 1: Specific heat in the normal (Cen) and superconducting (Ces) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (Tc) and above Tc. — superconductivity Figure 1: Specific heat in the normal (C_en) and superconducting (C_es) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (T_c) and above T_c.

Critical current density, denoted as $J_{c}$ , measures the maximum electric current per unit area that a superconductor can carry without losing its superconducting state and reverting to a normal state.1

In type II superconductors, the application of a magnetic field causes magnetic flux to enter the material in the form of quantized fluxoids. When a sufficiently large current passes through the material, these fluxoids are forced into motion. This movement generates energy dissipation, which produces heat and drives the material into the normal state. Consequently, manufacturers of superconducting magnets often attempt to pin these fluxoids in place by creating inhomogeneous material compositions to maintain high critical current densities. In high-temperature superconductors, which are highly anisotropic, the critical current density is also heavily dependent on the orientation of the current relative to the crystal structure.12