Critical Current

(in superconductors), the maximum value of the sustained direct current in a superconducting specimen at which the substance of the specimen passed into the normal, nonsuperconducting state. Since the substance in its normal state has a finite electric resistance, dissipation of the energy in the current takes place after the transition, thus heating the specimen.

In large superconductors of the first kind whose dimensions are considerably greater than the penetration depth of a magnetic field, the critical current I_c corresponds to the current that creates the critical magnetic field H_c on the surface of the superconductor. In this case the superconductor passes into an intermediate state, where part of it is in the normal state and part is in the superconducting state. While the current is present, the interfaces between the superconducting and the normal regions are in motion. Because of the Meissner effect, the magnetic field varies, an induction electric field develops, and energy dissipation occurs in the conductor.

In the case of the second kind of superconductors two values of critical current (I_c,₁ and I_c,2) are distinguished. In an ideal superconductor (which contains no lattice defects) at I_c,₁ the magnetic induction becomes nonzero and the magnetic field penetrates the superconductor. The penetrating field is in the form of filaments with a quantized magnetic flux, which are surrounded by circulating superconducting currents (vortex filaments). The energy dissipation in this case is associated with the time variation of the magnetic field caused by movement of the vortex filaments and with the corresponding induction electric field. In actual superconductors of the second kind (with lattice defects), an ohmic resistance develops when I_c,₂ > I_c, ₁, because the defects interfere with the movement of the vortex filaments.