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How does the Meissner effect expel magnetic fields from superconducting materials?

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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.

The Meissner effect is the phenomenon where a material, upon transitioning into a superconductor by cooling below its transition temperature, expels magnetic fields from its interior.1

When a material becomes superconducting, it loses all electrical resistance. In this state, the material generates surface currents that create an opposing magnetic field, effectively canceling out the external field within the bulk of the material. While weak magnetic fields are entirely repelled—except for a thin surface layer—the effect depends on the material type and field strength.1

In type I superconductors, the expulsion is typically complete under appropriate conditions. In type II superconductors, magnetic fields can penetrate the material in the form of quantized fluxoids when exposed to intermediate field strengths. If the external magnetic field becomes sufficiently strong, it destroys the superconducting state, and the Meissner effect ceases to occur.123