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What are the lower and upper critical magnetic fields of a type II superconductor?

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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.

In type II superconductors, the magnetic behavior differs from that of type I materials, as they do not transition abruptly to a normal state at a single critical field. Instead, they exhibit two distinct critical magnetic fields:1

Lower critical field ( $H_{c 1}$ ): This is the field strength below which the superconductor maintains a nearly complete Meissner effect, effectively excluding magnetic flux from its interior.1
Upper critical field ( $H_{c 2}$ ): This is the field strength above which the material abruptly ceases to be a superconductor and returns to a normal state.21

When a type II superconductor is exposed to an applied magnetic field between $H_{c 1}$ and $H_{c 2}$ , the magnetic field penetrates the material in the form of quantized fluxoids. In this intermediate state, the material remains superconducting, but the expulsion of the magnetic field is only partial.32