Critical field

Superconductivity is characterized both by perfect conductivity (zero resistance) and by the expulsion of magnetic fields (the Meissner effect). Changes in either temperature or magnetic field can cause the phase transition between normal and superconducting states. For a given temperature, the highest magnetic field under which a material remains superconducting is known as the critical field. The highest temperature under which the superconducting state is seen is known as the critical temperature. At that temperature even the smallest external magnetic field will destroy the superconducting state, so the critical field is zero. As temperature decreases, the critical field increases generally to a maximum at absolute zero.