Superconductivity is characterized both by perfect conductivity (zero resistance) and by the complete expulsion of magnetic fields (the Meissner effect).
Changes in either temperature or magnetic flux density can cause the phase transition between normal and superconducting states.
For a type-I superconductor the discontinuity in heat capacity seen at the superconducting transition is generally related to the slope of the critical field (
On a microscopic scale, the magnetic field is not quite zero at the edges of any given sample – a penetration depth applies.
This limit on current density has important practical implications in applications of superconducting materials – despite zero resistance they cannot carry unlimited quantities of electric power.
Along these flux cylinders, the material is essentially in a normal, non-superconducting state, surrounded by a superconductor where the magnetic field goes back to zero.
The upper critical field is the magnetic flux density (usually expressed with the unit tesla (T)) that completely suppresses superconductivity in a type-II superconductor at 0 K (absolute zero).