Meissner Effect
The Meissner effect is the phenomenon by which a superconductor expels all magnetic fields from its interior when it transitions below its critical temperature and becomes superconducting. This magnetic field expulsion occurs even if the material is placed in a magnetic field before cooling, distinguishing superconductors from perfect conductors. As a result, a magnet placed near a superconductor can experience levitation due to this repulsive effect.
Classification of Superconductors: Type I & Type II
Type I Superconductors
- Exhibit complete Meissner effect (full expulsion of magnetic fields).
- Have a single critical magnetic field, Hc. Above this field, superconductivity vanishes abruptly.
- Typically pure metals.
- Transition temperatures are usually below 10 K.
- No mixed state; transition is sharp.
- Examples: Lead (Pb), Mercury (Hg), Zinc (Zn).
Type II Superconductors
- Exhibit partial Meissner effect in intermediate fields, allowing magnetic field penetration in quantized vortices (mixed state).
- Have two critical fields: Hc1 and Hc2.
- Usually alloys, compounds, and ceramics; can have higher transition temperatures.
- Remain superconducting under relatively strong magnetic fields, making them useful for technology.
- Examples: Niobium–Titanium (NbTi), Niobium–Tin (Nb3Sn), Yttrium Barium Copper Oxide (YBCO), vanadium (V).
Takeaways
- Type I superconductors show a single critical field and complete magnetic field expulsion.
- Type II superconductors feature two critical fields and allow magnetic flux lines to penetrate in a mixed state, enabling applications in high-field technologies.