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).
Key Takeaways: Type I vs. Type II Superconductors
Figure 2: Infographic breaking down the magnetic profile, transition sharpness, and material phases of Type I vs. Type II superconductors.
- Type I Superconductors: Typically pure metals exhibiting a low Tc (< 10 K) and a sharp transition. They display a full Meissner effect, completely expelling magnetic fields up to a single critical threshold (Hc).
- Type II Superconductors: Typically complex alloys or compounds featuring a higher Tc. They characteristically present two critical fields (Hc1 and Hc2, entering a vortex-laden mixed state that allows partial magnetic penetration.