Superconductivity
Superconductivity is a phenomenon much different from conductivity. In fact, the best of the normal conductors, such as silver and copper, cannot become superconducting at any temperature, and the new ceramic superconductors are actually good insulators when they are not at low enough temperatures to be in a superconducting state.
The relationship between superconductivity and low temperatures is fundamental. For a material to become a superconductor, it must be cooled below a critical temperature called the superconducting transition temperature or critical temperature (Tc). This transition temperature is different for different superconducting materials and can range from a few Kelvin (K) to several hundred K.
At low temperatures, the atomic vibrations in a material decrease, which means that there are fewer impediments to the flow of electrons. This leads to the formation of Cooper pairs, which are pairs of electrons that are bound together by lattice vibrations. In a normal metal, electrons experience resistance when they collide with the atoms in the lattice structure of the material, but in a superconductor, the Cooper pairs move through the lattice structure without any resistance.
Types of Superconductors
Superconductors can be classified into two main types:
- Type I superconductors: These superconductors have a single critical magnetic field, below which they exhibit perfect conductivity, and above which they lose their superconducting properties abruptly. They are also called “soft” superconductors. Examples of type I superconductors include mercury, lead, and tin.
- Type II superconductors: These superconductors have two critical magnetic fields, and in between them, they exhibit a mixed state where only some parts of the material are superconducting. They are also called “hard” superconductors. Examples of type II superconductors include niobium-titanium, niobium-tin, and YBCO (yttrium barium copper oxide).
Type II superconductors are more widely used in practical applications because they can operate at higher magnetic fields and temperatures than type I superconductors. They can also maintain their superconducting properties in the presence of strong magnetic fields, which is essential for applications such as MRI machines and particle accelerators.
In addition to these two main types, there are also unconventional superconductors which do not fit into the conventional BCS (Bardeen-Cooper-Schrieffer) theory of superconductivity. These include high-temperature superconductors and heavy fermion superconductors.
Critical Temperature
The critical temperature (Tc) is a key parameter for superconductors, as it is the temperature below which the material exhibits zero electrical resistance and perfect diamagnetism.
For conventional superconductors, such as Nb3Sn and NbTi, the critical temperature is relatively low, ranging from about 9 K (-264 °C) to 18 K (-255 °C) depending on the material and the conditions of the sample.
In contrast, high-temperature superconductors, such as cuprates and iron-based superconductors, have much higher critical temperatures, ranging from about 30 K (-243 °C) to 138 K (-135 °C) for the record-high Tc material HgBa2Ca2Cu3O8+δ.
Superconductors – Materials
Here’s a table of 10 superconductors with their key characteristics.
| Superconductor | Chemical Formula | Type | Critical Temperature (K) | Critical Magnetic Field (T) |
| Tin (Sn) | Sn | Type I | 3.72 | 0.005 |
| Lead (Pb) | Pb | Type I | 7.19 | 0.015 |
| Mercury (Hg) | Hg | Type I | 4.15 | 0.091 |
| Niobium-titanium (NbTi) | NbTi | Type II | 10.4 | 12.5 |
| Niobium-tin (Nb3Sn) | Nb3Sn | Type II | 18.1 | 25 |
| Yttrium Barium Copper Oxide (YBCO) | YBa2Cu3O7-x | Type II | 92 | 0.2 |
| Bismuth Strontium Calcium Copper Oxide (BSCCO) | Bi2Sr2Ca2Cu3O10+x | Type II | 107 | 0.2 |
| Lanthanum Barium Copper Oxide (LBCO) | La1.85Ba0.15CuO4 | Type II | 40 | 0.2 |
| Magnesium Diboride (MgB2) | MgB2 | Type II | 39 | 0.2 |
| Iron-based superconductor (FeSe) | FeSe | Type II | 8 | 0.17 |
