Type I and Type II Superconductor

Type I superconductors are materials that can conduct electricity without any resistance when they are cooled below a certain temperature, known as critical temperature. When these materials are cooled below their critical temperature, they expel all magnetic fields from their interior, a phenomenon known as Meissner effect. They behave like diamagnets. Superconductivity stops once the applied magnetic field exceeds its critical magnetic field, and the material returns to a normal, resistive state.

Type I superconductors are typically elemental metals like lead and mercury.

The critical field for a Type I superconductor is temperature-dependent and can be described by the equation:

Where:

– H_C(T) is the critical field at temperature T.

– H_C(0) is the critical field at absolute zero temperature.

– T is the temperature of the superconductor.

– T_C is the critical temperature at which the material becomes superconducting.

Type II superconductors are a class of materials that also conduct electricity without resistance below their critical temperature. However, unlike Type I superconductors, they do not completely expel magnetic fields from their interior. Instead, they allow magnetic fields to penetrate through them in specific regions called vortices.

Vortices are tiny “tubes” of normal material surrounded by superconducting regions. Imagine a sponge soaked in water. The sponge (superconductor) has holes (vortices) where the water (magnetic field) can penetrate inside. It implies that the overall structure of the sponge remains intact, with the existence of specific regions where the normal (non-superconducting) state exists.

Type II superconductors have two critical magnetic field values: a lower critical field (H_c1) and an upper critical field (H_c2). Below H_c1, they behave like Type I superconductors and expel all magnetic fields (Meissner effect). Above H_c2, they lose their superconducting state entirely and become normal conductors. Between H_c1 and H_c2, they enter a region known as “mixed state”, where the vortices form but still retain their superconducting properties.

Type II superconductors include high-temperature superconductors like YBCO (yttrium barium copper oxide) and many niobium-based alloys.

Flux Pinning

Flux pinning is a phenomenon observed in Type II superconductors where magnetic flux lines are trapped within the material. This occurs because the superconductor contains structural defects, impurities, or grain boundaries that create regions where the vortices can become anchored. When an external magnetic field is applied, these vortices become trapped in these weaker areas, preventing their movement. This stabilization of the vortices helps the superconductor maintain its zero electrical resistance, even in the presence of an external magnetic field.

The key differences between Type I and Type II superconductors lie in their magnetic properties and critical field strengths. Below is a table summarizing these differences.