• Spins
• Susceptibility

Superconductor

A so-called superconductor has a special property: its ohmic resistance is zero. That is, superconducting materials can conduct an electric current without resistance. However, superconductors have another extremely interesting property: they are diamagnetic. A diamagnet can displace and weaken a magnetic field. The field lines run around a diamagneten. Ferromagnets and paramagnets do just the opposite: they amplify the external magnetic field because the field lines pass through them.

Superconductors in Research

Superconducting materials have long been an important and interesting area of ​​modern physics. This will not change in the foreseeable future: Too big is the desire for an easily manufactured superconductor. Therein lies the sticking point: Current superconductors are difficult to process and only superconducting when they are very cold. As a rule, superconductors must therefore be cooled, for example with liquid helium. After discovering quite a few materials that are superconducting at such low temperatures (<-100 ° C), today high-temperature superconductors are being studied. The breakthrough would be a material that is superconducting at room temperature (or higher).

Fascinating Effects

Because superconductors have perfect diamagnetic properties, they can be used for extremely interesting experiments. For example, a superconductor floats stably in a magnetic field, although it is not really a magnet at all. If, on the other hand, you wanted to levitate a magnet in a magnetic field, it would take a lot of effort to keep it stable.

Why a superconductor floats in the magnetic field

Diamagnetism is the answer to the question of how a superconductor can stably float in a magnetic field. By the way, water is also diamagnetic, but it does not float (or does it?). So what is the special feature? Diamagnets have no elemental magnets or electron spins that align themselves with a magnetic field. However, when a diamagnet is introduced into an external magnetic field, an induction effect occurs. In the diamagneten so a current is induced. It is this current that applies a magnetic field which is opposite to the external one (keyword: Lenz's rule). This creates a force that is slightly repellent. Now we can also answer the question of whether water floats: Experiments have shown that, for example, a frog (aquatic creature) can be held in suspension with an extremely strong magnet! In superconductors, the repulsive force between the two magnetic fields is very strong, which is why they are often referred to as perfect diamagnetic. Already weak magnets can therefore already let superconductors float.

Physical Background

The strength of a magnetization can be described with the magnetic permeability μ. For that, we first of all clarify the basic physical conditions: Thus, there is a magnetization M in the external magnetic fieldH₀. If this magnetic field multiplied by the magnetic permeability, one obtains the entire magnetic field H.

Here the formula:

H = H₀* μ (1)

The following applies for the magnetization M:

M = H - H₀ (2)

With (1) the following holds for the magnetization M:

M = μ * H₀- H₀= (μ - 1) * H₀

For ferromagnetic and paramagnetic substances, the permeability is greater than 1. For iron, it is between 1 and 2. For diamagnetic materials, the magnetic permeability is less than 1: this means that the magnetic field does not enter the superconductor. Another idea is that the magnetization is directed counter to the external field and of equal magnitude, which is why in the superconductor the external field is compensated. Thus, a superconductor has no magnetic flux density transmittance. On the contrary: The magnetic field is only outside the superconductor, because the magnetic resistance of the superconductor is infinitely large.