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Superconductor

What is a superconductor?

A superconductor is a material that can conduct electrical current without resistance. However, superconductors are not only infinitely good conductors but also perfect diamagnets.
A diamagnet displaces a magnetic field and weakens it internally. A superconductor can completely weaken the magnetic field and displace it from the interior. The field lines run completely around the superconductor. The opposite of diamagnets are paramagnets and ferromagnets, which strengthen an external magnetic field.
Table of Contents
Superconductivity is probably one of the most exciting discoveries in modern physics. It is already very fascinating that a material has no electrical resistance. But, what has largely become world-famous are images showing a superconductor floating above the pole of a permanent magnet or vice versa.

The illustration is a schematic depiction that is often shown in printed media: A small magnet hovers above a superconducting disc (black).
The illustration is a schematic depiction that is often shown in printed media: A small magnet hovers above a superconducting disc (black).
Many materials become superconducting at very low temperatures. Ordinary lead, for example, becomes superconducting at temperatures of liquid helium (4 K approx. – 270°C). These extremely low temperatures add to the fascination of superconductivity. Today, research is being carried out on high-temperature superconductors. However, the materials found still require very low temperatures. Ceramic materials with certain properties become superconducting at around -100°C. Yet, this still requires extreme cooling, e.g. with liquid nitrogen.

Anyone who has ever tried to levitate a permanent magnet by holding it with one pole over the same pole of a recumbent magnet (e.g. north pole against north pole) will know how difficult, if not impossible, this is. A superconductor, on the other hand, floats steadily in the magnetic field, even though it is not a magnet itself. If the superconductor is brought into contact with a ferromagnet (e.g. iron), no magnetic forces can be detected.

Why does a superconductor float in a magnetic field?

The reason for the repulsive magnetic forces between the superconductor and a magnetic field is the diamagnetism of the superconductor.

Many materials are diamagnetic, including water. Diamagnets do not have elementary magnets such as para- or ferromagnets, which can align themselves in an external magnetic field. However, when a diamagnet is placed in an external magnetic field, an induction effect occurs.

A current is induced in the material, causing a magnetic moment which, according to Lenz's law, is pointed in the opposite direction to the external magnetic field. This creates a weak repulsive force. Using extremely strong magnetic fields, it has even been possible to make a frog, a water-containing creature, float.

A diamagnet (e.g. water) is repelled when it is introduced into the magnetic field, albeit very weakly. The repulsive force between magnetic fields and diamagnetic materials is only strong in superconductors. Superconductors are, therefore, also known as "perfect diamagnets". They exhibit a magnetisation that completely displaces the magnetic flux density inside the superconductor. Due to the repulsive diamagnetic effect, the superconductor already floats above a relatively weak magnet.

Magnetic permeability of superconductors

Magnetic permeability μ is introduced to describe the strength of magnetisation.

Magnetisation M occurs in an external magnetic field H0. The total magnetic field H in the presence of the material is obtained by multiplying the external magnetic field H0 with the permeability μ: H= μ•H0.

This magnetic field is the sum of the external magnetic field H0 and the magnetisation M of the material:

H=M+H0.

Thus, the following applies to magnetisation: M=H-H0=μ•H0-H0=(μ-1)•H0.

The factor (μ-1) is also referred to as magnetic susceptibility χ and hence M=χ•H0. Para- and ferromagnetic substances have a permeability that is greater than 1. The magnetic susceptibility is therefore greater than zero. The permeability of diamagnetic materials is slightly less than 1 and the susceptibility is correspondingly less than zero. In the case of a superconductor, the magnetic permeability is μ=0 and the susceptibility χ=-1. This means that the magnetic flux no longer penetrates the superconductor. One can also conceive that the magnetisation of superconductors is equal to the external incident field, only in the opposite direction. Therefore, the external field is offset in the superconductor.

Consequently, a superconductor has no permeability for magnetic flux density. It has an infinitely high magnetic resistance. The superconductor completely displaces the magnetic flux from its interior.

The illustration shows the course of the field lines of the magnetic field H in the presence of a para- or ferromagnetic material (μ =2,χ=1) (left) and in the presence of a superconductor with (μ =0, χ =-1) (right). The original incident field is shown as a blue arrow and the magnetisation as a red arrow. In a ferromagnetic material, the magnetisation is positive and therefore aligned with the original field. This is always the case if χ > 0, i.e. the material
The illustration shows the course of the field lines of the magnetic field H in the presence of a para- or ferromagnetic material (μ =2,χ=1) (left) and in the presence of a superconductor with (μ =0, χ =-1) (right). The original incident field is shown as a blue arrow and the magnetisation as a red arrow.
In a ferromagnetic material, the magnetisation is positive and therefore aligned with the original field. This is always the case if χ > 0, i.e. the material "absorbs" the magnetic field in the same direction and thus amplifies it.
In a diamagnet, on the other hand, the magnetisation is pointed in the opposite direction to the incident field. The absorbed field is negative and therefore χ < 0.
While the positive field amplification can even be many times greater than the incident field, the negative attenuation is only possible up to the complete compensation of the field. This complete compensation occurs in superconductors. For the superconductor, χ = -1 applies. Thus μ = 0. The superconductor therefore does not allow any field to pass through. A superconductor is thus a "perfect diamagnet".
Fascinating projects can be found in our magnet applications with superconductors.


Portrait of Dr Franz-Josef Schmitt
Author:
Dr Franz-Josef Schmitt

Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.

The copyright for all content in this compendium (text, photos, illustrations, etc.) remains with the author, Franz-Josef Schmitt. The exclusive rights of use for this work remain with Webcraft GmbH, Switzerland (as the operator of supermagnete.gr). Without the explicit permission of Webcraft GmbH, the contents of this compendium may neither be copied nor used for any other purpose. Suggestions to improve or praise for the quality of the work should be sent via e-mail to [email protected]
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