Meissner effect

Under the Meissner effect is understood the property of superconductors in the Meissner phase to completely displace an externally applied magnetic field from its interior. This effect was discovered in 1933 by Walther Meissner and Robert Ochs field and can not be explained by classical physics. The macroscopic theoretical explanation of the Meissner effect supply the London equations.

Basics

The Meissner effect is a very characteristic for superconducting property. The external magnetic field penetrates about 100 nm far into the material, the deeper layers are field-free. This " forced expulsion " of the magnetic field is independent of whether the sample was superconducting before switching on the magnetic field only or is made ​​superconducting after the magnetic field was switched on (see next section).

The superconducting state is often detected via the Meissner effect and not about the disappearance of electrical resistance. It is also noteworthy that the effect does not depend on the history of the material, it is therefore reversible in the language of thermodynamics. Meissner and ox field as shown by indirectly that the superconducting state is a true thermodynamic state.

All superconductors show a complete Meissner effect, as long as the temperature does not exceed the critical temperature and the externally applied magnetic field remains below a critical field strength. Because of the full field displacement is also called perfect diamagnetic. Type II superconductor show above a critical field strength only an incomplete Meissner effect: in this so-called Shubnikov phase by the magnetic field penetrates the superconductor in thin tubes, so-called flux tubes, which arrange themselves in an equilateral triangular lattice. In this phase, the superconductor is not a perfect diamagnetic more; but superconductive he is still.

Is not too thin workpieces the Meissner effect depends on the purity and the homogeneity of the superconductor. A complete Meissner effect comes about only when the entire sample became superconducting. Otherwise, mixing states can make from normal and superconducting regions. The Meissner effect therefore, to assess the quality of a superconductor is. In contrast, the electrical resistance is already virtually zero when the critical temperature is reached.

Simplified declaration

A slightly simplified explanation goes as follows:

The interface is approximated by the plane x = 0. Links from the interface, ie for x < 0, normal conductive material and a homogeneous, vertical magnetic field Bz located. To the right of the interface. that is, for x> 0, the material is superconducting. Then flow along the interface, in a very thin surface layer to the width of λ ( " depth " ), the precise expansion can also be calculated, from the front rearward jy supercurrents. These produce according to the "right- hand rule " means a vertically downward magnetic field in order to minimize the field energy compensates for the upward vertical external magnetic field Bz exactly.

In the superconductor is so - apart from the mentioned surface layer of width λ - B = 0 everywhere

Contrast to the ideal conductor

Cutting away a plate of an ideal conductor ( a material with no electrical resistance ) on a permanent magnet from such currents are induced in platelets according to Lenz's law, which counteract the magnetic field and thereby allow the plates float above the magnet. Since there is no resistance, the currents are not attenuated and the wafer floats permanently. In a superconductor disappears below the critical temperature, its electrical resistance and it is an ideal conductor. Therefore, a superconducting plate floats over a magnet when it was pre- cooled to below its critical temperature.

However, the properties of a superconductor go beyond that of a perfect conductor. This can be shown by reversing the order of the experiment: If one the ( warm ) superconductor on the magnet plate and cools it off until then, it starts below the critical temperature to hover. For a classical ideal conductor nothing would happen, that is the so-called Meissner effect is a special property of superconductors. The effect can be explained by the fact that form during the phase transition into the superconducting state shielding that displace the magnetic field from the interior of the superconductor.

Applications

The Meissner effect is used, for example, in superconducting magnetic bearings or in superconducting switches, called Kryotronen.

The explanation of the Meissner effect in the form of the Ginzburg - Landau theory is used as a model for the so-called Higgs mechanism, the mass of the exchange particle of the electroweak interaction is generated by the high energy physics.