Meissner effect

The persistent currents not only prevent an applied magnetic field from entering the superconductor, but will also expel any magnetic field that may have been present when T is lowered below Tq- This expulsion of magnetic field, known as the Meissner effect, is a unique feature of the superconducting state. A normal conductor, when cooled to the point that its resistance approaches zero, will prevent an applied field from penetrating it, but will retain whatever magnetic field that was present before the conductor was cooled. 

---

It is perfectly diamagnetic, i.e. it completely excludes applied magnetic fields. This is the Meissner effect and is the reason why a superconductor can levitate a magnet. 

Meisenheimer s rearrangement, 2 466 Meissner diamagnetic current, 23 813 Meissner effect, 23 802—803 Meissner process, 77 163-164 Meitnerium (Mt), 7 492t MEK (methyl ethyl ketone) dehydration, 78 515... 

The authors of Ref. [12] reconsidered the problem of magnetic field in quark matter taking into account the rotated electromagnetism . They came to the conclusion that magnetic field can exist in superconducting quark matter in any case, although it does not form a quantized vortex lattice, because it obeys sourceless Maxwell equations and there is no Meissner effect. In our opinion this latter result is incorrect, since the equations for gauge fields were not taken into account and the boundary conditions were not posed correctly. 

This solution motivates the existence of a complete Meissner effect for all gauge fields inside quark superconductor. It corresponds to the absolute minimum value of free energy Fmin = Fn — 3a2/2A2 in the bulk. 

Twenty years later, in 1933, the German physicist Walter Meissner (together with his co-worker Robert Ochsenfeld) discovered that superconductors cannot be crossed by magnetic field lines. This property is today defined as the Meissner effect. 

The Meissner Effect and Levitation. Besides the absence of electrical resistance, a superconducting material is characterized by perfect diamagnetism. The exclusion of magnetic field lines from a material when it passes from a normal state to a superconducting state is shown schematically in Figure 3. 

The so-called phenomenon of levitation of a magnet placed above a superconductor, Figure 4, is a direct consequence of the Meissner effect. 

In order to understand how the Meissner effect leads to levitation we must recall the principles of electromagnetism. 

Figure 3 The Meissner effect. A superconductor (here in a circular section) excludes the magnetic field lines when it is frozen below the critical temperature...

Posselt H, Muller H, Andres K, Saito G (1994) Reentrant Meissner effect in the organic conductor k-(BEDT-TTF)2Cu[N(CN)2]C1 under pressure. Phys Rev B49 15849-15852... 

Figure 2 The Meissner Effect, or the levitation of a strong magnet by the internal diamagnetic field of a high Te superconductor. 

In one sample, the crossover from the metallic (Pauli paramagnetic) region to the diamagnetic state occurred at 32 K. The diamagnetism measured was rather weak, on the order of 1% Meissner fraction, as compared to a pure superconductor (-1/47T, the full Meissner effect). 

Paul" Chu, and others at University of Houston, also reproduced Zurich s I.B.M. research results (156). Bell Lab s confirmation of Bednorz and Muller s discovery of high Tc superconductivity in copper oxide compounds was published (157) in the Jan. 1987 issue of Physical Review Letters. The electrical resistivity data from their work showing an onset of superconductivity at 36.5 K for the composition Lax gSr0 2Cu04 is plotted as Figure 28. This product also showed a 60-70% Meissner effect. 

The discovery of thallium containing superconductors (4) was another important development. Several superconducting phases exist and consist of intergrowths of rock salt (TI-O) and perovskite layers. They have been reported with zero resistance and Meissner effect up to 125K, i.e., with the highest critical temperatures discovered so far. 

Figure 4 Flux exclusion (shielding) versus increasing temperature (solid triangles) and flux expulsion (Meissner effect) versus decreasing temperature (open triangles) in a 25 Oe external field for a superconducting YE Cu Oy.g single crystal. Exclusion and expulsion are equal for temperatures above the irreversibility point Tjrr (w90.5 K at 25 Oe) Tc is 92 K.

Figure 6.64 Schematic illustration of Meissner effect in which magnetic flux lines that (a) normally penetrate the material are (b) expelled in the superconducting state.

The Meissner effect is a very important characteristic of superconductors. Among the consequences of its linkage to the free energy are the following (a) The superconducting state is more ordered than the normal state (b) only a small fraction of the electrons in a solid need participate in superconductivity (c) the phase transition must be of second order that is, there is no latent heat of transition in the absence of any applied magnetic field and (d) superconductivity involves excitations across an energy gap. 

When the temperature drops below the critical temperature (ii), the magnetic flux is expelled from the interior. Both are called Meissner effects. 

Figure 7.22 Meissner effect in YBa2Cuj07 (From Johnston et a/., 1987).

Similarly, Eq. (911) becomes the equation of the Meissner effect in superconductivity ... 

---