Superconductors Meissner effect

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. 

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

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...

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). 

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. 

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. 

In addition to the zero resistivity, superconducting materials are perfectly diamagnetic in other words, magnetic fields (up to a limiting strength that decreases as the temperature rises toward Tc) cannot penetrate them (the Meissner effect). This is a consequence of the mobile, paired state of the electrons. Indeed, it is the demonstration of the Meissner effect, rather than lack of electrical resistivity, that is usually demanded as evidence of superconductive behavior. One entertaining consequence of the Meissner effect is that small but powerful magnets will float (levitate) above the surface of a flat, level superconductor.30... 

A superconductor is a material that loses all electrical resistance below a characteristic temperature called the superconducting transition temperature, Tc. An example is YBa2Cu307 (Tc = 90 K). Below Tc, a superconductor can levitate a magnet, a consequence of the Meissner effect. 

The repulsive force of the expulsion of these field lines, known as the Meissner effect, leads to the phenomenon of levitation — a magnet placed above a superconductor floats suspended above the top of the superconductor. 

If a Type I superconductor such as lead is placed in a small magnetic field (e.g. a few mT) and cooled, then at 7[. the magnetic field is expelled from the interior of the specimen. This is the Meissner effect, which is fundamental to the superconducting state it is not simply characteristic of a material which happens to be a (fictitious) perfect conductor. The total absence of an electric field in a... 

The irreversibility field (Hirr) Figure 4.56 illustrates the events as a magnetic field is applied to and then removed from a Type II superconductor below its critical temperature. At applied fields up to Hcl the response of the material is as expected for a Type I material. That is the field is excluded (the Meissner effect) and the material behaves as a perfect diamagnetic. At applied fields above Hcl there is some flux penetration and this increases until the material becomes normal at Hcl. 

A superconductor exhibits perfect conductivity (See Section 7.2) and the Meissner effect (See Section 7.3) below some critical temperature, Tc. The transition from a normal conductor to a superconductor is a second-order, phase-transition which is also well-described by mean-field theory. Note that the mean-field condensation is not a Bose condensation nor does it require and energy gap. The mean-field theory is combined with London-Ginzburg-Landau theory through the concentration of superconducting carriers as follows ... 

The Meissner effect is the exclusion of an external magnetic field from the bulk of the superconductor. By London theory the magnetic induction is... 

A two dimensional realization of doping is obtained by taking the parent electron-superconductor and hole-superconductor to consist solely of bipolarons and holes respectively. See Fig. 8.6. This model is consistent with the observed volume-fraction Meissner effect since the Meissner effect can occur only in condensed areas. The dependency of the Meissner effect on doping parallels the parabolic dependency of To See Section 8.6.3. 

The report of the Meissner effect stimulated the London brothers to develop the London equations, which explained this effect, and which also predicted how far a static external magnetic field can penetrate into a superconductor. The next theoretical advance came in 1950 with the theory of Ginzburg and Landau, which described superconductivity in terms of an order parameter and provided a derivation for the London equations. Both of these theories are macroscopic or phenomenological in nature. In the same year, 1950, the... 

There are two aspects to perfect diamagnetism in superconductors. The first is magnetic field exclnsion if a material in the normal state is zero field cooled (ZFC), that is, cooled below Tc to the superconducting state withont any magnetic field present, and then it is placed in an external magnetic field, the field will be excluded from the superconductor. The second aspect is magnetic field expulsion. If the same material in its normal state is placed in a magnetic field, the field will penetrate and have almost the same value inside and outside because the permeability fx is so close to the free space value fXo. If this material is then field cooled (FC), that is, cooled below E in the presence of this applied field, the field will be expelled from the material this is the Meissner effect that was mentioned earlier. 

In addition to catalytic applications, the perovskite backbone is a key component in modern high-temperature superconductive materials. By definition, a superconductor exhibits no resistance to electrical conductivity, and will oppose an external magnetic field, a phenomenon referred to as the Meissner effect (Figure 2.19). Many pure transition metals e.g., Ti, Zr, Hf, Mo, W, Ru, Os, Ir, Zn, Cd, Hg) and main group metals e.g., Al, Ga, In, Sn, Pb) exhibit superconductivity, many only when exposed to high-pressure conditions. These materials are referred to as Type I or soft superconductors. 

A superconductor is a material which conducts electricity without resistance and the exclusion of the interior magnetic field (Meissner effect) below a certain critical temperature Tc- Superconductivity occurs in a wide variety of materials, including elements, various metallic alloys and some heavily-doped semiconductors. Mixed metal oxides belong to the class of high-temperature superconductors (Tq > 30 K). 

Even an amateur without big bucks can break into the field—with a kit demonstrating the Meissner effect of magnetic levitation. For twenty-five dollars, the Institute for Chemical Education at the University of Wisconsin will send superconductor buffs a kit that includes a 1-inch... 

So, you drop that little disc of ceramic into the liquid nitrogen—you can put it all in a styrofoam cup—and put a little magnet on top. If the magnet floats, you ve got a superconductor, the Meissner effect. 

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