Superconducting materials 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. 

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.

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. 

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... 

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). 

Meissner effect The expulsion of lines of magnetic force from a material as it undergoes the transition to the superconducting phase. 

The synthesis of macroscopic amounts of C o and C70 (fullerenes) has stimuiated a variety of studies on their chemical and physical properties. We recently demonstrated that C o and C70 become conductive when doped with alkali metals. Here we describe iow-temperature studies of potassium-doped both as films and bulk samples, and demonstrate that this material becomes superconducting, Superconductivity is demonstrated by microwave, resistivity and Melssner-effect measurements. Both polycrystalline powders and thin-flim samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor. 

Strong diamagnetic materials exhibit a volume susceptibility close to the limit of x = — 1 (dimensionless). These obey perfect diamagnetic screening due to a superconducting current and in fact they are superconductors. Thus the measurement of magnetic susceptibility serves as a convenient and fast identification of superconductivity, even for powder materials (the Meissner effect). However, due to the considerable susceptibility value, the demagnetisation effect becomes substantial and thus a correct determination of... 

Fig. 9. Meissner effect in a sphere of superconducting material cooled in a constant applied magnetic field. At the transition from the normal to the superconducting phase the lines of induction are ejected from the sphere...

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