Superconducting materials magnets

Bardeen, J., 1992, in Concise Encyclopedia of Magnetic Superconducting Materials, ed. J. Evets (Pergamon Press, Oxford) p. 554. 

A superconductor. A pellet of superconducting material, previously cooled to 77 K with liquid nitrogen, floats above a magnet. 

They included five eligible materials classes, two favored (electronic-photonic and biomolecular) and three others (structural, magnetic, and superconducting materials). 

Electronic, Optoelectronic, Photonic, Magnetic, and Superconducting Materials... 

We can conclude that the new behaviour of the superconducting material is due to a new state for the electrons in fact, at the critical temperature, there is a jump of the electronic specific heat. In no external magnetic field, it is a second-order transition, which does not involve latent heat. 

One device is made up of samples of superconducting materials put inside a transformer a low-frequency small current in the primary creates a weak magnetic field. 

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. 

Scientists are interested in superconductors because they have a number of properties that arise from their lack of resistance. For example, once a current is induced in a circuit made with a superconducting material, the current continues to flow through the circuit indefinitely, without ever diminishing. Also, superconductors have the ability to completely repel magnetic field lines. This means that a magnet placed over a superconductor hovers in mid-air, as shown in Figure 4.25. 

The order parameter can be the magnetization in ferromagnetic materials, the electron pair amplitude in superconducting materials, the He amplitudes in superfluid He or the lattice distortion in crystals. 

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... 

Based on their difference in reacting to magnetic flux, superconducting material can be differentiated into Type-I and Type-II. The Type-11 superconductors are usually associated with intermetallic compounds instead of elements and their superconductivity is not easily affected even with a high magnetic field. 

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