Theory superconducting material magnetism

Hence, it seems safe to conclude that the next century will see considerable research on materials and alchemy using quantum theory. If the physical models improve and computations continue to get more accessible, many experimentalists will have access to these tools. In science, all decisions are ultimately made by experiment, and most new discoveries are made by observing physical systems. Perhaps it is not too outrageous to suggest that, if theory continues to improve as it has, theorists may discover new states of matter and properties such as superconductivity and magnetism using their computers and analytic modeling. 

As predicted by the Ginzburg-Landau theory and demonstrated later by experimental studies, superconducting materials can be classified according to their magnetic behavior into two distinct classes type I and type II superconductors. Only a few hundred superconducting materials (e.g., pure metals, alloys, ceramics, and, recently, organics compounds) are known today, and this leads one to consider superconductivity as a rare physical phenomenon. 

Fig. 5. Set of isotherms R(T= const, B) of amorphous InOj, films [4]. (a) Magnetic field normal to the film (b) magnetic field parallel to the film. In the fields region I the material remains superconducting, label III marks the region of negative magnetoresistance. The theory [9, 10] relates to the vicinity of the boundary between the regions I and II in the geometry (a).

One interesting application of the theory developed in the preceding sections involves the superconducting state. A transition from the normal to the superconducting state occurs in some materials at a fixed temperature Tc. Such a state is characterized not only by the complete disappearance of electrical resistivity, but also by the fact that, in type I, soft superconductors at least, the magnetic induction B is zero. Since B - H + 4irM, this means that for such superconductors M - - 4jtH. 

Binary and ternary alloys and oxides of these elements, as well as pure V, Nb, Gd, and Tc are referred to as Type II or high-field superconductors. In contrast to Type I, these materials exhibit conductive characteristics varying from normal metallic to superconductive, depending on the magnitude of the external magnetic field. It is noteworthy to point out that metals with the highest electrical conductivity (e.g., Cu, Au) do not naturally possess superconductivity. Although this behavior was first discovered in 1911 for supercooled liquid mercury, it was not until 1957 that a theory was developed for this phenomenon. 

---