The phenomenon of superconductivity

The following summary of the essentials of the phenomenon and theory of superconductivity serves to introduce terminology and permits the high transition temperatures (Tc) superconductors (HTSs) to be seen in a historical 

Since 1911 superconductivity has been observed in many metallic elements and intermetallics, principal among them being tin (3.7K), tantalum (4.5K), lead (7.2K), niobium (9.3K), Nb3Sn (18K) and Nb3Ge (20.9K). 

There are two types of superconductor, Type I and Type II, characterized by the way in which they respond to an applied magnetic field. 

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 

Below Tc the applied magnetic field can be increased up to a critical value Bc, when the superconductor changes to the normal state. This behaviour is characteristic of a Type I superconductor. 

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Nonstoichiometry is relatively common among mixed metal oxides, in which more than one metal is present. In 1986 it was discovered that certain compounds of this type showed the phenomenon of superconductivity on cooling to about 100 K, their electrical resistance drops to zero (Figure 20.9). A typical formula here is YBa2Cu30 where x varies from 6.5 to 7.2, depending on the method of preparation of the solid. 

Materials that exhibit the phenomenon of superconductivity enter into a new state below a critical temperature Tc (see Table 8.11). 

The phenomenon of superconductivity was discovered at the beginning of the twentieth century by the Dutch physicist H. Kamerlingh Onnes, during the first attempts to liquefy helium (which at atmospheric pressure boils at 4.2 K). After refining the technique of helium liquefaction, in 1911, Onnes attempted to measure the electrical resistance of metals at these extraordinary low temperatures, and realized that at 4 K the resistance of mercury, as well as that of other metals indicated in Figure 1, became too low to be measured. This change in electrical property became the indication of the new superconductive physical state. The temperature below which materials become superconducting is defined as the critical temperature, Tc. 

However, when pressure was applied to these metallic organic systems, the phenomenon of superconductivity was detected. The... 

The correlation as summarized in Fig.9 is remarkable in the sense that it supersedes the following factors that heretofore were invoked in one way or another to correlate to the phenomenon of superconductivity (a) transition or non-transition elements, (b) crystal structure type, (c) atomic number, (d) valence electron concentration. 

Any theory of the metallic state worth its salt must account for the phenomenon of superconductivity, in which the resistance of a substance is so small as not to be measurable. Indeed, anyone who has ever seen the manifestation of superconductivity will never forget his or her experience [104],... 

Superconducting materials have been known since the discovery of the phenomenon of superconductivity in 1911 by Kamerlingh Onnes. They are characterized by the absence of any measurable resistivity and by their perfect diamagnetic behavior below... 

The phenomenon of superconductivity is common for all fee transition metal nitrides of groups 4 and 5 and is also observed in fee MoNi- and hexagonal MoN. 5-NbNi x... 

The phenomenon of superconductivity is common in several particular types of compounds. Thus more than two dozen binary compounds with the fee sodium chloride (NaCl) stracture are superconducting. The carbides AC and nitrides AN, such as NbN with Tc = 17 K, have the highest transition temperatures of this group, and the metallic A atoms with values above 10 K were Nb, Mo, Ta, W, and Zr. The NaCl-type superconductors are compositionally stoichiometric but not structurally so. hi other words, these compounds have a small to moderate concentration of vacancies in the lattice. For example, YS has 10% vacancies, which means that its chemical formula should properly be written 0,980.9. Nonstoichiometric NaCl-type compounds such as Tai.oCo.ye also exist. Ordinarily the vacancies are random, but sometimes they are ordered. 

Like some other metals, mercury exhibits unusual behavior at extremely low temperatures, hi 1911, Dutch physicist Heike Kamerlingh Onnes discovered the phenomenon of superconductivity by freezing mercury to only a few degrees above absolute zero. At that temperature, mercury loses all of its natural resistance to the flow of electricity and becomes superconductive. 

Bardeen (along with Leon Neil Cooper and John Robert Schrieffer) won a second Nobel Prize in 1972 for their jointly developed theory of superconductivity, usually called (using the last initials of the three scientists) the BCS theory. In essence, BCS theory explains the phenomenon of superconductivity in Type I superconductors—metals, such as mercury, lead, and niobium. 

After the initial discovery by Onnes of superconductivity in mercury, tin, and lead, research focused on the discovery of new superconducting phases with even higher values. It was found that 25 % of the elements of the periodic table are superconductors and that a plethora of alloys exhibit superconductivity [16]. A theory to describe the phenomenon of superconductivity was introduced by Bardeen, Cooper, and Schrieffer (BCS) which, as originally formulated, placed an upper limit on Tc of about 35 to 40 K [19]. For a synopsis of the historical development of superconductor theory, see [20]. We shall use the term low temperature superconductor (LTS) as a reference to those materials which possess values less than the theoretical limit of 35 to 40 K imposed by the original BCS theory. 

Superconductors are materials that have the ability to conduct electricity without resistance below a critical temperature above absolute zero. The phenomenon of superconductivity was first seen in mercury at liquid helium temperatures. Great interest developed in this area in the late 1980s, when Muller and Bednorz discovered that even ceramic-like materials can exhibit superconductivity. C. W. Chu subsequently found yttrium barium copper oxide (YBCO) to be superconducting above liquid nitrogen temperatures. Indeed, various books are devoted to this subject. > In the following subsections we highlight representative force field applications that have aided the understanding of static and dynamic properties of superconductors. 

Superconductivity of Cuprates Despite the practical use of this phenomenon (see, e.g. below), the phenomenon of superconductivity has not yet been explained. In this situation, it is better to focus only on the experimental facts. For example, it was found that the motion of electrons contributing to superconductivity in the above-mentioned cuprates is parallel to the Cu-O planes of their laminar structures (Fig. 5.9), and the best superconducting properties are achieved in the case of thin layers of those compounds in which there is a parallel orientation of Cu-O planes to the direction of electric charge transport. 

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