Superconductors are substances possessing the property of superconductivity, e.i., the ability of conducting direct currents without any electrical resistance. Superconductivity can be observed only upon lowering the superconductors' temperature below some value which is characteristic for the particular superconductor and is designed as their critical temperature Tc.
The phenomenon of superconductivity was discovered in 1911 by the Dutch physicist H. Kamerlingh-Onnes in his work with mercury, for which the Tc value according to modern data is close to 4.17 K. Since then, this property was found in more than 20 elementary metals and in several different alloys and compounds. The highest Tc in case of elementary metals is that of niobium (8.7 K) and higher values of the order of 20 K have long been known for some compounds also containing niobium, the record (22.3 K) being held by the Nb3 (Al—Ge) compound. However, in 1987 a new group of complex compounds containing copper as well as oxides (e.g., Y, Ba, Bi, Th, etc.) was discovered for which Tc values could certainly be as high as 155 K and according to some quite recent estimates can reach 250 K, that is only slightly below room temperature. This new group of superconductors was since designated even as "high Tc" superconductors as opposite to the older types of "traditional" or "low Tc" ones.
The Tc values are functions of magnetic field strength to which superconductors can be exposed, decreasing in growing fields, so that superconductivity can be observed only in some specific limited range of magnetic field strengths and of temperatures. The values of maximum field strength also designated as critical ones (Hc) and corresponding Tc values can be well represented by a parabolic dependence Hc(T) = Hc(0)(1 − (Tc − Tc, max)2). Critical field values for elementary superconductors are relatively small and this, along with their low Tc values, has hindered practical applications of superconductors (Hc for pure Nb is no higher than 1.8 kOe). On the other hand, some superconducting alloys and compounds can be characterized by very high Hc values. Thus, for now most widely practically used alloys of Nb with Ti corresponding magnetic inductions (Bc) can reach 12 Teslas, for another "low Tc" compound Nb3Sn, that also finds somewhat limited applications, Bc can be as high as 35T, while some compounds are now known with Bc surpassing 100T. Even quite approximate determinations of Bc values can present substantial practical difficulties. New "high Tc" superconductors can also be characterized by high Bc values.
Superconductors have two possible behavioral patterns of superconductors with respect to magnetic fields, allowing them to be divided into two different classes. For superconductors of the first kind, the so called Meissner effect is observed which consists in total expulsion of magnetic field lines of force from the bulk of the superconductor, the effect being provided by screening corrents circulating in very thin layers near the surface of the specimen. It is essential that such expulsion must be produced regardless of the exact order in which the superconducting state is realized in a magnetic field. It is in this particular respect that the superconductors of the first kind differ from the hypothetical "ideal" conductor that does not prevent the magnetic field penetrating the bulk of the specimen prior to its going superconducting on lowering the temperature. It should be noted, however, that total magnetic field expulsion could be achieved only in very pure and otherwise perfect specimens having in addition regular smooth form. In impure, deformed specimen with sharp edges, etc. only partial expulsion is usually observed.
The superconductors of the second kind also exhibit Meissner effect, but only in relatively low magnetic fields, while with increasing field strength, partial penetration of the magnetic flux lines into the bulk of the specimen becomes more significant. A "mixed" state is then realized in the bulk of the superconductors with persistent currents circulating over the entire specimen volume in a kind of regular pattern consisting of so-called Abricosov's vortices otherwise also known as "fluxoids". This mixed state can be preserved in much higher fields up to some "second critical value", Hc 2, which can usually be found in reference tables.
Almost all pure elementary superconducting metals are first kind superconductors with rare possible exceptions (e.g., of Nb that can be considerd as intermediate or at most as not quite typical case). A very limited number of other alloys and compounds can also be included within the same group, while the bulk of other alloys and compounds fall typically into the second group of superconductors.
For many practical applications the problem of critical current values, e.i., of maximum currents that could flow through the superconductor without noticeable resistance, can be of prime importance. For the first kind superconductors, especially for pure, "ideal" ones very simple relation called Silsbee's rule has been formulated. According to this rule no resistance could be observed unless at any point on the superconductor surface, the magnetic field produced by both external sources and by the superconductor's own currents surpasses the critical value. In the case of superconductors of the second kind the problem of critical currents is much more complicated and, generally, only direct experimental determination of critical currents (or of current densities) as function of external fields and temperatures can provide a complete set of critical date (forming a kind of "critical surface" in the space of all necessary parameters).
For pure perfect specimens of type 2 superconductors, critical currents for the most part can be relatively low and materials find therefore rather limited applications. However, for highly deformed, nonuniform or doped type 2 alloys and compounds critical currents can reach extremely high values—up to 100 kA/ mm2. High critical currents permit one to use such substances for many practical applications, mainly in the form of superconducting coils (or magnets), that are now widely used in many different fields of science and industry (particle accelerators and other physical research devices, electric power technologies, magnetic ore separation, magnetic resonance imaging, etc.). Practical use of type 1 superconductors when compared to type 2 materials seem quite limited. However, due to their very unusual and specific properties such substances could also be used in some applications in high sensitivity measurements (magnetometers, radiation detectors), metrology (standard values reproduction and measuring equipment), UHF devices, etc. Certain expectations are also connected with the possible use of superconductors in very large memory arrays of big computers.
The problem of exact nature of physical mechanism leading to the occurrence of superconductivity in some particular substances is of very great interest for the modern science. In the "traditional" superconductors the very peculiar interaction between electrons in metals provided by the exchange of phonons, or sound wave quanta, is the most predominant if not the only possible form of such mechanisms. Low critical temperatures of these substances are the results of relatively small phonon energies, which are typically no higher than a few hundreds of Kelvins. Very high Tc values for new ceramic superconductors are presumably provided by some different interaction, the exact nature of which, in spite of very intense experimental research and a lot of proposed theoretical explanations, remains, however, quite obscure.