In 1933, one of the other surprising properties of the superconductivity was discovered by Meissner and Ochsenfeld: perfect diamagnetism. According to this theory; the magnetic flux is expelled from the interior of the superconductor sample when it is cooled below the critical temperature for superconductivity in weak external magnetic fields (Figure 1.2), which is called Meissner Effect.
Figure 1.2- Expulsion of a weak external magnetic field from the interior of the superconducting materialFollowing the discovery of the Meissner Effect, F. and H. London proposed a simple two–fluid model in 1934. The London model was functional enough either to explain Meissner effect or predict the penetration dept , a characteristic length of penetration of the static magnetic flux into superconductor.
In 1950 Vitaly Ginzburg and Lev Landau proposed an intuitive, phenomenological theory often called a macroscopic theory ‘ Ginzburg- Landau Theory ’ of superconductivity which replied to the characteristic properties of most interesting superconducting materials including high–Tc oxides. In 1957, Alexei Abrikosov also investigated the behavior superconductors in an external magnetic field and discovered that superconductor materials can be explained in two types: type-I and type-II superconductors. While the former expels magnetic flux completely from their interior, the latter do it completely only at small fields, but partially in higher external fields. In 1957 John Bardeen, Leon Cooper and Robert Schrieffer proposed a complete microscopic theory of superconductivity that is usually referred to as the BCS Theory , of which the basis depends on the interaction of conduction electrons with elastic waves of the crystal lattice. Thus, not only does the BCS theory provide the basis for our present understanding of superconductivity in conventional materials but also some extent of the reference theory plays an important role in the on-going search for correct description of superconductivity in high–Tc cuprate oxides.
In 1962 Brian Josephson postulated fascinating quantum tunneling effects that should occur when super current tunnels through an extremely thin layer roughly 10 Å of an insulator. The prediction was confirmed within a year and the effects are known as the Josephson effects. Superconducting technology based on these effects gradually evolved and in fact Josephson junction technology represents the basis of the promising superconducting electronics nowadays.
In that year, zero resistance and perfect diamagnetism were two main problems when properties of superconductivity were tried to use in the new technology. The former needs to get very low temperature for superconducting state. In these days, although people could supply this situation with using liquid helium but that was so expensive in those years. The latter is about the condition of disruption of superconductivity for any metal, when a magnetic field applied to, or current flowed through the sample so the practical value was decreasing in applications.
There would be a lot more practical uses for superconductivity unless it were for the very high cost of liquid helium coolant. Any gas will liquefy at sufficiently low temperatures; for example, oxygen becomes liquid at 90 K and nitrogen at 77 K. Moreover, it is far less costly to fluid these gases than to liquid helium. Liquid nitrogen has a much greater cooling capacity than liquid helium. For any application in which liquid nitrogen can replace liquid helium, the refrigeration cost will be about 1000 times less .
In 1973 powerful magnets were made with using Nb-Ti which is superconductor and in fact compound had the highest critical temperature, 23 Kelvin, in all superconductors. The high- temperature history of superconductivity started in late 1986 with J. George Bednorz and Karl Muller of the IBM research laboratory in Zurich, Switzerland. They had reported the observation of superconductivity in lanthanum copper oxides doped with barium or strontium (La-Ba-Cu-O) at temperatures up to 38 K. Therefore the upper limit barrier is nearly 30 K for superconductivity that had been theoretically predicted almost 25 years earlier. This ceramic compound with perovskite structure was noted to be the first high temperature superconductor. Its multiphase nominal compound has the formula of La5xBaxCu5O5(3-y) whose critical temperature changes with regard to the change of x value. In La2-x BaxCuO4 compound the highest Tc value is obtained for x=0.15. Moreover, by using strontium instead of barium, the value of Tc can dramatically be increased up to about 36 K .
Hundreds of scientists rushed to try various chemical compounds to see which one would give the highest critical temperature. In February 1987 research groups in Alabama and Houston coordinated by K. Wu Ashburn and Paul Chu, discovered yttrium barium copper oxide ,Y-Ba-Cu-O, ceramics by using yttrium oxide instead of lanthanum oxide, with Tc=92 K . This was an important discovery owing to the existence of a superconductor with a critical temperature above that of liquid nitrogen. It is very easy to prepare Y-Ba-Cu-O ceramics by mixing calcining and oxidizing the constituent powders (Figure1.3). The multiphase nominal compounds of this sample were noted to be Y1.2Ba0.3Cu2O4-x and Y0.6Ba0.4CuO3-x. Then scientists determined that the uniphase compound called as 1: 2: 3 or Y-Ba-Cu-O is YBa2Cu3O7 by checking the ratios of barium yttrium and cooper. The difference between La-Ba-Cu-O and Y-Ba-Cu-O is pointed out to be oxygen vacancy. Depending on the vacancies, there is possibility of changing the critical temperature .
In 1987, a new chain is added to the superconductivity studies by Michael et al. who discovered the superconductivity bismuth strontium calcium copper oxide (Bi-Sr-Cu-O) ceramics. In 1988, Maeda and his group observed superconductivity at 105 K by adding calcium to this system as Bi-Sr-Ca-Cu-O . Also Tokano and his group found a stable superconductor at 110 K by adding lead to the compound . In 1992 Quidwai and his group carried out the transition at 130-140 K by adding antimony to the system .
In 1988, Sheng and Parking observed superconductivity in thallium barium calcium copper oxide (Tl-Ba-Ca-Cu-O) compound at 110 K and 125 K. Nevertheless this compound either has carcinogenic effect or was unstable thus has to be produced each time. During these passing years, studies on different samples went on. In 1993 Schilling and his group observed superconductivity in mercury barium calcium copper oxide HgBa2CaCu3O1+x compound at 133 K; similarly, Chu and his group observed superconductivity in high pressure with Hg based compounds at 150 K .
In 2001, J. Nagamatsu, J. Akimitsu and their group observed superconductivity in MgB2 compound at 39 K, which was a very important discovery because of the fact that this critical temperature was quite high for Type-I superconductors and elastic material so it had the advantage of being easy to be adapted to technology.
Table 1.1 High Critical Temperature Superconducting Compounds
In 2008, the discovery of a new family of high critical temperature iron and arsenic superconductors (AsFe) marked a second major revolution in the world of superconductivity. The new compounds, which do not contain copper (Cu) but which have oxygen (O), flour (F) or arsenic (As) and iron (Fe), will help scientists to solve some of the mysteries in the area of solid state physics and to answer some questions such as “Is the AsFe superconductor family really so different from second type superconductor?”. This point is fundamental to define a unified approach to the two families of superconducting materials.
Figure 1.4- The critical temperature Tc of various superconductors plotted against their discovery date.
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