Superconductor device applications

Sapphire substrates, for II-Vl or III-V applications, can be found through the 100 mm range. Whereas, ZnSe, ZnTe, and GaN are found in the 1 cm range in research quantities. Efforts on ZnSe and related substrates increased with the demonstration of blue lasers in this material system. However, those efforts are foreshadowed by the blue LED and laser results in GaN. Presently there are several major research efforts underway to produce large area GaN substrates. The nitride substrate development effort also is motivated by high power, temperature and frequency device applications. For superconductors, lanthanum aluminate, strontium titanate, magnesium oxide, and other related substrates are found in the few to tens of square centimeter ranges. 

Since oxide high-temperature superconductors have a strong crystalline and electronic anisotropy, the materials must be synthesized in the form of oriented, virtually single crystal films. Furthermore, due to the extremely short coherence length of those materials, the quality of the films must be well controlled at the surface or interface as well as inside the film for device applications. The critical current density is one of the indications of the quality and the highestT obtained so far is >5 x 10 A/cm at 77K in OT. 

The re arch in catalysis is still one of the driving forces for interface science. One can certainly add to the topics of interface physics the whole new field of interface problems that is about to spring out of the new high Tc superconducting ceramics, i.e. the fundamental problem of the matching of the superconducting carriers wave-functions with the normal state metal or semiconductor electron states, the super-conductor-superconductor interfaces and so on, as well as the wide open discovery field for devices and applications. 

Liquid crystals have found widespread application in optical display devices as well as in detection of temperature uniformity and impurities. These properties are related to the orientational order of molecules in the temperature region between and the melting point. The possible applications of ferroelectric liquid crystals are promising. Superconductors (type II) can be used to create high magnetic fields at low power the ability of type I superconductors to trap magnetic flux within the domains of the normal material may also have applications. 

The prospective applications ofmolecular assemblies seem so wide that their limits are difficult to set. The sizes of electronic devices in the computer industry are close to their lower limits. One simply cannot fit many more electronic elements into a cell since the walls between the elements in the cell would become too thin to insulate them effectively. Thus further miniaturization of today s devices will soon be virtually impossible. Therefore, another approach from bottom up was proposed. It consists in the creation of electronic devices of the size of a single molecule or of a well-defined molecular aggregate. This is an enormous technological task and only the first steps in this direction have been taken. In the future, organic compounds and supramolecular complexes will serve as conductors, as well as semi- and superconductors, since they can be easily obtained with sufficient, controllable purity and their properties can be fine tuned by minor adjustments of their structures. For instance, the charge-transfer complex of tetrathiafulvalene 21 with tetramethylquinodimethane 22 exhibits room- temperature conductivity [30] close to that of metals. Therefore it could be called an organic metal. Several systems which could serve as molecular devices have been proposed. One example of such a system which can also act as a sensor consists of a basic solution of phenolophthalein dye 10b with P-cyciodextrin 11. The purple solution of the dye not only loses its colour upon the complexation but the colour comes back when the solution is heated [31]. 

The range of possibilities for semiconduction is very great, and the applications to the operation of transistors and related devices have revolutionized the electronics industry, but an extensive discussion of these topics is beyond the scope of this text.2 Note, however, that inorganic compounds are receiving intensive attention as the source of semiconductors, superconductors (page 285), and one-dimensional conductors (Chapter 16). 

But wire, monoliths, cables, and coils are not the only shapes of interest to materials scientists trying to shape the new superconductors into some useful form. Thin films, the form that the ceramic oxides would take in advanced computers and a host of electronic devices, are actually further along than the ceramic wire and indeed may be the very first of the new superconductors to see a practical application. 

Interest in the chemistry of thallium may be enhanced by the development of molecular devices with specific functionalities, such as superconductors, photosensitive devices, solar energy light-harvesting compounds, and ingredients in glass-fiber communication systems. Because of the potential applications, and for fundamental chemical reasons, the chemistry of thallium will continue to attract the attention of (and puzzle) chemists. 

The applications we discuss here, in magnets, power transmission, computer interconnections, Josephson devices and instrumentation, have almost all been studied before, during two decades of active work in applied superconductivity 41. Thought of simply as a standard superconductor with a higher transition temperature, YBaCuO does not by itself imply new kinds of applications, even though it may improve the commercial prospects for applications previously burdened by overhead costs associated with helium refrigeration. More novel applications may well emerge in the future. 

For applications at relatively low fields where little superconductor is used, refrigeration becomes a significant fraction of the overall cost, and YBaCuO could enjoy a cost advantage. The crossover points, in field and device size, below which YBaCuO is attractive will depend strongly on the device configuration and application. 

Another possible application for high temperature superconductors is as interconnects in computer systems with semiconducting devices (31). These could be called hybrid systems, since they involve both superconductors and semiconductors. In particular CMOS devices are well-known to have enhanced performance at 77 K and are thus potentially compatible with YBaCuO. 

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