Superconducting problems

W. Heinz, and P. Komarek, Investigations of Cryogenic and Superconductivity Problems for Future Synchronous Generators and Tokamak Fusion Reactors, paper presented at the World Electrotechnical Congress, Moscow, USSR, June 1977. 

Bersuker writes in connection with the implications of the J-T effect on the superconductivity problem An illustration of the JT approach to electron-phonon coupling in solids may be found in the modern attempts to explain the origin of high-temperature superconductivity (HTSC). Experimental data show that the electron-phonon interaction is essential in this phenomenon. And continues The... 

The Frohlich transformation is essential and it was applied to the superconductivity problem but it has not since been used in quantum chemistry problems. 

Nowadays there are many attempts to implement the J-T effect into the problem of superconductivity. But first something specific related to superconductivity has to be implemented into the J-T effect, viz. the Frdhlich transformation. Frdhlich did propose his transformation [10] almost 20 years after the first formulation of the J-T effect [33]. Unfortunately, this transformation is mostly known in solid-state physics (and moreover used exclusively in the superconductivity problem) and after more than a half of century it has not been integrated in the domain of quantum chemistry. It is very important for several reasons first in the explanation of the hypervibronic coupling mechanism in the J-T effect. It further takes into account not only the dependence of electronic states on the nuclear coordinates, as it is usual in the adiabatic case, but also on the nuclear momenta, which is inherent in the non-adiabatic one. This type of transformation leads to new fermion quasiparticles that... 

Hamiltonians equivalent to (1) have been used by many authors for the consideration of a wide variety of problems which relate to the interaction of electrons or excitons with the locaJ environment in solids [22-25]. The model with a Hamiltonian containing the terms describing the interaction between excitons or electrons also allows for the use of NDCPA. For example, the Hamiltonian (1) in which the electron-electron interaction terms axe taken into account becomes equivalent to the Hamiltonians (for instance, of Holstein type) of some theories of superconductivity [26-28]. 

HTS materials, because of their ceramic nature, are quite brittle. This has introduced problems relative to the winding of superconducting magnets. One solution is to first wind the magnet with the powder-in-tube wire before the ceramic powder has been bonded and then heat treat the desired configuration to form the final product. Another solution is to form the superconductor into such fine fila-... 

The electronic theory of metallic superconduction was established by Bardeen, Cooper and Schrieffer in 1957, but the basis of superconduction in the oxides remains a battleground for rival interpretations. The technology of the oxide ( high-temperature ) superconductors is currently receiving a great deal of attention the central problem is to make windable wires or tapes from an intensely brittle material. It is in no way a negative judgment on the importance and interest of these materials that they do not receive a detailed discussion here it is simply that they do not lend themselves to a superficial account, and there is no space here for a discussion in the detail that they intrinsically deserve. 

We have remarked that a temperature of zero on the absolute temperature scale would correspond to the absence of all motion. The kinetic energy would become zero. Very interesting phenomena occur at temperatures near 0°K (the superconductivity of many metals and the superfluidity of liquid helium are two examples). Hence, scientists are extremely interested in methods of reaching temperatures as close to absolute zero as possible. Two low temperature coolants commonly used are liquid hydrogen (which boils at 20°K) and liquid helium (which boils at 4°K). Helium, under reduced pressure, boils at even lower temperatures and provides a means of reaching temperatures near 1°K. More exotic techniques have been developed to produce still lower temperatures (as low as 0.001°K) but even thermometry becomes a severe problem at such temperatures. 

The discovery of high-temperature superconductivity in mixed oxides, such as the lanthanum-barium-copper oxide complexes, has created a great deal of interest in these materials. Superconductivity, that is, the absence of any resistance to the flow of electric current, is now possible at temperatures above the temperature of liquid nitrogen (77K). Many problems remain in the development of practical processes for these materials and commercialization is not likely to occur until these problems are solved. Among the several processing techniques now used, CVD appears one of the most successful. 

Consider now NajW03 or LiTi204. One might expect to find W(V)-W(VI) and Ti(III)-Ti(lV) MMCT. However, in these compounds all metal ions are equivalent and the d electrons available are spread out in a conduction band. The bronzes NajW03 are metallic. They become superconducting at 6K, whereas LiTi204 becomes superconducting at even 13 K [59]. Here we meet the central problem of mixed-valence compounds [60] which we will postpone till Sect. 5. 

The fabrication of logic elements using such devices allows in principle the construction of a large capacity, compact, high-speed computer [50], Major problems with the technology are that large fan-out ratios are difficult to achieve and that superconducting circuits have a very low inherent impedance and so are difficult to couple with conventional elements at room temperature. 

Keywords effective theory, dense QCD, color superconductivity, sign problem... 

The authors of Ref. [12] reconsidered the problem of magnetic field in quark matter taking into account the rotated electromagnetism . They came to the conclusion that magnetic field can exist in superconducting quark matter in any case, although it does not form a quantized vortex lattice, because it obeys sourceless Maxwell equations and there is no Meissner effect. In our opinion this latter result is incorrect, since the equations for gauge fields were not taken into account and the boundary conditions were not posed correctly. 

From BCS theory it is known, that in order to form Cooper pairs at T = 0 in a dense Fermi system, the difference in the chemical potentials of the Fermions to be paired should not exceed the size of the gap. As previous calculations within this type of models have shown [24], there is a critical chemical potential for the occurrence of quark matter pf > 300 MeV and values of the gap in the region A < 150 MeV have been found. Therefore it is natural to consider the problem of the color superconducting (2SC) phase with the assumption, that quark matter is symmetric or very close to being symmetric (pu pd). 

To overcome these practical problems, research aimed at the preparation of new materials exhibiting superconductivity at higher and higher temperatures and able to sustain this state even in the presence of strong magnetic fields (or, also in the presence of high electric currents). 

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. 

---