Mechanism of superconductivity

The gap in superconductivity between the fifth and sixth groups of the periodic table, discovered by Matthias,24 is seen to correspond to the transition from crest to trough superconductivity. It does not require for its explanation the assumption20- 25 that there are mechanisms of superconductivity other than the electron-phonon interaction. 

The Mechanism of Superconductivity. A satisfactory theory of superconductivity was proposed in 1957 by the American physicists John... 

High-pressure experiments on this oxide superconductor suggested (83) that a re-entrant superconducting transition occurred between the grains of the superconducting particles. The authors further concluded that the mechanism of superconductivity in this oxide system remained unknown. Other resistance measurements were also carried out (84) using pressures of -125 kbar indicating a possible onset of metallic behavior in this material at room temperature. 

Although we remain uncertain of the mechanism of superconductivity in the high Tc materials, the suggestion of metastability remains. 

Many layer-perovskites with Cu as the B cation are superconductors at relatively high temperatures ( 100 K). Although the mechanism of superconductivity is not well understood, a necessary condition is that the oxidation state of the copper ion be around +2.2. In many compounds, such as the well-known YBa2Cu307 (63324) this is achieved naturally through the relaxation of... 

The mechanisms of superconductivity in ceramics are not fully understood, and most progress is still being made by trial and error. Perhaps once the secrets are unlocked, the many obstacles posed by ceramic superconductors... 

The open issues mentioned here should legitimate the assumption that this class of compounds will provide further substantial and general insight into mechanisms of superconductivity and its interplay with magnetism. 

These C-T salts are metallic at room temperature and are also two-dimensional. Upon lowering the temperature some of these salts become superconducting, provided that they can avoid the electronic instability toward a metal-insulator transition. The actual mechanism of superconductivity is not known at present. 

This study has been motivated by the recent discovery and investigations of a new family of superconductors metal-intercalated chloronitrides. For example, the compound Liu.48(THF)yHfNCl has arelatively high value of Tc 25K [l]-[5]. The mechanism of superconductivity for these materials had remained a puzzle. Indeed, according to theoretical calculations [6] the electron-phonon interaction is not sufficient to provide the observed value of Tc. Analysis of the data on heat capacity [2], based on the dependence 7 (1 + A), see [7], has led to a similar conclusion (7 is the Sommerfeld constant, A is the electron-phonon coupling constant). 

The superconducting ability of [M(dmit)2] complexes, see Fig. 9, has prompted experimental and theoretical vibrational studies of [Ni(dmit)2]z and [Pd(dmit)2]z complexes (z is in the range 0-2—), in order to understand the mechanism of superconductivity in terms of electron-intramolecular and electron-intermolecular vibrational couplings (14, 21). These studies have... 

High Temperature Superconductors and Materials and Mechanisms of Superconductivity, Eds. J. Muller and J.L. Olsen, North-Holland, Amsterdam, 1988, See also various papers in Proc. Int. Conf. M2HTSC, Grenoble, Physica, vol. 235-240 (1994). 

Another interesting application of the unsynchronized-resonating-covalent-bond theory of metals to superconductivity is the elucidation of the mechanism of superconductivity in the substances K3C60 and RbaCgO) for which superconducting transition temperatures of 19.3 K [122] and 28 K [123], respectively, have been found [124], The crystal structure of K3C60 was reported in 1991 [125]. The salient features of the structure are shown in Figure 12. 

In Bi-system, there exist a series of superconducting phases. Among them the Bisystem 2223 phase possesses relatively high Tc (Tc=110K). It is a prospective superconductor material in practical application of large electric current. Therefore, the mechanism of superconductivity and formation of the 2223 phase have been of interest for a long time. 

For the ceramic oxide superconductors the mechanism of superconductivity is difficult to explain by standard BCS theory. For the organic superconductors, the superconductivity mechanism is probably of the BCS type. 

In this review we shall focus on some of these new forms of solid carbon. The emphasis is on the physical properties of electrically conducting fullerides, fulleride polymers and nanotubes, but the neutral fullerene polymers, dimers and onion-like structures are also included for completeness. This paper is by no means a review of all important work in the domain. We fully realize that in choosing the material we had to be subjective and we selected material best known to us. A few other short reviews have been published recently on fullerene polymers on the optical properties of polymeric fullerenes [15] and on the physical properties of conducting fullerenes [16,17]. There are extensive recent reviews on the pressure and heat induced polymers [18]. We did not include in the paper the physical and chemical properties of alkali fullerides with variously charged monomer ions. These are the subject of other reviews and are described in detail in a recent monograph [19]. In particular, there are comprehensive reviews [20,21] on experiments and theories aimed at the understanding of the mechanism of superconductivity. 

Another experimental result which might elucidate directly the mechanism of superconductivity is the shift of transition temperature Tc with isotopic substitution. This phenomenon had provided the first suggestion for the role of phonons in superconductivity well before the birth of the BCS theory. If the mass M of all of the atoms in a sample is increased, the lattice vibration frequencies will all decrease as w Looking back to Eq. (1),... 

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