The Mechanism of Superconductivity

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

Bardeen, Leon Cooper and John Robert Schrieffer, which, from their initials, was called BCS theory. 

The fundamental concept of the theory appears at first sight paradoxical electrons in the superconductor are mutually attracted forming so-called Cooper electron pairs . 

As the positive metal ions are heavier than the electron that caused the displacement, they return to their original positions fairly slowly. The relatively long time taken to re-establish the original positions allows the displaced metal ions to attract a second electron, which, given the proximity of the just passed first electron, leads to the formation of a Cooper pair. 

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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... 

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... 

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),... 

Our relaxation measurements in the normal state have possible implications for the mechanism of superconductivity. The ratio deriv from the... 

Insight into the changes in the intramolecular bonding of C, on intercalation, and the relevance of the various vibrational modes to the mechanism of superconductivity, can be obtained by comparing the low-temperature INS spectra of K,C,> and Cm,. The striking difference between the two systems is the redistribution of intensity that occurs between the radial and tangential modes. Carbon-60 has intense features up to — 110 meV, followed by many weaker and broader hands up to... 

We have ako charactciized the temperature dqien ncx A in both K3C50 and RbgC since thk can provide additional insight into the mechanism of superconductivity. Representative nmrmalizai conductance carves recorded on K-aOjo and theoretical fits to these data for 4.2 K < T < are shown in Fig. 3. 

For a further discussion of also symmetry aspects, possibly involved in the mechanism of superconductivity in oxide Cu VCu -solids, we refer to the literature [21,53,55],... 

To summarize, much more works, with taking various aspects into account, are needed to obtain a full understanding of the mechanism of superconductivity in sCeo. Even in the E (8> e and Tiu <8> hg model systems, setting aside the fullerides, the JT effect on superconductivity, especially in the presence of the Coulomb effect including the Hund s-rule coupling, is not known well. To some extent we shall address this issue in the model JT systems in Sect. 4 of this article. 

As the mechanism of superconductivity in these doped fullerites was not clear, the greatest attention was directed to the standard mechanism of superconductivity. The conventional theory is based on the electron-phonon interaction and generalized in the Eliashberg equations which include the Bardeen, Cooper and Schrieffer (BCS) equation as a weak-coupling limit [78]. For real superconductors the ratio for the weak-coupling limit is close to the value of 3.52, and for stronger coupling materials it increases up to about 5.1... 

The occurrence of holes predominately on oxygen is unprecedented in other metal oxide systems, and thus it is tempting to assume they play a key role in the mechanism of superconductivity in the cuprates. Both magnetic and phonon based mechanisms have been proposed that could attribute a... 

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