Electron superconductors

We have hitherto discussed cuprate superconductors with holes as charge carriers. Cuprates of T structure (Fig. 7.16) such as Nd2-jcCe Cu04 and Nd2Cu04 F are electron superconductors. There is however some symmetry in the phase diagrams of the electron and hole superconductors (Maple, 1990) as can be seen from Fig. 7.27. 

Challenges to Established Theories, It is interesting to note that some theoreticians struggle with describing how superconductivity occurs at high temperatures in the newer, ceramic superconductors. This is understandable because the classic theory of superconductivity is tied to metals. Most ceranuc superconductors discovered to date incorporate distinctive layers of copper and oxygen atoms, One question posed by some researchers, Is the mechanism of high-temperature superconductivity the same in hole superconductors as it is in electron superconductors ... 

Researchers at the Brookhaven National Laboratory, in applying X-ray techniques to a cerium-doped electron superconductor developed at the University of Tokyo, found that the holes of a hole superconductor... 

Maple 1990 Tokura et al. 19896). The maximum Tc that these compounds exhibit is around 25 K. Even though these cuprates are formally considered to be electron superconductors, the exact nature of the charge carriers is not fully certain. Some holes could also be present in such materials. In many ways, electron-super-... 

The parent (A2B0CU-O4) cuprates are antiferromagnetic semiconductors which, on doping to A2-xBxCu-04, become superconductors where x is called the doping parameter. The replacement of Nd3 by Ce4+ produces an electron superconductor... 

A two dimensional realization of doping is obtained by taking the parent electron-superconductor and hole-superconductor to consist solely of bipolarons and holes respectively. See Fig. 8.6. This model is consistent with the observed volume-fraction Meissner effect since the Meissner effect can occur only in condensed areas. The dependency of the Meissner effect on doping parallels the parabolic dependency of To See Section 8.6.3. 

Electron superconductors, such as Nd2 Ce CuO, do not have oxygen vacancies and the formal oxidation state of copper in these compounds is less than 2. Conduction is by the electron current. 

Although several other types of exotic superconductors (e.g., organic superconductors, heavy-fermion f-electron superconductors, magnetically ordered superconductors, multinary rare-earth, actinide, and transition-metal superconductors) have been investigated intensely since 1986, the cuprate superconductors have received by far the most attention because the highest values of the superconducting critical temperature are found in this class of materials. Rare-earth and actinide elements are key constituents of many of the high-temperature cuprate superconductors and have played a prominent role in the development of the first and some of the more important cuprate superconductors. 

There are no known Yb-based heavy-electron superconductors. 

In section 4, we discuss various theories for heavy electron superconductors, including possible pairing via phonons, charge fluctuations, and spin fluctuations, and emphasize the importance that the normal state electronic structure will play in determining the correct theory of this most unusual phenomenon. 

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