Fulleride superconductors

Yildirim T, Barbedette L, Fischer JE, Lin CL, Robert J, Petit P, Palstra TTM (1996) vs carrier concentration in cubic fulleride superconductors. Phys Rev Lett 77 167-170... 

Holczer, K., Klein, O., Huang, S.-M., Kaner, R. B., Fu, K.-J., Whetten, R. L. Diederich, F. 1991 Alkali-fulleride superconductors synthesis, composition, and diamagnetic shielding. Science, Wash. 252, 1154-1157. 

Table 3. Superconducting critical temperatures (/Kelvin, at 1 bar) or (/Kelvin, at applied pressure P/kbar) for organic, intercalated graphite, (SN),, and fulleride superconductors (updated from [71] TCE is 1,1,2-trichloroethylene BCDE is 1-bromo-1,2-dichloroethylene BDCE is 2-bromo-l,l-dichloroethylene DBCE is 1,2-dibromo-1-chloroethylene, TBE is 1,1,2-tribromoethylene).

Probably the cleanest measurement of an isotope effect in a fulleride superconductor was made on 99% enriched ving an exponent of a =... 

Ho91a K. Holczer et ai, Alkali-Fulleride Superconductors Synthesis, Composition, and Dia- Mu92... 

Alkali-Fulleride Superconductors Synthesis, Composition and Diamagnetic Shielding... 

Fullerene, black and shiny like graphite, is the subject of active current research because of its interesting electronic properties. When allowed to react with rubidium metal, a superconducting material called rubidium fulleride, Rb3C6o, is formed. (We ll discuss superconductors in more detail in Section 21.6.) Carbon nanotubes are being studied for use as fibers in the structural composites used to make golf clubs, bicycle frames, boats, and airplanes. On a mass basis, nanotubes are up to ten times as strong as steel. 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

The copper oxide ceramic superconductors are two-dimensional conductors They can conduct a current parallel to the layers of Cu and O atoms but not between the layers. The fullerides, by contrast, are three-dimensional conductors that conduct equally in all directions. Because of this property, they may prove to be superior materials for making superconducting wires. 

Table 12.3 summarizes many known organic superconductors [26,29] and a few other superconductors the stage-1 intercalated graphite KCs, polymeric (SN)X, and superconducting fullerides. 

When Cgo is treated with fluorine at elevated temperature, the reaction eventually leads to a product having the formula CToFeo. although fluorination under other conditions can lead to QoF - It is possible to reduce Ceo to produce negative ions known as fullerides. This can be accomplished electrochemically or by reactions with vapors of alkali metals. These compounds have the general formula M C60, and some compounds of this type are superconductors. It has also been possible to attach organic groups to the carbon atoms. 

The coherence length in the organic superconductors is of the order of a lattice constant (as it is for the ceramic oxide superconductors). The organic superconductors are of type II (they have two critical fields). The dimensionality is between 1 and 2 (it is 3-D for the fullerides). For organic superconductors, the isotope effect results are unclear it is not certain which phonon modes are important for the superconductivity. 

In this chapter on chemically modified fullerenes, I will discuss only a few representative molecules which incorporate fullerenes, and focus attention on the best-characterized compounds. A comprehensive review of chemical reactions with fullerenes has just been published by Taylor and Walton.[Ta93] It is not appropriate here to attempt a synthesis of the emerging field of the chemical properties of the fullerenes. Covered elsewhere in this book are ioni-cally bonded fullerides, such as the superconductor KsCeo (Chapter IV) and endohedral complexes, in which various atoms are captured inside the hollow carbon shell (Chapter VI). 

A range of metals, alloys and metallic compounds are Type I superconductors (Table 28.1). However, to put the practical limitations of working with the materials listed in Table 28.1 (and others including the superconducting fullerides described in Section 14.4) into perspective, we must compare the values of with the boiling points of available coolants, e.g. liquid He (4.2 K), H2 (20.1 K) and N2 (77 K). The low values of limit the possible applications of these materials, and illustrate why the so-called high-temperature superconductors described below are more important than superconducting metals and alloys. 

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