Superconducting lattice constants

Fig. 13. Dependence of Tc for various MsCao and Ms-iMi-Ceo compounds on the lattice constant a. Also included on the figure are data for superconducting samples under pressure [44].

The intent of this chapter is twofold. One is to give brief structural descriptions of many of the copper-oxide superconductors. For in-depth information, the reader is referred to the original publications. The second is to provide detailed crystallographic information (lattice constants, positional and thermal parameters, space groups, etc.), and compositional data on many of the superconductors discussed. Also, calculated x-ray powder diffraction patterns for these same compounds are tabulated. It is hoped that such information will prove useful to the superconductivity researcher. 

Superconductivity in ScNi2B2C is also based on a metastable phase (Ku et al., 1994) and single-phase samples could not be prepared. Tomilo et al. (2001b see also 1999, 2001a) found two tetragonal phases in their samples with rather different values of the lattice constants a and c and unit cell volume V (phase 1 ... 

Table 1. Superconducting properties of the samples label, nominal composition, lattice constants a and c, onset of superconducting transition as measured by susceptibility (1-3) or deduced from lattice parameters6 (4-5), isotropic shift CT so and fidl width at half maximum A...

The compound Ceo is not itself a superconductor, but when alkali metals are added it becomes superconducting. The doped compound forms a face-centered cubic lattice with a lattice constant of 10.04 A, and this structure has two tetrahedral holes and one octahedral hole per Ceo molecule. If all of these holes are occupied by alkali metals A, the resulting compound is A Ceo- An example of such a compound is K2RbC6o with potassium in the smaller tetrahedral holes and rubidium in the larger octahedral holes. The transition temperatures of these doped fullerenes range from 19 to 47 K. The compound RbsCeo was found to have an isotope effect exponent a = 0.37, somewhat less than the BCS value 0.5. 

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. 

Figure 5 Superconducting transition temperature as a function of the lattice parameter a for compounds with fee and simple cubic structures. The lattice constant was varied by changing the composition and by applying external pressure [33],...

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