Meissner fraction

In one sample, the crossover from the metallic (Pauli paramagnetic) region to the diamagnetic state occurred at 32 K. The diamagnetism measured was rather weak, on the order of 1% Meissner fraction, as compared to a pure superconductor (-1/47T, the full Meissner effect). 

The reported Meissner fraction was an important property to follow during the early days of superconductivity research. The magnitude of this diamagnetic effect, when properly measured, gives a good indication of the homogeneous nature of the materials being prepared in different laboratories. The first reported values were low, 2 to 15%, but as the composition, structure, and synthetic conditions became better known, the values increased to 70-80%— indicative of bulk superconductivity in an essentially pure product. 

Discovery of the 90+ K Superconductor "Paul" Chu and coworkers at the University of Houston (during October 1986) carried out the synthesis of (La1.xBax)CuOs.y (Type I) and (La1.xBax)2 Cu04.y (Type II) compounds and isolated superconducting phases exhibiting a sharp decrease in resistivity at 32 K. The best materials, however, showed only a 2% Meissner fraction. By applying pressure to one such product, their forte in superconductor research, they observed an increase in transition temperature of 8 degrees at 14 kbar pressure (see Figure 29). Chu, et al., submitted (156) these results to Physical Review Letters on 15 December 1986, and the publication appeared in the January 26, 1987 issue. 

The purpose of this chapter is not to address the continuing controversy about the electronic nature of the electron-doped superconductors, but rather to review the crystal chemistry of the T -Nd2Cu04 system and give practical details on how to prepare crystallographically pure electron-doped superconductors in ceramic or single-crystal form with high Meissner fractions. 

At present it is not known whether the superlattice or the undistorted T phase or both are responsible for superconductivity, although there are reports that single crystals of Nd185Ce015CuO4 with high Meissner fractions (>60%) do not contain any superlattice (28). 

The two-phase nature of all ceramic samples of the electron-doped superconductors probably accounts for their less desirable properties, including low Meissner fractions in many samples, low and nonmetallic conductivity in the normal state, and... 

If the entire body is superconducting, the Meissner fraction will be reduced due to flux pinning (40). How can one determine whether a low Meissner fraction is due to flux pinning and/or due to a sample problem like superconducting "shells" One approach... 

The bulk-phase superconductors have a transition temperature of 70-85 K with a 20% or higher diamagnetic Meissner fraction. The lattice parameters are a = 5.37 k,b = 5.42 A, and c = 15.79 A for orthorhombic, superconducting PbjSrjYo.sCao.sCujOg. 

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