Electron-doped superconductors

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. 

However, at this point we should note that the electron-doped superconductors are not strictly single phase. Electron diffraction studies on both single crystals and ceramic superconducting samples have shown that they almost invariably, contain two phases one phase has the undistorted T -Nd2Cu04 structure,... 

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... 

At the time of writing it still remains unclear whether or not the ab-plane response of the electron-doped superconductor Nd2 ,CeyCu04 is also characterized by residual losses. Microwave measurements seem to indicate conventional electrodynamic response consistent with an s-wave gap (Wu et al. 1993). But this assertion was not supported by a FIR transmission experiment which did not show any of the expected features of the s-wave BCS superconductor (Choi et al. 1996). The conclusion of an isotropic BCS gap with a magnitude between 4.1 and 4.9A 7 (Stadlober et al. 1995) was inferred from Raman... 

The La2-xMj Cu04 and RBa2Cu307 5 compounds are hole-doped superconductors, as are most of the high-Tc cuprates. The first system of electron-doped cuprate superconductors discovered was Ln2-.TMxCu04 y (Ln=Pr, Nd, Sm, Eu M = Ce, Th 2i 0.1-0.18 y 0.02) (Tokura et al. 1989, Markert and Maple 1989, Markert et al. 1989b, Early et al. 1989). The highest values of Tc found among the electron-doped superconductors are 25 K. 

On the other hand, microwave penetration depth (Anlage et al. 1994) and Raman spectroscopy measurements (Stadloper et al. 1995) on the electron-doped superconductor Ndi 85Ceo.i5Cu04-, indicate that the superconducting order parameter of this material has s-wave symmetry. 

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