Systems electron-doped

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. 

More than 15 years after the discovery of high-Tc superconductivity in layered cuprates its mechanism is still under debate. This has to do with the asymmetry of physical properties between the electron-doped and hole-doped side of the complex phase diagram, temperature vs. doping, T(x), and with the fact that no consensus has been reached about the question what are the key experiments a theory of high-Tc superconductivity must be able to explain. In this paper we argue that the elementary excitations and their interdependence with spin excitations in the cuprates are of central interest in order to learn more about the correlations in general and, in particular, about the mechanism for Cooper-pairing in these systems. 

This always holds when the semiconductor is clean, without any added impurities. Such semiconductors are called intrinsic. The balance (4.126) can be changed by adding impurities that can selectively ionize to release electrons into the conduction band or holes into the valence band. Consider, for example, an arsenic impurity (with five valence electrons) in gennanium (four valence electrons). The arsenic impurity acts as an electron donor and tends to release an electron into the system conduction band. Similarly, a gallium impurity (three valence electrons) acts as an acceptor, and tends to take an electron out of the valence band. The overall system remains neutral, however now n p and the difference is balanced by the immobile ionized impurity centers that are randomly distributed in the system. We refer to the resulting systems as doped or extrinsic semiconductors and to the added impurities as dopants. Extrinsic semiconductors with excess electrons are called n-type. In these systems the negatively charged electrons constitute the majority carrier. Semiconductors in which holes are the majority carriers are calledp-type. 

YBa2Cu307 5 is around 97°, the exact value dependent on 8, and in the orthorhombic form of La, Sr Cu04 the octahedra are tilted with respect to each other. In the electron-doped systems the planes are unusual in that there are no axial oxygen atoms at all. There does not seem to be any correlation of with the Cu-O-Cu angle and thus superconductivity is found in both flat and distorted sheets. 

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