Electron-doped cuprate superconductors

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. 

All the cuprates described till now are hole superconductors. The nature of holes has been subject of considerable discussion (Chakraverty et al. 1988 Rao et al. 19896 Sarma Rao 1989). There has been no experimental evidence for the presence of Cum type species in the doped cuprates. Instead, there is considerable evidence from electron and X-ray spectroscopies for the presence of hybridized oxygen holes which can be represented as O-. The detailed description of the holes in terms of the d and p characters has been investigated (Bianconi 1990). Essentially, the mobile holes in the cuprates are present in the in-plane n band which has 0-2p character. The concentration of holes (in all but the T1 cuprates) are easily determined by iodometry or Fen-Fem titrations (Rao el al. 1991a Shafer Penney 1990). Since the Hall coefficients are temperature dependent, the chemical titration method becomes invaluable. 

Electron—hole asymmetry is encountered in cuprate superconductors.3 In the cuprates, superconductivity occurs in the electron-doped regime, although not as prominently as in the hole-doped regime. The electron-... 

Cuprate superconductors exhibit complicated phase diagrams which are functions of the doping parameter, x which controls the amount of the electron-transfer into or out of the cuprate plane. See for example Fig. 8.2. 

Studies on other high-temperature superconductors Positron annihilation measurements across Tc, coupled with the calculations of PDD have been carried out in a variety of hole-doped superconductors that include YBa2Cu40g [48], Bi-Sr-Ca-Cu-0 [49], and Tl-Ba-Ca-Cu-0 [50, 51] systems. We will not labor with the details here, except to state that a variety of temperature dependencies are seen and these can be rationalized when the results are analysed in terms of positron density distribution and the electron-positron overlap function [39]. These calculations show that the positron s sensitivity to the superconducting transition arises primarily from the ability to probe the Cu-O network in the Cu-0 layer. The different temperature dependencies of lifetime, i.e., both the increase and decrease, can be understood in terms of a model of local electron transfer from the planar oxygen atom to the apical oxygen atom, after taking into account the correct positron density distribution within the unit cell of the cuprate superconductor. 

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