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We describe XANES studies of electron-doped superconductors based on neodymium cuprate doped with cerium. The mechanism for observed changes in the transition temperature on co-doping with divalent Sr is investigated using measurements of the Nd, Cu, Ce and Sr absorption edges.

Thermoelectric power (S) studies on a Nd1.82-xSrxCe0.18CuOy:x less than or equal to 0.18 superconducting system in the temperature range 35-250 K are reported here. In the x=0.09 sample, synthesized in the reduced environment, the small magnitude of S is highly metalliclike and its sign is negative, a characteristic of electron conduction. The sign of S for the x=0.18 sample shows a crossover below 75 K from negative to positive, in apparent conflict with electronic conduction. Interestingly, after oxygenation this sample exhibits a broadened but positive phonon draglike peak. This oxygenated sample shows overcompensation of the carrier (electron) concentration. Critical analysis of the data suggests that Sr doping seemingly causes a competition between electron- and holelike conduction. The slope dS/dT is, in general, negative suggesting that the main contribution is coming from the diffusive part. The observed thermopower features seem to fall in line with the theoretical curves of Durczewski and Ausloos [Z. Phys. B 85, 59 (1991); Phys. Rev. B 53, 1762 (1996)] based on the inelastic scattering of quasifree electrons by phonons. (C) 1996

A direct evidence is obtained for the local oxygen non-stoichiometry in T ′-type Nd 2− x Ce x CuO 4−δ using a high resolution analytical SEM technique. A correlation between the oxygen non-stoichiometry and superconductivity is established. It is found that the oxygen content increases from the grain boundary towards the interior of a superconducting grain, suggesting a gradual fall in T c towards the core. Our results reveal that critical levels of carriers and oxygen vacancies are essential for the T ′-type cuprates to exhibit superconductivity.

The extent of solubility of Li + in monoclinic CuO and its effect on the crystal structure and electrical behaviour have been studied employing different experimental techniques. It is found that (i) the extent of lithium incorporation in CuO is limited to ≤ 25at%, (ii) the defect compounds, Cu 1−x Li x O 1− δ , are formed without altering the crystal structure and (iii) the resistivity decreases with increase in Li concentration. Further, it is observed that these compounds continue to exhibit semiconducting ( d ϱ dT ) behaviour.

We report a refined phase diagram for the electron-doped Nd2-xCexCuOy superconducting system in the range 0.120 less-than-or-equal-to x less-than-or-equal-to 0.190 with DELTAx=0.005. It is found that the highest T(c)(rho=0) approximately 17 K is confined to the very narrow range 0.145 less-than-or-equal-to x less-than-or-equal-to 0.150; a T(c onset) approximately 24 K with sharp superconducting transition is also observed. An attempt is made to understand the T(c) dependence on the variation of the oxygen content (y) in the Nd2-xCexCuOy system.

We describe EXAFS studies of electron-doped superconductors based on neodymium cuprate doped with cerium. The mechanism for observed changes in the transition temperature on co-doping with divalent cations is investigated using measurements of the Nd, Cu, Ce, Ba, Ca and Sr absorption edges. Changes in the close-neighbour cation-oxygen distances and the Debye-Waller factors are reported and correlated with the observed changes in Tc.

The revival of superconductivity in the electron-excess T'-type compound Nd 1.82 Ce 0.18 CuO 4-δ by hole doping has been observed. The normal-state resistivity (ρ) changes from semiconducting (dρ/dT 0) as the hole doping increases up to a certain level.