Nd2 xCexCuO4 Research Articles

Crystal and Defect Structure of Nd1.9Ce0.1CuO4±y

The crystal and defect structure of neodymium cerium cu‐prate (Nd1.9 Ce0.1 CuO4±y) has been studied as a function of temperature (1200–1500 K) and oxygen pressure (100–103 atm) via X‐ray diffractometry, powder neutron diffraction, and thermogravimetry methods. The oxygen nonstoichiom‐etry changes from oxygen‐excess (positive y values) to oxygen‐ deficient oxides (negative y values). The absolute values of oxygen nonstoichiometry y are calculated from the neu‐tron diffraction and thermogravimetry data. The materials adopt the tetragonal structure (space group I4/mmm). A defect model is proposed for Nd2‐x Cex CuO4±y. It is con‐cluded that oxygen vacancies VÖ (which occupy O(1) crys‐tallographic positions (in planes)) and interstitial oxygen O(i (which occupy O(3) “apical” positions) are present simultaneously. Gaseous oxygen intercalates into the Nd1.9 Ce0.1 CuO4±y crystals and replaces oxygen vacancies in the O(1) position and interstitial oxygen atoms in the O(3) positions. The theoretical model and the experimental data are in good agreement with each other, and the equilibrium constants of defect formation have been calculated.

Growth and anisotropic superconducting properties of Nd2–xCexCu04–y single crystals

ELECTRON-DOPED copper oxide superconductors have recently been discovered in the system Ln2–xCexCuO4–y, where Ln is Pr, Nd or Sin1,2. Here we report the growth of single crystals of Nd2–xCexCuO4–y, and measurements of their anisotropic superconducting properties. A sharp superconducting transition is observed at 22.5 K in Nd1.84Ce0.16CuO4–y. The resistivity shows a metallic temperature dependence both parallel and perpendicular to the basal plane. Applying a magnetic field perpendicular to the basal plane causes a parallel shift of the resistive transition curve to lower temperatures, as in conventional type II superconductors but unlike the hole-doped copper oxide superconductors. The Ginzburg–Landau coherence lengths estimated from the resistively defined upper critical magnetic field are 70.2 A in the basal plane and 3.4 A along the c axis, showing an anisotropy factor of 21. This value is much larger than that in La2–xSrxCuO4–y despite their apparent structural similarities. Crystallographic differences from the hole-doped systems and the concomitant changes in electronic properties may offer clues to the role of the Cu–O planes in the microscopic pairing mechanism of high-temperature superconductors.

Nature of the charge carriers in electron-doped copper oxide superconductors

The charge carriers in all of the well-known layered copper oxide high-temperature superconductors1– are holes located in the CuO2planes, the common structural element of these systems. Recently, Tokura et al.5 have reported that doping of Ln2CuO4, where Ln represents Pr, Nd or Sm, with formally tetravalent Ce can produce a high-temperature superconductor with electron carriers. To determine the character of the carrier states in Nd2–xCexCuO4, we have performed X-ray absorption measurements at the Cu K and Ce L3 edges for several different Ce concentrations. We find clear evidence that the electrons introduced by doping fill 3d holes on the Cu atoms, thus converting some Cu2+ ions to Cu+. Hence the nature of the states nearest to the Fermi energy (that is, those responsible for conduction) is qualitatively different from that in the hole-doped superconductors, in which the holes move in a band formed predominantly from O 2p states8–10. As a result, the details of the electron-pairing mechanism could be very different in the two systems11.