Properties of infinite-layer and T′-phase electron-doped copper-oxide superconductors

Abstract

An overview of recent research on electron-doped infinite-layer compounds (e.g., Sr_1−x La_xCuO₂) and electron-doped T′-phase compounds (e.g., Ns_2−x Ce_xCuO₄) is presented. Studies of steric effects indicate that superconductivity disappears in both systems for values of the in-plane lattice constant below a critical values a _cr ≈3.92 Å. Data from the Nd_1−yY_y)_2−xCe_xCuO₄ system confirm that steric effects are responsible for the loss of superconductivity (i.e., not magnetic interactions, as has been recently proposed). Previous Mössbauer work has emphasized the presence of a distribution of antiferromagnetic clusters in Nd−Ce−Cu−O, as well as the strong 2-D nature of the magnetism in that system; thus, we have modelled our anisotropic resistivity data (Montgomery method) using a spin-fluctuation scattering model. The extracted parameters indicate a gap to spin excitations (Δ≈3.4 meV), reasonable scattering lengths (l _‖≈200 Å; l _perp ≈4 Å), and a mass anisotropy m _⊥/m _‖≈570. A remarkably linear dependence of the extracted Fermi wavevector on the residual resistivity provides values of the impurity scattering cross section and impurity concentration. Also discussed are properties of La_2−x Y_xCuO₄, the first T′-phase compound without a magnetic rare-earth element. Various other investigations of these two known electrondoped copper-oxide structures are presented or reviewed.