Growth and properties of thin films of the doped infinite-layer compounds Ca 1− xLn xCuO 2 (Ln=Sm, Nd, Y)

Abstract

The pulsed laser deposition (PLD) technique has been utilized for thin film synthesis of lanthanide-doped infinite-layer compounds Ca 1− x Ln x CuO 2 (Ln=Sm, Nd, and Y). The films are grown epitaxially on (100)-oriented SrTiO 3 substrates in a low pressure oxygen ambient with in situ monitoring using reflection high-energy electron diffraction (RHEED). For Sm 3+ and Nd 3+ substitutions, the infinite-layer structure of CaCuO 2 is retained for x≤0.15, with a small but systematic decrease in the c-axis parameter of the films being observed with increasing dopant concentration. For higher substitution levels, a defect structure analogous to Sr 14Cu 24O 41 is stabilized. Doping of the infinite-layer phase has also been achieved with Y 3+ substitution, however the phase transformation to the defect structure occurs at lower concentrations ( x<0.10). A significant decrease in the room-temperature resistivity, with metallic temperature dependence, is observed for optimally doped CaCuO 2 films after they are post-annealed in vacuum. Nevertheless, no evidence of superconductivity has been detected in any of the films down to 5 K. Possible reasons for the absence of superconductivity in the Ca 1- x Ln x CuO 2 films are discussed.