The origin of superconductivity in nominally ‘undoped’ T′-La2−xYxCuO4 films

We have systematically studied the transport properties of La2−xYxCuO4 (LYCO) films with T′-phase (0.05≤x≤0.30). In this nominally ‘undoped’ system, superconductivity was acquired in a certain Y doping range (0.10≤x≤0.20). Measurements of resistivity, Hall coefficient in normal states and resistive critical field (Hc2ρ) in superconducting states of the T′-LYCO films show similar behavior as the known Ce-doped n-type cuprate superconductors, indicating the intrinsic electron doping nature. The charge carriers are thought to be induced by oxygen deficiency. Non-superconducting Y-doped Pr- or Nd-based T′-phase cuprate films were also investigated for comparison, suggesting a crucial role of the radii of A-site cations in the origin of superconductivity in the nominally ‘undoped’ cuprates. A reasonable scenario is put forward for the microscopic reduction process to explain the experimental observations.