Preparation of undoped superconducting T'-\textit{RE}$_{2}$CuO$_{4}$ by MBE with ex-situ post-reduction

It has recently been revealed that the optimally-reduced end-member compounds T'-\textit{RE}$_{2}$CuO$_{4}$ (\textit{RE} = Pr, Nd, Sm, Eu, Gd) show superconductivity with $T_{c}$ over 30K [1] although they are commonly believed as Mott insulators. The superconducting specimens were produced by metal organic decomposition (MOD) with elaborated reduction procedures, where the advantage of thin-films, large surface-to-volume ratio, is fully utilized to achieve the optimal oxygen configuration. Their single-crystalline thin films prepared by UHV-based process may have a further advantage of providing a good opportunity for elucidating the highly important controversy in these materials using powerful but surface-sensitive probes such as ARPES and STM. With this motivation in mind, we grew 100-nm-thick T'-\textit{RE}$_{2}$CuO$_{4}$ (\textit{RE} = Pr, Nd, Sm) films on SrTiO$_{3}$ substrates by MBE. The as-grown films were semiconducting. However, with a post-reduction treatment in a tubular furnace, which is essentially identical to that for the MOD films, the MBE films became metallic and showed superconductivity at $\sim $ 30K, indicating that the superconducting films are potentially obtainable through in-vacuo process by tuning up the reduction conditions. [1] O. Matsumoto et al., Physica C 468 (2008) 1148; M. Naito et al., J. Phys: Conf. Ser. 108 (2008) 012037.