Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan.
Phys Rev Lett. 2018 Jun 22;120(25):257001. doi: 10.1103/PhysRevLett.120.257001.
In order to realize superconductivity in cuprates with the T^{'}-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T^{'}-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T^{'}-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting Nd_{2-x}Ce_{x}CuO_{4} (x=0, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO_{2} plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd_{2}CuO_{4} film is doped with electrons despite the absence of the Ce substitution.
为了在 T^{'}型结构的铜酸盐中实现超导性，不仅需要化学取代（Ce 掺杂），还需要后生长还原退火。然而，对于薄膜来说，经过精心设计的还原退火，即使没有 Ce 掺杂，也可以在 T^{'}型铜酸盐中诱导出超导性。为了揭示 Ce 未掺杂 T^{'}型铜酸盐超导性的起源，我们对超导和非超导 Nd_{2-x}Ce_{x}CuO_{4}（x=0、0.15 和 0.19）薄膜进行了体敏感硬 x 射线光电子能谱和软 x 射线吸收光谱研究。通过后生长退火，核心层谱表现出剧烈的变化，我们将其归因于 CuO_{2}平面中芯孔屏蔽的增强以及化学势随能带填充的变化而沿其移动。结果表明，尽管没有 Ce 取代，超导 Nd_{2}CuO_{4}薄膜仍掺杂有电子。
