Undoped cuprate superconductors — band superconductors or oxygen-doped Mott-Hubbard superconductors?

Undoped cuprates have long been considered to be antiferromagnetic Mott insulators (charge-transfer insulators). In this article we report that one group of cuprates, namely RE₂CuO₄ with the Nd₂CuO₄ (T') structure, become superconducting without doping. Our discovery was achieved by metal-organic decomposition, an inexpensive and easy-to-implement thin-film process. The highest T_c of undoped T'-RE₂CuO₄ is over 30 K, substantially higher than their 'electron-doped' analogs. Remarkably, Gd₂CuO₄, even the derivatives of which have not shown superconductivity so far, gets superconducting with T_c^onset as high as 20 K. Our discovery contradicts with the past results supporting undoped cuprates to be generally insulating. The clue to understanding the sharp contrast between the past and our results is impurity oxygen (O_ap) at the apical site. The implication of our discovery is briefly discussed.