Topped With Copper-Oxide Films*

We have grown CuO thin films on Bi₂Sr₂CaCu₂O_8+x substrates by ozone-assisted molecular beam epitaxy and investigated the electronic structure by in-situ angle-resolved photoemission spectroscopy. We observe similar low-energy band structure compared as in Bi₂Sr₂CaCu₂O_8+x, while the wide energy spectrum is consistent with an insulating state,even with different amount of CuO. The superconducting gap structure fits well to the d_X²-Y² gap form. Our results suggest that the d-wave is robust in this system and that the reported U-like gap by STM is possibly a minority and “local” state, to which ARPES is not very sensitive.