Nodeless pairing in superconducting copper-oxide monolayer films on BiSrCaCuO.

The pairing mechanism of high-temperature superconductivity in cuprates remains the biggest unresolved mystery in condensed matter physics. To solve the problem, one of the most effective approaches is to investigate directly the superconducting CuO layers. Here, by growing CuO monolayer films on BiSrCaCuO substrates, we identify two distinct and spatially separated energy gaps centered at the Fermi energy, a smaller U-like gap and a larger V-like gap on the films, and study their interactions with alien atoms by low-temperature scanning tunneling microscopy. The newly discovered U-like gap exhibits strong phase coherence and is immune to scattering by K, Cs and Ag atoms, suggesting its nature as a nodeless superconducting gap in the CuO layers, whereas the V-like gap agrees with the well-known pseudogap state in the underdoped regime. Our results support an s-wave superconductivity in BiSrCaCuO, which, we propose, originates from the modulation-doping resultant two-dimensional hole liquid confined in the CuO layers. [ABSTRACT FROM AUTHOR]