Two-dimensional superconductivity in single-band correlated 2H -type NbO2 layers

The oxide superconductor ${\mathrm{Li}}_{1\text{\ensuremath{-}}x}\mathrm{Nb}{\mathrm{O}}_{2}$ has two-dimensional (2D) $\mathrm{Nb}{\mathrm{O}}_{2}$ layers consisting of edge-shared triangular prisms. This structure is a unique oxide analog to $2H$-type transition-metal dichalcogenides (TMDs), yet its electronic properties have not received any significant attention. Using ${\mathrm{Li}}_{1\text{\ensuremath{-}}x}\mathrm{Nb}{\mathrm{O}}_{2}$ epitaxial films, we find the 2D superconductivity associated with the Berezinskii-Kosterlitz-Thouless transition and a large anisotropy of the upper critical field. The temperature-independent anisotropy strongly suggests single-band superconductivity, in contrast to TMDs. Correspondingly, the largely isolated single ${d}_{z2}$ state revealed by synchrotron-radiation photoelectron spectroscopy and density functional theory is explained in terms of strong ligand field splitting. These results indicate that superconductivity in ${\mathrm{Li}}_{1\text{\ensuremath{-}}x}\mathrm{Nb}{\mathrm{O}}_{2}$ occurs in 2D $\mathrm{Nb}{\mathrm{O}}_{2}$ layers with a narrow and correlated single band, reminiscent of superconductivity in cuprates. Our study provides insight into the unconventional correlation in the Hubbard system with a 2D triangular lattice.