Correlation effects in the triangular lattice single-band system Li_xNbO₂

Superconductivity in hole-doped Li_xNbO₂ has been reported with T_c≈5 K in the range 0.45⩽x<0.8. The electronic structure is based on a two-dimensional triangular Nb lattice. The strong trigonal crystal field results in a single Nb d_z² band isolated within a wide gap, leading to a single-band triangular lattice system. The isolated, partially filled band has a width W=1.5 eV with dominant second neighbor hopping. To identify possible correlation effects, we apply dynamical mean field theory and quantum Monte Carlo (QMC) using on-site Coulomb repulsion U=0–3 eV and check selected results using determinant QMC. For U as small as 1 eV, the single particle spectrum displays a robust lower Hubbard band, suggesting the importance of correlation effects in Li_xNbO₂ even for U⩽W. At half-filling (x=0), a Mott transition occurs at U_c≈1.5 eV. Coupling between the O A_g phonon displacement and correlation effects is assessed.