Correlation Effects in the Triangular Lattice Single-band System Li_xNbO_2

TL;DR

This study investigates correlation effects in a single-band triangular lattice superconductor Li_xNbO_2, revealing a Mott transition at moderate Coulomb interaction and highlighting the significance of electronic correlations even at low U values.

Contribution

It provides the first detailed analysis of correlation effects and Mott transition in Li_xNbO_2 using DMFT and QMC methods, emphasizing the role of electron-electron interactions.

Findings

Robust lower Hubbard band observed at U as low as 1 eV.

Mott transition occurs at U_c ~ 1.5 eV at half-filling.

Correlation effects are significant even for U less than the bandwidth.

Abstract

Superconductivity in hole-doped Li_xNbO_2 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(z^2) 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 DMFT(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_xNbO_2 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.