Mott transition on a triangular lattice

Abstract

We study the paramagnetic side of the phase diagram of the cobaltates, ${\text{Na}}_{x}{\text{CoO}}_{2}$, using an implementation of the cellular dynamical mean-field theory with a noncrossing approximation impurity solver for the one-band Hubbard model on a triangular lattice. At low doping we find that the low-energy physics is dominated by a quasi-dispersionless band generated by strong correlation physics. At half filling, we find a metal-insulator transition at a critical value of the on-site interaction ${U}_{c}=5.6\ifmmode\pm\else\textpm\fi{}0.15t$ which depends weakly on the cluster size. The onset of the metallic state occurs through the growth of a coherence peak at the chemical potential. Away from half filling, in the electron-doped regime, the system is metallic with a large continuous Fermi surface as seen experimentally. Upon hole doping, a quasi-non-dispersing band emerges at the top of the lower Hubbard band and controls the low-energy physics. This band is a clear signature of non-Fermi-liquid behavior and cannot be captured by any weakly coupled approach. This quasi-dispersionless band, which persists in a certain range of dopings, has been observed experimentally. We also investigate the pseudogap phenomenon in the context of a triangular lattice and propose a general framework for discussing the pseudogap problem. This framework involves a momentum-dependent characterization of the low-energy physics and links the appearance of the pseudogap to a reconstruction of the Fermi surface without invoking any long-range order or symmetry breaking. Within this framework we predict the existence of a pseudogap for the two-dimensional Hubbard model on a triangular lattice in the weakly hole-doped regime.