Phase transitions in the Hubbard model and ionic Hubbard model at half-filling on the honeycomb lattice are investigated in the strong coupling perturbation theory which corresponds to an expansion in powers of the hopping t around the atomic limit. Within this formulation we find analytic expressions for the single-particle spectrum, whereby the calculation of the insulating gap is reduced to a simple root finding problem. This enables high precision determination of the insulating gap that does not require any extrapolation procedure. The critical value of Mott transition on the honeycomb lattice is obtained to be Uc ≈ 2.38t. Studying the ionic Hubbard model at the lowest order, we find two insulating states, one with Mott character at large U and another with single-particle gap character at large ionic potential, ∆. The present approach gives a critical gapless state at U = 2∆ at lowest order. By systematically improving on the perturbation expansion, the density of states around this critical gapless phase reduces.

We study the Anderson disordered Hubbard model on the honeycomb lattice. The Hubbard term is handled with strong-coupling perturbation theory which encodes the Mott transition physics into a rich dynamical structure of a local self-energy. The local nature of self-energy allows us to combine it with kernel polynomial method and transfer matrix methods. The locality of self-energy combined with the analytic nature of the strongcoupling perturbation theory enables us to study lattices with millions of sites. The transfer matrix method in the ribbon geometry is essentially free from finite size errors and allows us to perform a careful finite size scaling of the width of the ribbon. This finite size scaling enables us to rule out the possibility of metallic phase in between the Mott and Anderson insulating phases. We therefore find a direct transition between Anderson and Mott insulators when the disorder strength W is comparable to the Hubbard interaction U . For a fixed disorder W , we obtain an interaction dependent nonmonotonic behavior of the localization length which reflects interaction induced enhancement of the localization length for weak and intermediate interaction strengths. Eventually at strong interactions U , the Mott localization takes over and the localization length becomes comparable to the lattice scale. This is reminiscent of the holographic determination of the Mott state where the system at IR recognizes its UV lattice scale.

In the binary-alloy with composition AxB1−x of two atoms with ionic energy scales ±∆, an apparent Anderson insulator (AI) is obtained as a result of randomness in the position of atoms. Using our recently developed technique that combines the local self-energy from strong-coupling perturbation theory with the transfer matrix method, we are able to address the problem of adding a Hubbard U to the binary alloy problem for millions of lattice sites on the honeycomb lattice. By adding the Hubbard interaction U , the resulting AI phase will become metallic which in our formulation can be clearly attributed to the screening of disorder by Hubbard U . Upon further increase in U , again the AI phase emerges which can be understood in terms of the suppressed charge fluctuations due to residual Hubbard interaction of which the randomness takes advantage and localizes the quasi-particles of the metallic phase. The ultimate destiny of the system at very large U is to become a Mott insulator (MI). We construct the phase diagram of this model in the plane of (U, ∆) for various compositions x.

Integer quantum Hall (IQH) states and quantum anomalous Hall (QAH) states show the same static (dc) response but distinct dynamical (ac) response. In particular, the ac anomalous Hall conductivity profile σyx(ω) is sensitive to the band shape of QAH states. For example, dispersive QAH bands shows resonance profile without a sign change at the band gap while the IQH states shows the sign change resonance at the cyclotron energy. We argue by flattening the dispersive QAH bands, σyx(ω) should recover to that of flat Landau bands in IQH, thus it is necessary to know the origin of the sign change. Taking a topological lattice model with tunable bandwidth, we found that the origin of the sign change is not the band gap but the Van Hove singularity energy of the QAH bands. In the limit of small bandwidth, the flat QAH bands recovers σyx(ω) of the IQH Landau bands. Because of the Hall response, these topological bands exhibit giant polarization rotation and ellipticity in the reflected waves (Kerr effect) and rotation in the order of fine structure constant in the transmitted waves (Faraday effect) with profile resembles σyx(ω). Our results serve as a simple guide to optical characterization for topological flat bands.