Energy scales and quasi-particle behavior of fermions in the normal state of flat-band systems*

We explore the energy scales and quasi-particle behavior in the paramagnetic normal state of the repulsive Hubbard model on the Lieb lattice. The special geometry of the Lieb lattice, a face-centered 2D square lattice, has a flat dispersion, which leads to various novel electronic phases with the inclusion of many-body interactions [1, 2]. Singularity in the density of states can lead to the breakdown of quasi-particle behavior at finite temperature. The interplay of the singularity and the interaction determine the energy scale to observe such non-Fermi liquid behavior. We address this using dynamical mean field theory (DMFT), a very well-established methodology for correlated many-body systems. We have used continuous-time quantum Monte Carlo (CTQMC), an exact impurity solver, within DMFT. We also comment on particle-hole mapping between attractive and repulsive Hubbard model with a unitary transformation and the corresponding electronic properties.
[1] Temperature and doping induced instabilities of the repulsive Hubbard model on Lieb lattice, Physical Review B 96, 245127 (2017)
[2] Spin-imbalanced pairing and Fermi surface deformation in flat bands, Phys. Rev. B 97, 214503 (2018)