Polaron spectra and edge singularities for correlated flat bands

Single- and two-particle spectra of a single immobile impurity immersed in a fermionic bath can be computed exactly and are characterized by divergent power laws (edge singularities). Here we present the leading lattice correction to this canonical problem by embedding both impurity and bath fermions in bands with nonvanishing Bloch band geometry, with the impurity band being flat. By analyzing generic Feynman diagrams, we pinpoint how the band geometry reduces the effective interaction which enters the power laws; we find that for weak lattice effects or small Fermi momenta, the leading correction is proportional to the Fermi energy times the sum of the quantum metrics of the bands. When only the bath fermion geometry is important, the results can be extended to large Fermi momenta and strong lattice effects and cross-validated by analysis of S-matrix eigenvalues. We numerically illustrate our results on the Lieb lattice and draw connections to various spectroscopy experiments.