Singlet and triplet bipolarons on the triangular lattice

摘要

We study the Coulomb-Fr\"ohlich model on a triangular lattice, looking inparticular at states with angular momentum. We examine a simplified model ofcrab bipolarons with angular momentum by projecting onto the low energysubspace of the Coulomb-Fr\"ohlich model with large phonon frequency. Such aprojection is consistent with large long-range electron-phonon coupling andlarge repulsive Hubbard $U$. Significant differences are found between the bandstructure of singlet and triplet states: The triplet state (which has a flatband) is found to be significantly heavier than the singlet state (which hasmass similar to the polaron). We test whether the heavier triplet statespersist to lower electron-phonon coupling using continuous time quantum MonteCarlo (QMC) simulation. The triplet state is both heavier and larger,demonstrating that the heavier mass is due to quantum interference effects onthe motion. We also find that retardation effects reduce the differencesbetween singlet and triplet states, since they reintroduce second order termsin the hopping into the inverse effective mass.