Dynamical density-density correlations in one-dimensional Mott insulators

The dynamical density-density correlation function is calculated for the one-dimensional, half-filled Hubbard model extended with nearest-neighbor repulsion using the Lánczos algorithm for finite-size systems and analytically for large on-site repulsion compared to hopping amplitudes. At the zone boundary an excitonic feature exists for any finite nearest-neighbor repulsion and exhausts most of the spectral weight, even for parameters where no exciton is visible at zero momentum.