Theory of the density fluctuation spectrum of strongly correlated electrons

The density response function $N(\mathrm{q}, \omega )$ of the two-dimensional $t-J$ model is studied starting from a mixed-gauge formulation of the slave-boson approach. Our results for $N(\mathrm{q}, \omega )$ are in remarkable agreement with exact diagonalization studies, and provide a natural explanation of the anomalous features in the density response in terms of the spin-polaron nature of the charge carriers. In particular, we have identified unexplained low-energy structures in the diagonalization data as arising from the coherent polaron motion of holes in a spin liquid.