Fractional Hall physics from large $N$ interacting fermions

Abstract

We solve models of $N$ species of fermions in the lowest Landau level with $U(N)$-invariant interactions in the $N\gg 1$ limit. We find saddles of the second quantized path integral at fixed chemical potential corresponding to fractional Hall states with filling $ \frac{p}{q}$ where the integers $p$ and $q$ depend on the chemical potential and interactions. On a long torus there are $q$ such states related by translation symmetry, and $SU(N)$-invariant excitations of fractional charge. Remarkably, these saddles and their filling persist as extrema of the second-quantized action at $N=1$. Our construction gives a first-principles derivation of fractional Hall states from strongly interacting fermions.