Dynamical Cluster Dual Fermion expansion

The dual fermion series is a diagrammatic approach for correlated lattice models that includes non-perturbative local and perturbative non-local dynamic correlations. Unfortunately, strong non-local fluctuations, such as those present near half filling in the two-dimensional Hubbard model, render the local approximation less suitable as a starting point. In this talk we present a cluster dual fermion method based on an expansion around the dynamical cluster approximation. Unlike previous formulations, our method does not employ a coarse graining approximation to the interaction, which we show to be the leading source of error at high temperature, and converges to the exact result independent of the size of the underlying cluster.