Instanton gas approach to the Hubbard model

In this article we consider a path integral formulation of the Hubbard model based on a SU(2)-symmetrical Hubbard-Stratonovich transformation that couples auxiliary field to the local electronic density. This decoupling is known to have a regular saddle-point structure: each saddle point is a set of elementary field configurations localized in space and imaginary time which we coin instantons. We formulate a classical partition function for the instanton gas that has predictive power. Namely, we can predict the distribution of instantons and show that the instanton number is sharply defined in the thermodynamic limit, thus defining a unique dominant saddle point. Despite the fact that the instanton approach does not capture the magnetic transition inherent to the Hubbard model on the honeycomb lattice, we were able to describe the local moment formation accompanied by short-ranged anti-ferromagnetic correlations. This aspect is also seen in the single particle spectral function that shows clear signs of the upper and lower Hubbard bands. Our instanton approach bears remarkable similarities to local dynamical approaches, such as dynamical mean field theory, in the sense that it has the unique property of allowing for local moment formation without breaking the SU(2) spin symmetry. In contrast to local approaches, it captures short-ranged magnetic fluctuations. Furthermore, it also offers possibilities for systematic improvements by taking into account fluctuations around the dominant saddle point. Finally, we show that the saddle point structure depends upon the choice of lattice geometry. For the square lattice at half-filling, the saddle point structure reflects the itinerant to localized nature of the magnetism as a function of the coupling strength. The implications of our results for Lefschetz thimbles approaches to alleviate the sign problem are also discussed.