Fluctuation diagnostic of the nodal/antinodal dichotomy in the Hubbard model at weak coupling: a parquet dual fermion approach*

We apply a dual fermion parquet scheme to the half-filled Hubbard model on a square lattice at small interaction. Our results establish that in this regime nonlocal vertex corrections play an important role in the formation of the pseudogap. Namely, in comparison to the simpler ladder approximation, these additional vertex corrections included in the parquet equations enhance the coupling of spin fluctuations with the quasiparticles. The pseudogap thus opens already
at a higher temperature, in quantitative agreement with the numerically exact diagrammatic Monte Carlo. We perform a fluctuation diagnostic of the self-energy, which implies that nodal and antinodal fermions are affected equally by spin fluctuations with the exact commensurate nesting vector (π, π). However, the antinode couples more efficiently to incommensurate fluctuations than the node, leading to the nodal/antinodal dichotomy.