Bond excitations in the pseudogap phase of the Hubbard Model

TL;DR

This paper investigates bond excitations in the pseudogap phase of the 2D Hubbard model, revealing quantum criticality, bond order formation, and bond singlet dimerization through calculations of correlation functions.

Contribution

It provides the first evidence of diverging bond susceptibility and bond order in the pseudogap phase using the dynamical cluster approximation.

Findings

Bond susceptibility diverges at T=0 in the pseudogap region.

Bond singlet dimerization is suggested by strong correlations between neighboring bonds.

A quantum critical point separates the pseudogap phase from a Fermi liquid.

Abstract

Using the dynamical cluster approximation, we calculate the correlation functions associated with the nearest neighbor bond operator which measure the z component of the spin exchange in the two-dimensional Hubbard model with $U$ equal to the bandwidth. We find that in the pseudogap region, the local bond susceptibility diverges at T=0. This shows the existence of degenerate bond spin excitation and implies quantum criticality and bond order formation when long range correlations are considered. The strong correlation between excitations on parallel neighboring bonds suggests bond singlet dimerization. The suppression of divergence for $n< \approx 0.78$ implies that tor these model parameters this is quantum critical point which separates the unconventional pseudogap region characterized by bond order from a conventional Fermi liquid.