We present the finite-difference parquet method that greatly improves the applicability and accuracy of two-particle correlation approaches to interacting electron systems. This method incorporates the nonperturbative local physics from a reference solution and builds all parquet diagrams while circumventing potentially divergent irreducible vertices. Its unbiased treatment of different fluctuations is crucial for reproducing the strong-coupling pseudogap in the underdoped Hubbard model, consistent with diagrammatic Monte Carlo calculations. We reveal a strong-coupling spin-fluctuation mechanism of the pseudogap with decisive vertex corrections that encode the enhanced, energy-dependent scattering amplitude between electrons and antiferromagnetic spin fluctuations.
@article{arxiv.2505.20116,
title = {The finite-difference parquet method: Enhanced electron-paramagnon scattering opens a pseudogap},
author = {Jae-Mo Lihm and Dominik Kiese and Seung-Sup B. Lee and Fabian B. Kugler},
journal= {arXiv preprint arXiv:2505.20116},
year = {2026}
}