# Limitations of spin-fermion models in studying underdoped cuprates

## TL;DR

This paper critically examines the spin-fermion model for underdoped cuprates, emphasizing the importance of strong coupling and local constraints, and proposes a refined model to better capture strong correlation physics.

## Contribution

It provides a microscopic foundation for the spin-fermion model and introduces a strong-coupling version that accounts for local no double occupancy constraints.

## Key findings

- Spin-fermion coupling enforces local no double occupancy.
- Strong coupling is essential for accurate physics of underdoped cuprates.
- Proposed a new strong-coupling spin-fermion model as a better starting point.

## Abstract

A microscopic basis is provided for the spin-fermion model used to describe the physics of the underdoped cuprates. In this way, a spin-fermion coupling is shown to take care of the local no double occupancy constraint, which is ignored in the weakly coupled regime. This ingredient is proved to be essential to capture the physics of strong correlations, however. We elaborate further on our recent proposal for the strong-coupling version of the spin-fermion model that may prove to be the correct starting point to deal with the strong correlation physics displayed by the underdoped cuprates.