# Exact diagonalization study of the Hubbard-parametrized four-spin ring exchange model on a square lattice

## TL;DR

This study uses exact diagonalization to explore how four-spin ring exchange influences quantum phase transitions and spin correlations in a square lattice Hubbard model with antiferromagnetic interactions.

## Contribution

It provides the first detailed numerical analysis of the impact of ring exchange on the phase diagram and excitation spectrum of the Hubbard-parametrized four-spin model.

## Key findings

- Ring exchange induces a quantum phase transition between different magnetic orders.
- Quantum fluctuations lower the critical ring exchange ratio from mean field predictions.
- A pseudo-continuum appears at the quantum critical point in the dynamical correlation function.

## Abstract

We have used exact numerical diagonalization to study the excitation spectrum and the dynamic spin correlations in the $s=1/2$ next-next-nearest neighbor Heisenberg antiferromagnet on the square lattice, with additional 4-spin ring exchange from higher order terms in the Hubbard expansion. We have varied the ratio between Hubbard model parameters, $t/U$, to obtain different relative strengths of the exchange parameters, while keeping electrons localized. The Hubbard model parameters have been parametrized via an effective ring exchange coupling, $J_r$, which have been varied between 0$J$ and 1.5$J$. We find that ring exchange induces a quantum phase transition from the $(\pi, \pi)$ ordered Ne\`el state to a $(\pi/2, \pi/2)$ ordered state. This quantum critical point is reduced by quantum fluctuations from its mean field value of $J_r/J = 2$ to a value of $\sim 1.1$. At the quantum critical point, the dynamical correlation function shows a pseudo-continuum at $q$-values between the two competing ordering vectors.