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

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 Neèl 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.

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Cited by in corpus (1)

• Possible quantum paramagnetism in compressed Sr${}_2$IrO${}_4$