Microscopic origin of isotropic non-Heisenberg behavior in highly correlated systems

TL;DR

This paper investigates the microscopic origins of isotropic deviations from the Heisenberg model in correlated systems, highlighting the role of excited atomic non-Hund states which are not captured by traditional biquadratic operators.

Contribution

It demonstrates that non-Hund states significantly contribute to deviations from the Heisenberg model, challenging the adequacy of biquadratic operators in describing such effects.

Findings

Non-Hund states cause deviations comparable to biquadratic exchange.

Excited atomic states influence macroscopic properties of extended systems.

Biquadratic operators cannot fully describe isotropic deviations.

Abstract

We have reanalyzed the microscopic origin of the isotropic deviations that are observed from the energy spacings predicted by the HDVV Hamiltonian. Usually, a biquadratic spin operator is added to the HDVV Hamiltonian to account for such deviations. It is shown here that this operator cannot describe the effect of the excited atomic non-Hund states which brought the most important contribution to the deviations. For systems containing more than two magnetic centers, non-Hund states cause additional interactions that are of the same order of magnitude as the biquadratic exchange and should have significant effects on the macroscopic properties of extended systems.