TDAE-\c60: Can a Mott insulator be a ferromagnet?

TL;DR

This paper investigates the magnetic and orbital properties of TDAE- extc{60} by deriving a superexchange Hamiltonian, analyzing its phase diagram, and predicting magnetic transition temperatures in the Mott-insulating phase.

Contribution

It introduces a multicomponent superexchange model for TDAE- extc{60} and explores its phase diagram, including SU(4) and ferromagnet-antiferromagnet points, with predictions for magnetic ordering temperatures.

Findings

Identification of SU(4) antiferromagnetic point.

Derivation of Curie temperature for ferromagnetic regime.

Prediction of Néel temperature for isospin ordering.

Abstract

Motivated by the structure of TDAE-\c60, we derive a multicomponent superexchange Hamiltonian for the spins and orbital (``isospin'') degrees of freedom in the Mott-insulating phase. We explore its phase diagram and identify points of special interest: an SU(4) antiferromagnet (solved by Sutherland) and two ferromagnet$\otimes$antiferromagnet points where the ground state is known. For the ferromagnetic regime, we apply interchain mean field theory and derive an expression for the Curie temperature where spin ordering occurs and a lower Ne{\'e}l temperature for a conjectured isospin ordering.