Evidence for multiferroicity in TTF-CA organic molecular crystals

TL;DR

This paper demonstrates through ab-initio calculations that the organic crystal TTF-CA exhibits multiferroic behavior, with spontaneous ferroelectric and magnetic orderings driven by electronic and structural transitions.

Contribution

It provides the first theoretical evidence of multiferroicity in TTF-CA, combining ab-initio and Hubbard model analyses to elucidate the microscopic mechanisms.

Findings

TTF-CA exhibits simultaneous ferroelectric and magnetic orderings.

Magnetic ordering enhances and can reverse the ferroelectric polarization.

The multiferroic ground state is confirmed by both ab-initio and Hubbard model calculations.

Abstract

We show by means of ab-initio calculations that the organic molecular crystal TTF-CA is multiferroic: it has an instability to develop spontaneously both ferroelectric and magnetic ordering. Ferroelectricity is driven by a Peierls transition of the TTF-CA in its ionic state. Subsequent antiferromagnetic ordering strongly enhances the opposing electronic contribution to the polarization: it is so large that it switches the direction of the total ferroelectric moment. Within an extended Hubbard model we capture the essence of the electronic interactions in TTF-CA, confirm the presence of a multiferroic groundstate and clarify how this state develops microscopically.