In-situ Strain Control of Electronic Ferroelectricity in Triangular Lattice Mott Insulators κ-(BEDT-TTF)₂Hg(SCN)₂X (X=Br, Cl)

We demonstrate strain tuning of electronic ferroelectricity in organic Mott Insulators based on the BEDT-TTF molecule. In these materials, charge localized to (BEDT-TTF)₂ dimers can result in a ferroelectric “charge order” state without lattice distortion, where the charge distribution breaks inversion symmetry. In the case where inversion symmetry is preserved, S= 1/2 spins on a triangular lattice exhibit strong antiferromagnetic interactions between neighboring sites. In this work, we study two materials of this class which realize ground states on opposite sides of the phase border between a charge order insulator and an antiferromagnetic Mott insulator. We demonstrate strain control of the transition into and out of the ferroelectric charge order state using Raman scattering. By applying in-situ uniaxial strain, we are able to demonstrate both enhancement and suppression of the charge order transition, establishing strain control of the phase transition. For κ-(BEDT-TTF)₂Hg(SCN)₂Cl, we demonstrate enhancement of the charge order transition from 30 K at ambient conditions up to 55 K under a uniaxial strain of 0.53% along the b-axis, nearly doubling the transition temperature. For the quantum dipole liquid κ-(BEDT-TTF)₂Hg(SCN)₂Br, we implement biaxial strain, controlling one axis of strain in-situ, to explore strain engineering of electronic ferroelectrcity and spin liquid behavior.