Reversible strain induced electronic ferroelectricity in the triangular lattice organic Mott Insulators κ-(BEDT-TTF)₂Hg(SCN)₂X (X=Br, Cl)

The BEDT-TTF (BEDT-TTF = bis(ethylenedithio)tetrathiafuvalene) based Mott insulators of the form κ-(BEDT-TTF)₂Hg(SCN)₂X (X=Br,Cl) host a correlated electron system on a triangular lattice of (BEDT-TTF)₂⁺¹ dimers. Each dimer lattice site carries one hole and one S=1/2 spin which interacts antiferromagnetically. Anionic substitution has been shown to tune across a charge order transition, where unequal share of the hole between the molecules of a dimer site results in electronic ferroelectricity. We demonstrate successful tuning across the charge order transition via uniaxial strain. We use Raman scattering spectroscopy to study the charge state, collective dipole fluctuations, and lattice response to charge order. By applying a tensile strain of 0.4% along the b-axis of the quantum dipole liquid material κ-(BEDT-TTF)₂Hg(SCN)₂Br, we induce charge order at 39 K. We suppress charge order down to 10 K in κ-(BEDT-TTF)₂Hg(SCN)₂Cl by applying a tensile strain of 1.6% along the c-axis. We measure the emergence of vibronic molecular vibrations near the charge order transition, suggesting a renormalization of electronic and spin interactions. We identify the softening of collective dipole fluctuations through the charge order transition, consistent with electronic ferroelectricity. We demonstrate a reversible strain induced second-order ferroelectric phase transition, providing a promising outlook for fast-switching behavior in correlated systems.