Evidence for Electronically Driven Ferroelectricity in a Strongly Correlated Dimerized BEDT-TTF Molecular Conductor

Abstract

By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor κ-(BEDT-TTF)_{2}Hg(SCN)_{2}Cl, we provide evidence for order-disorder type electronic ferroelectricity that is driven by the charge order within the (BEDT-TTF)_{2} dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and nondimerized or weakly dimerized systems at 1/4 filling, exhibiting a charge order. Our results indicate that intradimer charge degrees of freedom are of particular importance in correlated κ-(BEDT-TTF)_{2}X salts and can create novel states, such as electronically driven multiferroicity or charge-order-induced quasi-one-dimensional spin liquids.

Low‐Frequency Charge Carrier Dynamics in Ferroelectric κ‐(BEDT‐TTF)2X – A Comparative Study of X = Cu[N(CN)2]Cl and X = Hg(SCN)2Cl

“…In contrast to κ ‐(ET) 2 Cu[N(CN) 2 ]Cl, for the ET‐based salt with X = Hg(SCN) 2 Cl (in short κ ‐Hg‐Cl) clear evidence for charge order was found by vibrational spectroscopy, however with a comparatively low value of charge disproportionation of in the insulating state, which is in reasonable agreement with an estimated size of (with Å being the intermolecular distance within the dimer) for the individual dipoles from measurements of the dielectric constant . In this salt, the charge localization on one molecule within the dimer, causing charge‐rich and charge‐poor sites, results in a metal‐insulator transition at K. The reason why charge order is more pronounced in this compound can be attributed to a weaker dimerization compared to other dimer‐Mott systems (as e.g., κ ‐Cl), as demonstrated by density functional theory calculations . Thus, the charge disproportionation results from strong on‐site Coulomb repulsion and intersite interaction parameters U and V , respectively, and the system is considered in between the extrem cases of strongly dimerized (half‐filled) Mott systems like κ ‐Cl and quarter‐filled charge‐ordered systems like some θ ‐phase compounds .…”

“…In summary, we have studied the quasi‐two‐dimensional organic charge‐transfer salts κ ‐(ET) 2 Cu[N(CN) 2 ]Cl and κ ‐(ET) 2 Hg(SCN) 2 Cl by measuring their resistance/conductance noise power spectral densities. κ ‐Hg‐Cl undergoes a first‐order charge‐ordering metal‐insulator transition at K accompanied by long‐range ferroelectric order . κ ‐Cl is an antiferromagnetic Mott insulator at low temperatures, where K coincides with a ferroelectric transition at , where in different samples both long‐range dipolar order and relaxor‐type ferroelectricity have been observed .…”

“…Now we turn to the system κ ‐(ET) 2 Hg(SCN) 2 Cl, where charge ordering accompanied by long‐range ferroelectric order is established . The resistance measurements of two samples of κ ‐(ET) 2 Hg(SCN) 2 Cl, shown in Figure a, were performed along the out‐of‐plane a axis in a 4‐point configuration.…”

“…Thus, the charge disproportionation results from strong on‐site Coulomb repulsion and intersite interaction parameters U and V , respectively, and the system is considered in between the extrem cases of strongly dimerized (half‐filled) Mott systems like κ ‐Cl and quarter‐filled charge‐ordered systems like some θ ‐phase compounds . Since the combination of dimerization and charge order results in a net dipole moment, the metal‐insulator transition in κ ‐Hg‐Cl is accompanied by long‐range ferroelectric order of order‐disorder type at K as recently demonstrated by measurements of the dielectric constant . Although such a new mechanism referred to as electronic ferroelectricity opens the possibility for multiferroicity, measurements of ESR and specific heat in κ ‐Hg‐Cl have shown no signatures of magnetic order, and a charge order‐induced quasi‐1D spin‐liquid state is suggested instead …”

Melting of charge order in the low-temperature state of an electronic ferroelectric-like system

“…A compound discussed in this work κ-(BEDT-TTF) 2 Hg(SCN) 2 Cl (k-Hg-Cl) shows a structure where (BEDT-TTF) +1 2 sites form a slightly anisotropic triangular lattice within the layer. In this compound electronic ferroelectricity is observed in a charge ordered state below 30 K [11,12]. In this work we experimentally detect a gradual melting of this charge order as the material is cooled down below 15 K.Typically, if a system undergoes a phase transition into a broken symmetry state, such as a ferroelectric state, this state is a ground state of the system.…”