Electrodynamics in Organic Dimer Insulators Close to Mott Critical Point

Abstract

Organic layered charge-transfer salts κ-(BEDT-TTF) 2 X form highly frustrated lattices of molecular dimers in which strong correlations give rise to Mott insulating states situated close to the metal-to-insulator phase boundary. The salts κ-(BEDT-TTF) 2 Cu 2 (CN) 3 and κ-(BEDT-TTF) 2 Ag 2 (CN) 3 have been considered as prime candidates for a quantum spin liquid, while κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl has been suggested as a prototypical charge-order-driven antiferromagnet. In this paper, we summarize and discuss several key results, including some not reported previously, obtained in search to clarify the competition of these two ground states. The origin of anomalous dielectric response found at low temperatures in all three salts is also discussed. We conclude by pointing out the relevant new insights into the role of frustration and random disorder in the suppression of magnetic ordering and formation of the spin liquid state.

Hall effect study of the κ -(ET) 2X family: Evidence for Mott-Anderson localization

“…25 This proposal remains to be clarified in a future study. It is worth noting that recent dc resistivity and dielectric spectroscopy measurements by our group on κ-(ET) 2 Cu[N(CN) 2 ]Cl, 18 which belongs to the same family as the three compounds investigated here and with all CN groups ordered, also pointed towards the presence of disorder within conducting ET layers the origin of which for now remains a mystery. Also, a recent magnetotransport study of α-(ET) 2 I 3 , 40 which has a different arrangement of ET molecules, revealed hopping conductivity and negative magnetoresistance at low temperatures that were both ascribed to disorder within insulating I 3 layers which is transferred to conducting layers via hydrogen bonds.…”

“…There is now a growing body of evidence that this charged disorder, besides the geometric frustration, plays an important role in formation of the quantum spin liquid ground state. [17][18][19] The presence of hopping conductivity indicates that κ-(ET) 2 Cu 2 (CN) 3 cannot be considered as a simple Mott insulator, the insulating properties of which stem from the existence of a correlation gap. In fact, κ-(ET) 2 Cu 2 (CN) 3 is more similar to Anderson insulators which are gapless but have disorder-induced localized states at the Fermi level giving rise to hopping conductivity.…”

Donor–anion interactions in quarter-filled low-dimensional organic conductors

“…2b) which changes the amount of static disorder between ethylene group conformations of BEDT-TTF (see section V.A.2) [32][33] In addition, recent studies show also that the physics is not so simple and that one has to go beyond the single-site Mott description to understand the physical properties of κ-(BEDT-TTF) 2 X. Firstly the occurrence of a structural transition at ≈200K in both hydrogenated and deuterated κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br [34] induces an incommensurate modulation of electronic density, [35] which renders the dimers non-equivalent at low temperature. Secondly, from the observation of a mutiferroicity phenomenon in κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl around T N [36], and the recent report of a ferroelectric instability around 10K in some samples [37], it appears that charges are not frozen at low temperature, in contradiction with the use of a localized Mott-Hubbard phase diagram.…”

Advances in Organic Conductors and Superconductors

“…The low-temperature anomaly observed in the quantum spin liquid compound κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , however, remains an open issue [6]. In any case, the electrodynamic properties of these organic dimer Mott insulators appear to be fairly well understood by now [7,8]. The Mott transition observed in the two-dimensional organic κ-phase BEDT-TTF salts is the focus of the detailed pressure-dependent measurements of the heat capacity [9] and thermal expansion [10].…”