Lattice vibrations of the charge-transfer saltκ−(BEDT-TTF)2Cu2(CN)3: Comprehensive explanation of the electrodynamic response in a spin-liquid compound

Abstract

The dimer Mott insulator κ-(BEDT-TTF)2Cu2(CN)3 exhibits unusual electrodynamic properties. Numerical investigations of the electronic ground state and the molecular and lattice vibrations reveal the importance of the Cu2(CN) − 3 anion network coupled to the BEDT-TTF molecules: The threefold cyanide coordination of copper and linkage isomerism in the anion structure cause a loss of symmetry, frustration, disorder, and domain formation. Our findings consistently explain the temperature and polarization-dependent THz and infrared measurements, reinforce the understanding of dielectric properties and have important implications for the quantum spin-liquid state, which should be treated beyond two-dimensional, purely electronic models.In the field of quasi-two-dimensional strongly correlated electron systems, the κ-salts of the BEDT-TTF family have attracted considerable interest [1] because their triangular arrangement of the dimers is close to geometrical frustration leading to the first realization of spinliquid behavior in κ-(BEDT-TTF) 2 Cu 2 (CN) 3 [2]. The antiferromagnetic exchange coupling among the dimers is rather strong (J ≈ 250 K), however, neither the magnetic order nor structural distortions have been detected down to lowest temperatures. Despite enormous theoretical and experimental efforts, the nature of this phase is still inconclusive and the question of a possible gap in the spin excitations remains open [3][4][5][6]. From the residual spin susceptibility and the power-law temperature dependence of the NMR spin-lattice relaxation rate, low-lying spin excitations are concluded, while their contribution to the optical conductivity is still under discussion [7][8][9].Interacting spins on a triangular lattice cannot order in a simple antiferromagnetic ground state, quantum fluctuations prevent a stable spin-liquid phase in the presence of finite hopping t and electronic repulsion U . More advanced microscopic models have been put forward to explain the properties of κ-BEDT-TTF salts [10-13] based on the presence of charge dipoles on the dimers which couple to spin degrees of freedom and thereby prevent ordering. While the electronic order has been proven in related compounds [14] in which the intra-dimer Coulomb repulsion U is reduced with respect to inter-site interaction V , the ratio V /t is of minor relevance in the case of κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , called κ-CN (Fig. 1). The anomalous dielectric response observed in the kHz and MHz range below 60 K [20, 21] evidences some unexpected charge excitations in the spin-liquid state which must be explained satisfactorily. Recently, time-domain optical measurements rendered a rather broad absorption band around 1 THz that develops below T ≈ 80 K. Itoh et al. [22] attributed it to collective excitation of intra-dimer electric dipoles. However, NMR, Raman and infrared spectroscopies rule out any sizable charge imbalance all the way down to low temperatures [24][25][26].In order to shed light on the excitation properties in the quantum spin-liquid state in...

Anion effects on electronic structure and electrodynamic properties of the Mott insulatorκ−(BEDT−TTF)2Ag2(CN)3Pinterić1, Lazić2, Pustogow3 et al. 2016Phys. Rev. BSelf Cite414860View full textAdd to dashboardCite

“…5. Its overall features resembles the κ-CuCN-analogue; 22,34,35 in particular the cation-derived character around the Fermi level is supported by the rigid band shift of the cation subsystem alone. Here, the shift and bandwidth are smaller by 76 meV and 20 meV, respectively, and the full charge transfer of 2 electrons takes place, in contrast to only 1.93 for κ-CuCN.…”

“…So far no global structural changes and no charge disproportionation between molecular dimer sites larger than 2δ ρ ≈ ±0.01e that could break the symmetry have been found. 20,21 In the case of the QSL κ-CuCN extensive numerical and experimental investigations 22 have recently revealed that the inversion-symmetry is broken on a local scale due to linkage isomerism in the anion layer; the relaxed structure exhibits the low P1 symmetry. Absorption bands in the THz frequency range -previously attributed to collective excitation of intradimer electric dipoles 11 -are traced back to coupled anion-dimer vibrations.…”

“…1) the anions also affect the overall electronic structure, but to a lower degree compared to κ-CuCN. 22 We have carried out dielectric, dc resistivity and vibrational spectroscopy measurements and supplemented them by density functional theory calculations. In both systems the relaxed struc-…”

“…1) the anions also affect the overall electronic structure, but to a lower degree compared to κ-CuCN. 22 We have carried out dielectric, dc resistivity and vibrational spectroscopy measurements and supplemented them by density functional theory calculations. In both systems the relaxed struc- ture consists of quasi-degenerate electronic states related to different cyanide configurations; however, in κ-AgCN the higher-symmetry space groups P2 1 and P c are preserved and we resolve a slight charge inequality between two non-equivalent sites.…”

Metal-insulator transition in the dimerized organic conductor κ−(BEDT-TTF)2Hg(SCN)2Br

“…[1][2][3] Subsequent theoretical approaches [4][5][6][7] suggest that also the Coulomb interaction V between the dimers has to be included in the description of possible chargeordering phenomena in these dimerized (BEDT-TTF) salts. Although numerous experimental results, [8][9][10][11][12][13] rule out appreciable charge disproportionation in κ-(BEDT--TTF) 2 Cu[N(CN) 2 ]Cl, κ-(BEDT-TTF) 2 Cu 2 (CN) 3 and other copper-based κ-(BEDT-TTF) 2 X salts, these considerations might be relevant for dimerized salts in general. Unfortunately, charge-order phenomena in κ-phase salts with an effectively half-filled band are scarce and only a few compounds are reported exhibiting charge order, such as κ-(BEDT-TTF) 4 PtCl 6 ·C 6 H 5 CN, the triclinic κ-(BEDT-TTF) 4 [M (CN) 6 ][N(C 2 H 5 ) 4 ]·3H 2 O and the monoclinic κ-(BEDT-TTF) 4 [M (CN) 6 ][N(C 2 H 5 ) 4 ]-·2H 2 O (with M = Co III , Fe III , and Cr III ) salts.…”