Purely electronic terahertz polarization in dimer Mott insulators

Abstract

We theoretically discover purely electronic polarization modes in THz frequency region in dimer Mott insulators κ-(BEDT-TTF)2X. The unusual low-frequency modes arise from the coupling between the oscillation of intradimer electric dipole moments and that of alternating interdimer bond orders. These collective motions play an important role in the dynamical dielectric properties of the dimer Mott insulators. Near the phase boundary of the dimer Mott transition, the ferroelectric ground state is realized by introducing electron-lattice coupling.PACS numbers: 71.30.+h, 78.30.Jw Low-dimensional strongly correlated electron systems with a quarter-filled band have recently attracted much attention. BEDT-TTF salts are typical two-dimensional systems with anisotropic triangular lattices, and the strong correlation and frustration act cooperatively to generate exotic phases. Among the BEDT-TTF salts, κ-(BEDT-TTF) 2 X (X: a counter anion) exhibit the Mott insulator phase [1][2][3][4][5], exotic superconducting phase [6], spin liquid phase [7][8][9], and so on. The BEDT-TTF molecular lattice is distorted to form dimers in κ-(BEDT-TTF) 2 X. Hybridized two sites can be effectively treated as a single site, and the valence band is regarded as a half-filled one. As a result, κ-(BEDT-TTF) 2 X exhibits the Mott insulator phase if the effective on-site Coulomb energy is large enough. The state is called the dimer Mott insulator.Recently quite interesting charge excitation phenomena have been observed in the dimer Mott insulators: photoinduced phase transition to a metallic state [10], and anomaly in dielectric properties [11]. In particular, the latter suggests that low-energy charge excitation exists in the dimer Mott insulators in contrast to the case of the Mott insulators.In this letter, we show for the first time that the dimer Mott insulator has THz electric polarization, which purely arises from the electron dynamics. The ferroelectricity which arises from the electron dynamics has been observed in some materials, and these ferroelectrics have unconventional origins such as charge order generation [12][13][14][15][16]. We propose another new origin of ferroelectricity in this paper.We adopt the 1/4-filled extended Hubbard Hamiltonian for holes on the two-dimensional anisotropic triangular lattice, which includes the degrees of freedom in a dimer. It is given by H = <n,m> (β n,mpn,m + V n,m n n n m ) + U n n n,↑ n n,↓ , (1) where < n, m > denotes the neighbor site pairs, c n,σ (c † n,σ ) is the annihilation (creation) operator for a hole of spin σ at the site n, β n,m (V n,m ) is the transfer integral (the Coulomb interaction energy) between the sites n and m,p n,m = σ (c † m,σ c n,σ + c † n,σ c m,σ ), U is the on-site Coulomb interaction energy, n n,σ = c † n,σ c n,σ , and n n = σ n n,σ . The 4 × 4 cluster of the system size N = 16 with the periodic boundary condition is used.We calculate the quantum state excited by light pulse expressed by the vector potential A(t) = eA exp(−(t/D)2 ) cos(ω p t) of a Gaussian profile....

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

“…Similar considerations have been put forward by other groups. 5,6 Mazumdar, Clay and collaborators 7 could show that in these system a frustration-induced transition occurs from a Néel antiferromagnetism to a spin-singlet state in the interacting quarter-filled band on an anisotropic triangular lattice. In the spin-singlet state the charge on the molecules becomes unequal: a paired-electron crystal is formed with pairs of charge-rich sites separated by pairs of charge-poor sites.…”

“…Depending on the strength of the on-site Coulomb repulsion U with respect to the bandwidth W , a metallic and superconducting ground state develops at low temperatures, or a Mott insulator, which might be antiferromagnetically ordered or behaves like a spin liquid due to strong frustrations. [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.…”

Paired electron crystal: Order from frustration in the quarter-filled band

“…Several recent theoretical works [94][95][96] have also considered intradimer charge degrees of freedom in κ-CN, in view of the possible phase transition at 6 K, and in particular, the dielectric anomaly reported in reference 88. References 94 and 95 investigated the strong-dimer limit of the ρ = 1 2 extended Hubbard model, where the Hilbert space in lowest order is restricted to states with one hole per dimer, and interactions between dimers are treated as perturbations.…”

“…The author's consider the ρ = 1 2 extended Hubbard model specifically for the κ-lattice (thus the strongly frustrated region of our model), and find from exact diagonalization that for nonzero electron-lattice coupling the ground state was a CO ferroelectric near the AFM-PM phase boundary 96 . The possibility of unequal charge distributions within the dimers [94][95][96] is in agreement with our results. The key differences from our work with these theories are that frustration as well as strength of the intradimer bonds are variables within our model, and our determination that the formation of the CO is driven by the tendency to the form spin-singlets.…”

Anion effects on electronic structure and electrodynamic properties of the Mott insulatorκ−(BEDT−TTF)2Ag2(CN)3Pinterić1, Lazić2, Pustogow3 et al. 2016Phys. Rev. B414860View full textAdd to dashboardCite

“…On the other hand, the Mott dimer insulators κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl and κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , called κ-CuCN, have been thoroughly studied because they are discussed as prototypes of a molecular multiferroic and quantum spin liquid (QSL) systems. 9,15 It turns out to be extremely challenging to reconcile the idea of quantum electric dipoles on molecular dimers interacting via dipolar-spin coupling [16][17][18][19] with the experimentally evidenced absence of any considerable charge imbalance. 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.…”