Importance of van der Waals interactions and cation-anion coupling in an organic quantum spin liquid

Abstract

The Mott insulator β ′ -EtMe3Sb[Pd(dmit)2]2 belongs to a class of charge transfer solids with highly-frustrated triangular lattice of S = 1/2 molecular dimers and a quantum-spin-liquid ground state. Our experimental and ab initio theoretical studies show the fingerprints of strong correlations and disorder, important role of cation-dimer bonding in charge redistribution, no sign of intra-and inter-dimer dipoles, and the decisive van der Waals contribution to inter-dimer interactions and the ground state structure. The latter consists of quasi-degenerate electronic states related to the different configurations of cation moieties which permit two different equally probable orientations. Upon reducing the temperature, the low-energy excitations slow down, indicating glassy signatures as the cation motion freezes out. PACS numbers:Quantum spin liquid (QSL), a highly correlated fluctuating quantum spin state, is a long-standing intriguing phenomenon in physics [1,2]. It is expected to appear in geometrically frustrated systems when strong quantum fluctuations suppress the long-range magnetic order. Recently the QSL has been realized in materials with frustrated lattices at the insulating side of Mott transition [3]. Because of this exciting discovery considerable efforts have been devoted to the studies of organic Mott insulators -charge transfer crystalline solids in which electrons are strongly correlated and confined to two dimensions. They form layers with triangular structures of molecular pairs-dimers with an odd number of electrons separated by non-conducting inorganic moities. Three organic Mott systems with different degrees of correlationsexhibit an anomalous electrodynamic response below 60 K [4-8], while at very low temperatures the magnetic and thermodynamic response exhibits unconventional behavior ascribed to the QSL ground state [9][10][11]. The full understanding of the nature of QSL is missing primarily because frustrated triangular lattices on their own are unable to destroy the long-range magnetic order [12]. It was suggested that an additional, exotic spin-dipolar coupling in the presence of geometrical frustration would suffice to induce quantum fluctuations and suppress magnetic ordering, however its experimental confirmation is still lacking [13][14][15][16][17]. The mechanism of dipolar-spin coupling relies on the Coulomb interactions within the sublattice of molecular dimers only, thereby completely neglecting the role of inorganic moieties to which the molecular sublattice is strongly hydrogen-bonded. For κ-CuCN and κ-AgCN the decisive role of cation-anion hydrogen bonding is revealed by the experimental results combined with ab initio numerical calculations. This points to a significant anion-driven renormalization of the electronic properties due to disorder by cyanide isomorphism [7,8]. Notably, despite the fact that single crystals of these two BEDT-TTF materials are nominally pure, the variable-range hopping, relaxor dielectric response and anomalous terahertz response are observed wit...

Electrodynamics of quantum spin liquids

“…Below approximately 200 cm −1 , however, the optical conductivity exceeds the power-law extrapolation considerably, giving evidence for an additional contribution to the electrodynamic response. At much lower frequencies, σ 1 (ω) gradually levels off toward the dielectric data [117], comparable to what is plotted in figure 8 for κ-(BEDT--TTF) 2 Cu 2 (CN) 3 . The transition regime is consistent with a σ 1 (ω) ∝ ω 2 behavior as suggested by Ng and Lee [62], that catches up the interpolated value of the correlated electrons rather quickly.…”

Molecular quantum materials: electronic phases and charge dynamics in two-dimensional organic solids

“…An anomalous dielectric peak shows up as an ubiquitous property independent of the nature of the ground state in the spin sector [458,523,576,608,610,620] The relaxor-type dielectric response suggests a short-range ferroelectric-like order [576,621] and the existence of domains of low symmetry equivalent with the average high symmetry, in accord with density functional (DFT) calculations and structural refinements [71,72,523,556,620]. However, both spectroscopic [490,523,610,620] and structural measurements [71,72] consistently demonstrate the equally distributed charge in quantum spin liquid compounds. Moreover, the persistence of strong quantum fluctuations due to charge or dipolar fluctuations in the kHz-MHz frequency range is unlikely because no Barrett-like behavior of the dielectric constant [cf.…”

“…In the BEDT-TTF-based quantum spin liquids, this structure originates from the cyanide groups located at the inversion centers bridging the polymeric CuCN/AgCN chains in the anionic layer. In the β -EtMe 3 Sb[Pd(dmit) 2 ] 2 with quantum spin liquid ground state, disorder occurs due to two equally probable orientations of the Et groups (static disorder) and to internal rotational degrees of freedom of the Me groups (dynamic disorder); both Et and Me groups reside in the non-conducting cations [620]. Interestingly, a quantum-spin-liquid state is not established in β -Me 4 P[Pd(dmit) 2 ] 2 and β -Me 4 Sb[Pd(dmit) 2 ] 2 : here static disorder is absent, but dynamic disorder is still present and causes a relaxor peak in ε (T ) [621].…”

Electrodynamics in Organic Dimer Insulators Close to Mott Critical Point

“…The previous DFT studies on different organic charge-transfer salts have neither included nor discussed the van der Waals (vdW) interactions at all [64][65][66][67][68][69][70]. The only exception is found in our work [23,24,39].…”

“…The role of random disorder in this competition may also be important, as indicated by an experimental study on κ-Cl showing that X-rays suppress magnetic order and allow a spin liquid to form [38]. Finally, we have recently suggested that the non-conducting anions which contain frustration and disorder in the organic spin liquids may be a critical ingredient needed for the prevalence of quantum spin liquid over magnetic ordering [39].…”