Lattice effects in the quasi-two-dimensional valence-bond-solid Mott insulator EtMe3P[Pd(dmit)2]2

Abstract

The organic charge-transfer salt EtMe 3 P[Pd(dmit) 2 ] 2 is a quasi-two-dimensional Mott insulator with localized spins S = 1/2 residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients α i along the in-plane i = a and c axis as well as along the out-of-plane b axis for temperatures 1.4 K T 200 K. Particular attention is paid to the lattice effects around the phase transition at T VBS = 25 K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-range antiferromagnetic correlations can be expected. The salient results of our study include (i) the observation of strongly anisotropic lattice distortions accompanying the formation of the valence-bond-solid phase, and (ii) a distinct anomaly in the thermal expansion coefficients in the paramagnetic regime around 40 K. Our results demonstrate that upon cooling through T VBS the in-plane c axis, along which the valence bonds form, contracts while the second in-plane a axis elongates by the same relative amount. Surprisingly, the dominant effect is observed for the out-of-plane b axis which shrinks significantly upon cooling through T VBS . The pronounced anomaly in α i around 40 K is attributed to short-range magnetic correlations. It is argued that the position of this maximum, relative to that in the magnetic susceptibility around 70 K, speaks in favor of a more anisotropic triangular-lattice scenario for this compound than previously thought.

Thirty-Year Anniversary of κ-(BEDT-TTF)2Cu2(CN)3: Reconciling the Spin Gap in a Spin-Liquid Candidate

“…Taken together, the anomalies around T = 6 K in the magnetic, thermodynamic, thermal transport, and structural properties of κ-(BEDT-TTF) 2 Cu 2 (CN) 3 summarized in Figures 3-6 are consistent with a spin-gapped VBS state, similar to that reported in the layered organic Mott systems κ-(BEDT-TTF) 2 B(CN) 4 [72,73] and EtMe 3 P[Pd(dmit) 2 ] 2 [68][69][70][71]. Notably, already small amounts of disorder and defects are sufficient to conceal the intrinsic response of valence electrons in various magnetic probes [82], precluding a proper identification of the magnetic ground state in the past.…”

“…Following its observation as a hump-like feature in C/T [26], the '6K anomaly' of κ-(BEDT-TTF) 2 Cu 2 (CN) 3 showed up in thermal expansion measurements as a pronounced lattice distortion [81], indicated in Figure 6a. The concomitant occurrence of structural modifications with the opening of a spin gap is, together with its similar anisotropy and size, reminiscent of the VBS transition in EtMe 3 P[Pd(dmit) 2 ] 2 [71] and the spin-Peierls transitions in linear-chain compounds like (TMTTF) 2 X and GeCuO 3 [104,105]. Since the author is not aware of any QSL state coupled to such pronounced structural anomalies [2][3][4][5], it is concluded that the ground state of κ-(BEDT-TTF) 2 Cu 2 (CN) 3 is a nonmagnetic VBS phase.…”

“…(c) Due to the lack of AFM order down to a few millikelvin [34,37,39,67], the frustrated triangular-lattice compounds (t /t ≈ 1) from (a) have been intensely discussed as QSL candidates [2][3][4][5]7]. (d) EtMe 3 -P[Pd(dmit) 2 ] 2 [68][69][70][71] and κ-(BEDT-TTF) 2 B(CN) 4 [72,73] exhibit a VBS state below T . Due to the Clausius-Clapeyron relation, the large spin entropy of the paramagnetic Mott state yields a positive slope of the insulator-metal boundary in the T-p phase diagram [6,7,45], whereas dT MI /dp < 0 for AFM [51,66] or VBS [69,70].…”

“…In this case, the spins of neighboring sites form a singlet state, which is usually accompanied by a Peierls-like lattice distortion. Such nonmagnetic phases occur close to t /t ≈ 1, e.g., in EtMe 3 P[Pd(dmit) 2 ] 2 [68][69][70][71], and, in particular, for quasi-1D (t /t > 1) exchange interactions, like for κ-(BEDT-TTF) 2 B(CN) 4 with t /t > 1.4 [72,73,77]. Other prominent examples are the spin-Peierls phases in linear-chain compounds [78][79][80].…”

Charge and Lattice Fluctuations in Molecule-Based Spin Liquids

“…Because of the fluctuation, no ordering is considered to be realised, even though the 2D layer slightly deviates from that of an equilateral triangular lattice. Without the dynamical fluctuation, a subtle deviation from the equilateral triangular lattice should produce a CO state accompanied by VBO, which was the case for m -salt and Sb-salt 3 32,34–38,40,49,50,53 . The broad and small peak in the normalized C p T −1 vs T plot obtained from the specific heat measurement of Sb-salt 2 is in agreement with the fluctuation involving the tetramers and octamers because the tetramers and octamers induce the small entropy releasing 43 .…”