The thermal conductivity κ of the quasi-2D organic spin-liquid candidate EtMe3Sb[Pd(dmit)2]2 (dmit-131) was measured at low temperatures, down to 0.07 K. We observe a vanishingly small residual linear term κ0/T , in κ/T vs T as T → 0. This shows that the low-energy excitations responsible for the sizeable residual linear term γ in the specific heat C, seen in C/T vs T as T → 0, are localized. We conclude that there are no mobile gapless excitations in this spin liquid candidate, in contrast with a prior study of dmit-131 that reported a large κ0/T value [Yamashita et al., Science 328, 1246(2010]. Our study shows that dmit-131 is in fact similar to κ-(BEDT-TTF)2Cu2(CN)3, another quasi-2D organic spin-liquid candidate where a vanishingly small κ0/T and a sizeable γ are seen. We attribute heat conduction in these organic insulators without magnetic order to phonons undergoing strong spin-phonon scattering, as observed in several other spin-liquid materials. arXiv:1904.10402v3 [cond-mat.str-el]
A quasi-one-dimensional organic charge-transfer salt (TMTTF)_{2}PF_{6} undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a "structureless transition," and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer δ_{CO}=0.20e. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.
We investigated the precise crystal structures and electronic states in a quasi-two-dimensional molecular conductor α-(BETS)2I3 at ambient pressure. The electronic resistivity of this molecular solid shows a metal-to-insulator (MI) crossover at 𝑇 MI = 50 K. Our x-ray diffraction and 13 C nuclear magnetic resonance experiments revealed that α-(BETS)2I3 maintains the inversion symmetry below 𝑇 MI . The first-principles calculations found a pair of anisotropic Dirac cones at a general k-point, where the degenerated contact points are located at the Fermi level. Furthermore, the origin of the insulating state in this system is explained by a small energy gap of ~2 meV opened by a spin-orbit interaction, in which the Z2 topological invariants indicate a weak topological insulator. Our results suggest that α-(BETS)2I3 is a promising material for studying the bulk Dirac electron system in two-dimension.
1T-TiSe 2 has a semimetallic band structure at room temperature and undergoes phase transition to a triple-q charge density wave (CDW) state with a commensurate superlattice structure (2a × 2a × 2c) below T c ≈ 200 K at ambient pressure. This phase transition is caused by cooperative phenomena involving electron-phonon and electron-hole (excitonic) interactions, and cannot be described by a standard CDW framework. By Cu intercalation or the application of pressure, this phase transition temperature is suppressed and superconductivity (SC) appears. However, it is not clear what kind of order parameters are affected by these two procedures. We investigated the crystal structure of Cu x TiSe 2 and pressurized 1T-TiSe 2 around the SC state by synchrotron x-ray diffraction on single crystals. In the high-temperature phase, the variation of structural parameters for the case of Cu intercalation and application of pressure are considerably different. Moreover, the relationship between the critical points of the CDW phase transition and the SC dome are also different for the two cases. The excitonic interaction appears to play an important role in the P−T phase diagram of 1T-TiSe 2 , but not in the x−T phase diagram.
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