Search Results (9)

We review the magneto-structural properties of electron–electron correlated quasi-one- dimensional (1D) molecular organics. These weakly localized quarter-filled metallic-like systems with pronounced spin 1/2 antiferromagnetic (AF) interactions in stack direction exhibit a spin charge decoupling where magnetoelastic coupling picks up spin 1/2 to pair into S = 0 singlet dimers. This is well illustrated by the observation of a spin-Peierls (SP) instability in the (TMTTF)2X Fabre salts and related salts with the o-DMTTF donor. These instabilities are revealed by the formation of a pseudo-gap in the spin degrees of freedom triggered by the development of SP structural correlations. The divergence of these 1D fluctuations, together with the interchain coupling, drive a 3D-SP ground state. More surprisingly, we show that the Per2-M(mnt)2 system, undergoing a Kondo coupling between the metallic Per stack and the dithiolate stack of localized AF coupled spin ½ (for M = Pd, Ni, Pt), enhances the SP instability. Then, we consider the zig-zag spin ladder DTTTF2-M(mnt)2 system, where unusual singlet ground state properties are due to a combination of a 4k_F charge localization effect in stack direction and a 2k_F SP instability along the zig-zag ladder. Finally, we consider some specific features of correlated 1D systems concerning the coexistence of symmetrically different 4k_F BOW and 4k_F CDW orders in quarter-filled organics, and the nucleation of solitons in perturbed SP systems. Full article

We investigated the superconductivity of (TMTTF)₂TaF₆ (TMTTF: tetramethyl-tetrathiafulvalene) by conducting resistivity measurements under high pressure up to 8 GPa. A cubic anvil cell (CAC) pressure generator, which can produce hydrostatic high-pressure, was used for this study. Since the generalized temperature-pressure (T-P) diagram of (TMTCF)₂X (C = Se, S, X: monovalent anion) based on (TMTTF)₂PF₆ (T_CO = 70 K and spin-Peierls: SP, T_SP = 15 K) was proposed by Jérome, exploring superconductivity states using high-pressure measurement beyond 4 GPa has been required to confirm the universality of the electron-correlation variation under pressure in (TMTTF)₂X (TMTTF)₂TaF₆, which has the largest octahedral-symmetry counter anion TaF₆ in the (TMTTF)₂X series, possesses the highest charge-ordering (CO) transition temperature (T_CO = 175 K) in (TMTTF)₂X and demonstrates an anti-ferromagnetic transition (T_AF = 9 K) at ambient pressure. A superconducting state in (TMTTF)₂TaF₆ emerged after a metal-insulator transition was suppressed with increasing external pressure. We discovered a superconducting state in 5 ≤ P ≤ 6 GPa from T_c = 2.1 K to 2.8 K, whose pressure range is one-third narrower than that of X = SbF₆ (5.4 ≤ P ≤ 9 GPa). In addition, when the pressures with maximum SC temperatures are compared between the PF₆ and the TaF₆ salts, we found that (TMTTF)₂TaF₆ has a 0.75 GPa on the negative pressure side in the T-P phase diagram of (TMTTF)₂PF₆. Full article

In the quasi-one-dimensional (TMTTF)${}_2$X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions and cations. Here, we investigate the charge-ordered state through partial substitution of the anions in (TMTTF)${}_2$[AsF${}_6$]${}_{1-x}$[SbF${}_6$]${}_x$ with $x\approx 0.3$, determined from the intensity of infrared vibrations, which is sufficient to suppress the spin-Peierls state. Our dc transport experiments reveal a transition temperature $T_{\mathrm{CO}}$ = 120 K and charge gap ${\Delta }_{\mathrm{CO}}$$=430$ K between the values of the two parent compounds (TMTTF)${}_2$AsF${}_6$ and (TMTTF)${}_2$SbF${}_6$. Upon plotting the two parameters for different (TMTTF)${}_2$X, we find a universal relationship between $T_{\mathrm{CO}}$ and ${\Delta }_{\mathrm{CO}}$ yielding that the energy gap vanishes for transition temperatures $T_{\mathrm{CO}}$$\le 60$ K. While these quantities indicate that the macroscopic correlation strength is continuously tuned, our vibrational spectroscopy results probing the local charge disproportionation suggest that $2\delta$ is modulated on a microscopic level. Full article

New radical-cation salts based on tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenefulvalene (TMsTSF) with metallacarborane anions (TMTTF)[3,3′-Cr(1,2-C₂B₉H₁₁)₂], (TMTTF)[3,3′-Fe(1,2-C₂B₉H₁₁)₂], and (TMTSF)₂[3,3′-Cr(1,2-C₂B₉H₁₁)₂] were synthesized by electrocrystallization. Their crystal structures were determined by single crystal X-ray diffraction, and their electrophysical properties in a wide temperature range were studied. The first two salts are dielectrics, while the third one is a narrow-gap semiconductor: σ_RT = 5 × 10⁻³ Ohm⁻¹cm⁻¹; E_a ≈ 0.04 eV (aprox. 320 cm⁻¹). Full article

The electronic properties in the quasi-one-dimensional Fabre salts are strongly affected by electronic correlations along the molecular stacks, but also by the interactions with the anions located in a cage that is formed by the methyl end groups. We systematically compare the charge transport in deuterated and protonated (TMTTF)${}_2$X salts with the anions X = Br, PF${}_6$, SbF${}_6$, and ClO${}_4$, ranging from Mott and Efros–Shklovskii variable-range hopping to activated band transport with a temperature dependent energy gap. The strong dependence of charge localization and ordering on the anion size and deuteration confirms the subtle structural involvement of the anions in the charge transport along the TMTTF stack. Full article

The physical properties of molecular crystals are governed by the frontier orbitals of molecules. A molecular orbital, which is formed by superposing the atomic orbitals of constituent elements, has complicated degrees of freedom in the crystal because of the influence of electron correlation and crystal field. Therefore, in general, it is difficult to experimentally observe the whole picture of a frontier orbital. Here, we introduce a new method called “core differential Fourier synthesis” (CDFS) using synchrotron X-ray diffraction to observe the valence electron density in materials. By observing the valence electrons occupied in molecular orbitals, the orbital state can be directly determined in a real space. In this study, we applied the CDFS method to molecular materials such as diamond, C₆₀ fullerene, (MV)I₂, and (TMTTF)₂X. Our results not only demonstrate the typical orbital states in some materials, but also provide a new method for studying intramolecular degrees of freedom. Full article

Comprehensive measurements of the pressure- and temperature-dependent dc-transport are combined with dielectric spectroscopy and structural considerations in order to elucidate the charge and anion orderings in the quasi-one-dimensional charge-transfer salts (TMTTF) ${}_{2}X$ with non-centrosymmetric anions X = BF ${}_4$ , ClO ${}_4$ and ReO ${}_4$ . Upon applying hydrostatic pressure, the charge-order transition is suppressed in all three compounds, whereas the influence on the anion order clearly depends on the particular compound. A review of the structural properties paves the way for understanding the effect of the anions in their methyl cavities on the ordering. By determining the complex dielectric constant $\widehat{ϵ}(\omega ,T)$ in different directions we obtain valuable information on the contribution of the anions to the dielectric properties. For (TMTTF) ${}_2$ ClO ${}_4$ and (TMTTF) ${}_2$ ReO ${}_4$ , ${ϵ}_{b^{\prime }}$ exhibits an activated behavior of the relaxation time with activation energies similar to the gap measured in transport, indicating that the relaxation dynamics are determined by free charge carriers. Full article

Charge ordering in the (TMTTF)₂X salts with centrosymmetric anions (X = PF‾₆ , AsF‾₆ , SbF‾₆ ) leads to a ferroelectric state around 100 K. For the first time and in great completeness, the intra- and intermolecular vibrational modes of (TMTTF)₂X have been investigated by infrared and Raman spectroscopy as a function of temperature and pressure for different polarizations. In this original paper, we explore the development and amount of charge disproportionation and the coupling of the electronic degrees of freedom to the counterions and the underlying lattice. The methyl groups undergo changes with temperature that are crucial for the anion cage formed by them. We find that the coupling of the TMTTF molecules to the hexafluorine anions changes upon cooling and especially at the charge-order transition, indicating a distortion of the anion. Additional features are identified that are caused by the anharmonic potential. The spin-Peierls transition entails additional modifications in the charge distribution. To complete the discussion, we also add the vibrational frequencies and eigenvectors based on ab-initio quantum-chemical calculations. Full article