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Carbon-13 nuclear magnetic resonance ( 13 C NMR) measurements have been carried out in the quasione-dimensional organic conductor (TMTTF) 2 SbF 6 , which exhibits charge ordering (CO) transition and antiferromagnetic (AF) transition at 157 K and 6 K at ambient pressure, respectively. We estimated the degree of charge disproportionation, ρ = ρ rich − ρ poor , in the CO state to be 0.25e from the analysis of the NMR shift and found the broadening of the NMR spectral peaks caused by the inhomogeneous distribution in association with the AF fluctuation at charge rich site below 20 K in the CO state. Moreover, we determined the magnetic structure with two kinds of staggered moments in the AF phase of (TMTTF) 2 SbF 6 , which suggests that the CO state remains even in the AF phase. Consequently, we found the difference between two AF phases appearing on both sides of the spin-Peierls phase in the universal P -T phase diagram.

␣-͑BEDT-TTF͒ 2 I 3 has a double column structure with three crystallographically independent molecules, A, B, and C, and becomes an insulator below 135 K due to a charge-ordering ͑CO͒ transition. The CO state is suppressed under pressure and shows weak temperature dependence of the electrical resistance. The decline of the carrier density is observed over 1.8 GPa, suggesting a zero-gap-semiconductor state under high pressure. The local susceptibility on each site provides important information about the electronic band structure. In our previous work, molecular site B became spin poor and molecular site C became spin rich in the metallic state, suggesting the disproportionation of the local spin density. Therefore, it is worthwhile to investigate the behavior of the electron properties of each molecular site. We accessed the 13 C-NMR in this salt at ambient pressure and at 2.1 GPa. The analysis of the local spin susceptibility suggests that the disproportionation of the spin susceptibility at 2.1 GPa is enhanced from that at ambient pressure, and the spin susceptibility vanishes at low temperature at 2.1 GPa. Moreover, from the temperature dependence of T 1 −1 , we obtain the relation of.

Quasi-One and quasi-two dimensional organic conductors consisting of TTF derivatives such as BEDT-TTF (bis-(ethylene-dithio)-tetra-thia-fulvalene) and TMTCF (C = S; TMTTF: tetra-methyl-tetra-thia-fulvalene, C = Se; TMTSF: tetra-methyl-tetraselena-fulvalene) have been well investigated in condensed matter physics because of interest in the emerging electric and magnetic properties, such as the spin density wave, charge order, superconductivity, anti-ferromagnetism, and so on. To probe the electronic state, nuclear magnetic resonance (NMR) is one of the most powerful tools as the microscopic magnetometer. A number of 13 C-NMR studies have been performed of the double-site central 13 C= 13C bond substituted molecules. However, problems with the coupled spin system of 13 C= 13 C complicated the interpretation for observations on NMR.Therefore, single-site 13 C-enriched molecules are desired. We summarize the problem of Pake doublet and the preparation of the single-site 13 C-susbstituted BEDT-TTF and TMTCF molecules. We also demonstrate the superiority of 13 C-NMR of the single-site 13 C-susbstituted molecule utilizing the hyperfine coupling tensor.

Quasi-one-dimensional organic conductors (TMTCF) 2 X have various electric and magnetic properties. Although theoretical and experimental studies have suggested that (TMTTF) 2 Br has the properties of commensurate antiferromagnetism, details of the magnetic structure of this compound are unclear. Two types of antiferromagnetism are expected, one due to a localized electron and the other to the nesting of the Fermi surface. We therefore assessed the antiferromagnetic structure of (TMTTF) 2 Br using 13 C NMR. Site assignment of the observed antiferromagnetic spectrum confirmed that there were two magnetic molecular sites, with staggered moments and amplitude of 0.11μ B /molecule, as well as nonmagnetic molecular sites. A commensurate structure with antiferromagnetic ordering of (↑ • ↓ •) along a one-dimensional chain would be expected as the freezing of antiferromagnetic fluctuations in the charge-ordered phase of the (TMTTF) 2 AsF 6 salt. The presence of a nodal site is strongly suggestive of the nesting type antiferromagnetism. The fine structure of the antiferromagnetic spectrum suggests superlattice along the interchain direction. We could not observe line broadening due to the charge order above the antiferromagnetic transition.

Among quasi-two-dimensional organic conductors, ͑BEDT-TTF͒ 2 X, what form the two column structure of ␣ or is thought to show a charge-ordering insulating state. The ␣Ј modification is the other candidate for a charge-ordering state. To determine the existence of a charge-ordering state and electron magnetism in the ␣Ј phase, which has a symmetry that differs from that of the ␣ and phases, we utilized 13 C-NMR to examine ␣Ј-͑BEDT-TTF͒ 2 IBr 2 in which one side of the central carbon on the BEDT-TTF molecule is substituted with 13 C nuclei. We observed changes in its spectrum at 200 K. The angular dependence of the NMR shift at 15 K indicated that the charge-ordering pattern is intracolumn disproportionation and the ratio of charge disproportionation on the spin-poor site and spin-rich sites is almost 1:0. The angle dependence of NMR spectrum at 60 K suggests that the hyperfine coupling tensor in the paramagnetic charge-ordering state is determined not only by the charge density of on-site molecules but also by the contribution of off-site molecules. The change in hyperfine coupling tensor must be taken into account in NMR studies of the paramagnetic CO state.

The electronic state of (TMTTF) 2 SbF 6 was investigated by the 1 H and 13 C NMR measurements. The temperature dependence of T 1 -1 in 1 H NMR shows a sharp peak associated with the antiferromagnetic transition at T AF =6 K. The temperature dependence of T 1 -1 is described by the power law T 2.4 below T AF . This suggests the nodal gapless spin wave excitation in antiferromagnetic phase. In 13 C NMR, two sharp peaks at high temperature region, associated with the inner and the outer carbon sites in TMTTF dimer, split into four peaks below 150 K. It indicates that the charge disproportionation occurs. The degree of charge disproportionation ∆ρ is estimated as (0.25±0.09)e from the chemical shift difference. This value of ∆ρ is consistent with that obtained from the infrared spectroscopy. In the antiferromagnetic state (AFI), the observed line shape is well fitted by eight Lorentzian peaks. This suggests that the charge order with the same degree still remains in the AF state. From the line assignment, the AF staggered spin amplitude is obtained as 0.70µ B and 0.24µ B at the charge rich and the poor sites, respectively. These values corresponding to almost 1µ B per dimer are quite different from 0.11µ B of another AF (AFII) state in (TMTTF) 2 Br with effective higher pressure. As a result, it is understood that the antiferromagnetic staggered spin order is stabilized on the CO state in the AFI phase of (TMTTF) 2 SbF 6 .

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