Electron spin resonance studies on the organic linear-chain compounds (TMTCF)2X (C = S,Se; X = PF6, AsF6, ClO4, Br)

We have conducted comprehensive electron spin resonance (ESR) investigations on single crystals of the one-dimensional organic compounds (TMTTF)(2)PF6, (TMTTF)(2)ClO4, (TMTTF)(2)Br, (TMTSF)(2)PF6, and (TMTSF)(2)AsF6 in the temperature range from 4 to 500 K and additionally, (TMTSF)(2)ReO4 and (TMTSF)(2)ClO4 at room temperature. In contrast to the selenium analogs TMTSF which are one-dimensional metals, the sulfur salts are semiconductors with localized spins on the TMTTF dimers. Taking into account the thermal expansion of the crystals at high temperature (T>20 K) the ESR intensity of all sulfur compounds can be described as a spin-1/2 antiferromagnetic Heisenberg chain with exchange constants 420 less than or equal to J less than or equal to 500 K. Although the TMTSF compounds are one-dimensional organic metals down to 10 K, the temperature dependence of the spin susceptibility can also be described within the framework of the Hubbard model in the limit of strong Coulomb repulsion with J approximate to 1400 K. By modeling (TMTTF)(2)ClO4 as an alternating spin chain, the change of the alternation parameter at the first-order phase transition (T-AO=72.5 K) indicates a tetramerization of the chain. (TMTTF)(2)PF6 undergoes a spin-Peierls transition at T-SP=19 K which can be well described by Bulaevskii's model with a singlet-tripler gap Delta(sigma)(0)= 32.3 K. We find evidence of antiferromagnetic fluctuations at temperatures well above the magnetic ordering in (TMTTF)2Br, (TMTSF)2PF6, and (TMTSF)(2)AsF6 which follow the critical behavior expected for three-dimensional ordering. (TMTTF)(2)PF6 and (TMTTF)(2)Br show one-dimensional lattice fluctuations.