Coexistence of charge order and antiferromagnetism in (TMTTF)2SbF6: NMR study

Abstract

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 .