Structural investigation of the spin-singlet phase in (TMTTF)2I

Abstract

To elucidate the electronic state that exists in the boundary region between the spin-singlet phase and the high-pressure side antiferromagnetic phase in the modified generalized phase diagram, we carried out x-ray diffraction, electron spin resonance, and nuclear magnetic resonance measurements of the tetramethyl-tetrathiafulvalene (TMTTF) family salt, (TMTTF) 2 I. The unit-cell volume of (TMTTF) 2 I is between that of (TMTTF) 2 Br and (TMTTF) 2 PF 6 . We found that (TMTTF) 2 I undergoes a spin-singlet phase transition at 21 K without involving paramagnetic charge-ordering phase. Therefore, the charge-ordering transition is not necessary for the spin-singlet transition of TMTTF salts as seen in other conventional spin-singlet salts. Finally, we discuss possible mechanisms of the spin-singlet phase transition.

13C NMR study of the magnetic properties of the quasi-one-dimensional conductor, (TMTTF)2SbF6

“…[30], though the transition temperature has not been established. If this state is a non-magnetic sP phase, the sequence of the ground states contradicts with the pressure effect on the dimensionality of the system [4,37]. In our experiment, the expected reduction of the Knight shift has not been observed below T SG .…”

“…The diminishing of the site dependence of the NMR shift is caused by the reduction of spin susceptibility, χ s → 0, considering K s = Aχ s . AsF 6 , PF 6 , and I salts have similar spectral shape below T sP (or spin-singlet transition temperature T SS ) [31,32,[37][38][39]41]. A previous 13 C 2D NMR study obtained split spectra under 6 kbar on the dipolar coupling axis, that is attributed to the sP state [30].…”

“…At the lower pressure portion of the generalized phase diagram presented in ref. [3], there is the sP (or SG) phase [32][33][34][35][36][37]. (TMTTF) 2 PF 6 shows the CO transition above the temperature of the sP transition.…”

“…The ground state at ambient pressure of (TMTTF) 2 SbF 6 is an AF (T AF ∼7.5 K) [44,45]. The striking observation of the AF phase rather than the sP phase at ambient pressure has cast doubt on the assumption that the dimensionality of the system increases with increasing pressure [4,37]. The sP phase transition can be explained by the one-dimensional Heisenberg dimer model [46].…”

The optical conductivity for a spin-Peierls ground state of (TMTTF)$_{2}$PF$_{6}$ with tetramer formation

“…[11], t 1 /t 2 = 0.862 and t 3 /t 2 = 0.833 are calculated within the framework of the extended Hückel method [15] with structural parameters of (TMTTF) 2 PF 6 observed by X-ray diffraction experiments at 7 K. In contrast to transfer integrals, to determine Coulomb repulsive interaction strengths U and V is much difficult in general. However, we employ 0.2 ≤ V /U ≤ 0.6 for U/t 2 = 5, 10 as typical values of (TMTTF) 2 PF 6 [8,16,17,18,19,20,21] in this article.…”