Deuteration Effects on the Transport Properties of (TMTTF)2X Salts

Abstract

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)2X salts with the anions X = Br, PF6, SbF6, and ClO4, 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.

Tuning Charge Order in (TMTTF)2X by Partial Anion Substitution

“…Using partial substitution of the anions, it will be interesting to inspect the length scales of long-range correlations and short-range CO modulations and associated disorder effects-in particular in the deuterated compounds where CO is affected from a change in TMTTF-anion coupling [14,39]. In addition, the interplay of CO and anion order can be studied via mixing of octahedral and tetrahedral anions, which can possibly stabilize novel forms of structural order at commensurate stoichiometries.…”

“…To that end, we determine the CO contribution from the temperature-dependent energy gap ∆(T) = T ln ρ, following the procedure applied in [37]. ∆(T) of (TMTTF) 2 -[AsF 6 ] 0.7 [SbF 6 ] 0.3 is shown in Figure 5a, together with the energy gaps of (TMTTF) 2 X with X = PF − 6 (here we show the deuterated compound with T CO = 90 K from [39]), AsF − 6 , and SbF − 6 [37]. Note that the data have been shifted vertically and in all cases ∆(T 0 ) = 0 by definition.…”

“…From our present transport results, we obtain ∆ CO = 430 K for (TMTTF) 2 [AsF 6 ] 0.7 [SbF 6 ] 0.3 . x for x = 0 [37], 0.3, and 1 [37], and for deuterated (TMTTF) 2 PF 6 with T CO = 90 K [39]. The total gap size, measured by the resistivity, increases as ∆ CO adds up in quadrature ∆(T) = T ln ρ = ∆ 2 CO + ∆ 2 (T CO ) in the CO state.…”

“…(b) ∆ CO of (TMTTF) 2 [AsF 6 ] 1−x [SbF 6 ] x (blue star) is plotted versus T CO together with the data (solid squares) of (TMTTF) 2 X with X = PF − 6 , AsF − 6 , SbF − 6 and ReO − 4 [37]. Open squares indicate measurements on pristine and deuterated PF − 6 and SbF − 6 compounds [39]. The data follow an approximately linear relationship until ∆ CO vanishes for T CO ≤ 60 K. Alternatively, this universal behavior can be interpreted as a negative ∆ CO for T CO → 0.…”

“…In Figure 5b, we plot ∆ CO as a function of T CO for the data shown in (a), i.e., (TMTTF) 2 -[AsF 6 ] 1−x [SbF 6 ] x with x = 0.3 (star) and PF − 6 , AsF − 6 , SbF − 6 , together with ReO − 4 (solid squares) [37]. Included are also additional datasets for pristine and deuterated compounds from [39], indicated by the open squares. As we find, all data points fall on a universal line that yields ∆ CO = 0 around T CO = 60 K. It is tempting to relate this temperature scale with D/k B ≈ 60 K reported in Figure 8b of [14].…”