Deuteration Effects on the Transport Properties of (TMTTF)2X Salts
Round 1
Reviewer 1 Report
The paper submitted by Rohwer and Dressel reports the electronic properties of (TMTTF)2X salts with hydrogenated and deuterated TMTTF donor molecules. They measured and analyzed the temperature dependence of the resistivity to obtain the information of the relation between the chemical substitution effect of X anions and deuteration and degrees of charge localization.
They found a tendency of characteristic temperature T_CO, T_rho and energy gaps Delta_0, Delta_CO upon anion size and deuteration, i.e., subtle structural involvement. The proposed general trend is interesting and important for understanding a system of (TMTTF)2X although some of uncertain observed phenomena are not cleared in this paper.
The reviewer would ask revision of the analysis in figures for showing more convincing evidence of the characteristic temperatures T_CO. The authors should explain the definition of T_CO obtained from the temperature dependence of the resistivity as shown in Figures 7 and 8. For example, reviewer could not found that T_CO of H12-(TMTTF)2Br and H12-(TMTTF)2PF6 are indicated at any characteristic structure on rho(T) and Delta(T) curves. At least, authors should put auxiliary strait guide lines on rho(T) curves, which linear fit temperature regions are listed in Table A1, to show clearly the definition of T_CO.
Minor suggestions;
In the figure captions of Fig. 7, three transport models are listed; Arrhenius (left), three-dimensional variable-range hopping (middle) and variable-range hopping (right). Is three-dimensional VR hopping correct? It should be ES VR hopping.
Some typos are found; “pronotated” in captions of Fig. 8 and “pronotnated” in captions of Table.3 should be “protonated”.
Author Response
We thank the referee for pointing out the incorrect figure caption of Fig. 7 which has been changed to
“Comparison between three transport models, namely, Arrhenius (left column), three-dimensional variable-range hopping with Coulomb gap considered according to Efros and Shklovskii (central column) and three-dimensional Mott variable-range hopping without electronic correlations (right column), for all four protonated and deuterated salts (top to bottom). A given model is relevant in regions where the corresponding curves are linear. Supplementary data regarding the linear fits are provided in Table A1 (see Appendix).“
The mistypes in the captions of Fig. 8 and Tab. 3 have been corrected.
We thank the referee for making us aware that our original wording concerning the CO transition, more specifically how TCO was obtained, should be more clear (lines 95 – 102). In the compounds in which the TCO transition temperature was less pronounced and, therefore, could not be unambiguously inferred from our resistivity data, i.e., samples containing the anions Br and PF6, TCO were found in literature and indicated in our graphs for completeness and clarity. The exact phase transition temperatures were obtained from previous studies, where they were measured with methods targeting this feature, e.g., NMR, dielectric permittivity, x-ray, EPR measurements. Our aim was to shed light on the transport properties in the different phase regimes rather than confirming phase transition temperatures, i.e., studying the behavior of the samples inside the colored regions in Fig. 1, rather than obtaining the positions of the lines separating them. Therefore, the TCO markers in our data analysis -- Fig. 3, 7, and 8 – serve to indicate the known transition temperatures from literature demarcating the relevant temperature regions investigated here. We adapted the text (lines 95 – 102) to read as follows:
“For most Fabre salts, a transition to a 4kF charge ordered state occurs at TCO. In our compounds containing centrosymmetric anions, this transition is visible as a steepening in the slope of the resistivity curve observed upon cooling, and is especially strong in (TMTTF)2SbF6, while being less pronounced in (TMTTF)2PF6 and (TMTTF)2Br, see Figure 3. This observation is in agreement with results previously reported [7]. For (TMTTF)2PF6 and (TMTTF)2Br the transitions in deuterated and protonated crystals are difficult to infer from the raw data r(T), but can be identified in the temperature dependent energy gap plotted in Figure 8. The CO transition temperatures, as marked in our data, are listed in Table 1 [6,7,13,14,19].”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The subject matter of this paper, the roles of electron-electron and electron-ion interactions in the Fabre salts have been of continuing interest to the organic charge-transfer salts for a long time. The authors compare the resistivities of protonated versus deuterated (TMTTF)2X salts of four different anions, X = Br, PF6, SbF6 and ClO4. They are able to confirm earlier relationship between anion size and intrastack dimerization, and anion size and charge-ordering transition temperature. The authors then go on to analyze transport behavior and obtain temperature-dependent behavior of the energy gap (Fig 8). These are new results that will be of interest to the charge-transfer solids community.
This referee's attention was drawn however to the behaviors of the energy gaps at low temperature. In all the materials, but most significantly in X = PF6 and SbF6 the authors' plots suggest/indicate that the energy gap is lowered very strongly in these two materials. The authors should comment on whether this interpretation is correct. This is not unimportant. In the case of the PF6 salt T. Nakamura has shown that the amplitude of the charge-ordering is severely reduced as the material enters from the high temperature charge-ordered phase to the spin-Peierls phase (J. Phys. Soc. Jpn. 2006 and 2007). R. T. Clay et al have shown theoretically that the pattern of the charge-order changes at the low temperature 2kF transition in PF6 and AsF6, and simultaneously the amplitude of the charge-order is reduced (Phys. Rev. B 2007). Is it possible that the reduction in the energy gap in Fig 8 of current manuscript is showing the same thing, reduction in charge-order amplitude, which in that case would be perhaps the dominant contributor the gap?
There is no need for further review. But the authors should certainly comment on the reduction in the gap at low T. That is not optional. Whatever the reason is, it is conveying some important message that would be of interest to the readership.
Author Response
The referee raises an interesting question on the transition from the charge-order to spin-Peierls state. Unfortunately, our present setup does not allow us to go to sufficiently low temperatures in order to address this point. Due to the huge resistance, the experiments on the deuterated crystals of (TMTTF)2PF6 and (TMTTF)2SbF6 have been limited to T > 35 K. We hope to get back to that at a later point.
The reduction of the energy gap at low temperatures was previously explained by Köhler et al. [7]: At very low temperatures hopping conduction between localized carriers dominated the charge transport. In order to extract pure activated transport, this contribution has to be subtracted. An example is shown in Ref. [7].
The referee is correct that the charge order is affected by the spin-Peierls transition as best seen by vibrational spectroscopy and discussed in our review [4] in full length. We are presently conducting an infrared study on deuterated crystals to clarify this issue.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript presents a detailed investigation on deuteration effects on the transport properties of (TMTTF)2X salts. Both the development of the research and the report of results are scientifically sound and, in my opinion, the manuscript may be published in its present form.
I would, however, suggest the authors, if possible, to explain with some more detail the structural changes (relevant changes in distances, angles, ...) induced by the H -> D replacement. I think this is relevant information that would be nice to find in the same paper where changes in transport properties are reported.
Author Response
Unfortunately, no structural studies are available on deuterated crystals at low temperatures. Furthermore, in general the position of the hydrogen atoms cannot be refined. Vibrational spectroscopy could give some indications on the position of the terminal ethylene groups and coupling to the anions. This will be subject of future research.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
I have read the authors' response and also note that the author list now includes Dr. T. Nakamura whose work I was quoting.
Under the circumstances, I agree to the paper being published, as long as the editors are satisfied with the responses to the comments by the other reviewers.
