Superconductivity and Fermi Surface Studies of β″-(BEDT-TTF)2[(H2O)(NH4)2Cr(C2O4)3]·18-Crown-6
, David Graf 2, Lee Martin 3, Toby J. Blundell 3 and Charles C. Agosta 1,*
Round 1
Reviewer 1 Report
The two-dimensional organic superconductor β”(ET)Cr was a very good candidate for observation of the exotic superconducting FFLO state. A specific feature of this material is an exceptionally large separation between conducting ET layers, as described in Ref. [1]. The present paper is an interesting continuation of the superconductivity research presented in Ref. [1]. The radio frequency penetration depth was investigated by using a contactless tunnel diode oscillator technique. Unique results, obtained at very low temperatures as a function of magnetic field (up to 32 T) oriented at different angles with respect to the conducting layers, are reported. Although no FFLO state has been found, the paper presents valuable and interesting data, such as the critical magnetic field Hc2 and Shubnikov-de Haas oscillations up to 28 T, both measured for different magnetic field orientations. I agree with the Author’s opinion that the discovery of high Hc2 anisotropy is a remarkable result. The results are thoroughly discussed and compared with literature data for other two-dimensional organic superconductors. I think that it is a very good and very well-written paper which should be accepted for publication in Magnetochemistry.
My small remarks related to the text are as follows:
- In Fig.1, Fig. 2a, Fig. 3, and Fig. 4a the penetration depth is given in arbitrary units, nevertheless in these figures we see numbers in the ordinate axes, without any units. I think that it would be better to remove the numbers.
- Page 4, line 122: We see an abbreviation NHMFL. For a reader its meaning can be unclear .
- Page 5, Fig. 4c: In the ordinate axis we see “SdH freq” but in the figure caption and also in the text “FSdH”.
- Page 5, line 131: We read about “Lifshitz-Kosevich formula” but without any suitable citation. I seems to me that it would be better to give this formula or at least a citation where it can be found.
Author Response
We thank the reviewers for their careful review of our manuscript and the timely responses that were provided. We address the referees’ comments (R1-R4) below. As requested by the editor, edits are highlighted in yellow in the updated LaTeX file.
R1: “We see an abbreviation NHMFL. For a reader its meaning can be unclear.”
-We now clarify this by defining the abbreviation (NHMFL) earlier, in the Materials and Methods section of the text.
R1: “In Fig.1, Fig. 2a, Fig. 3, and Fig. 4a the penetration depth is given in arbitrary units, nevertheless in these figures we see numbers in the ordinate axes, without any units. I think that it would be better to remove the numbers.”
-We agree this could be confusing but changed the figures in a different way than suggested. The y-axes of these graphs are the TDO frequencies, which we believe are relevant to include for numeric comparisons, so we have adjusted the y-axis labels of these figures to reflect that. The TDO frequencies are proportional to rf-penetration depth, which is stated in the introduction, and is now made clearer above Fig. 1 in the text, and on the y-axis of Fig. 1. To eliminate unnecessary clutter, we change Fig. 2a, 3, and 4a to frequency only, with the understanding from the reader that frequency corresponds to penetration depth. We believe these changes still address the reviewer’s concern.
R1: “Page 5, line 131: We read about “Lifshitz-Kosevich formula” but without any suitable citation. It seems to me that it would be better to give this formula or at least a citation where it can be found.”
-Added citation (Shoenberg).
R1: “Page 5, Fig. 4c: In the ordinate axis we see “SdH freq” but in the figure caption and also in the text “FSdH”.”
-The axis label and legend has been updated to match the text.
Reviewer 2 Report
The authors a very nice paper on their penetration depth and Fermi surface measurements of some molecular superconductor. The manuscript is written well and contains some interesting points related to the strong anisotropy and worth to be published. Although the authors obviously address a small audience, I suggest to extend the introduction and start with a more general paragraph on these compounds and why they are interesting. This should be supplemented with some more general references to molecular quantum materials and organic superconductors.
Author Response
We thank the reviewers for their careful review of our manuscript and the timely responses that were provided. We address the referees’ comments (R1-R4) below. As requested by the editor, edits are highlighted in yellow in the updated LaTeX file.
R2: “I suggest to extend the introduction and start with a more general paragraph on these compounds and why they are interesting. This should be supplemented with some more general references to molecular quantum materials and organic superconductors.”
-An expanded introduction has been added with the goal of explaining to a wider audience why layered organic superconductors are of interest to study.
Reviewer 3 Report
This manuscript reports SdH studies on an organic superconductor with the thickest insulating layers, expecting observation of an FFLO state. Although they did not observe FFLO states, the manuscript is clear and concise, providing an important piece of information for those interested in related work. Thus I would like to recommend publication of this manuscript and suggest some minor points for the authors to kindly consider in possible revision. (1) L. 131 (P. 5) g = 1.89; The estimated g-values are rather small for related molecular conductors (the chalcogen donor salts, in particular). Although the present authors refer to the small effective mass of the compound, they should add some comments on the validity of thus estmated g-value. (2) L. 177 (P. 7) Orbital critical field calculations. It appeared ambiguous to this reviewer that which values in Table 1 should be compared with H_c2_vertical. H0_orb(T)? If so, please state explicitly in the main text (L. 177). (3) L. 198 (P. 7) The conclusion may be unclear. It appears to this reviewer that the manuscript does not have conclusion in the present form. (4) There are some mistypo in letters representing physical parameters like B, g and T, which are not written in italic in some sentences and figures.
Author Response
We thank the reviewers for their careful review of our manuscript and the timely responses that were provided. We address the referees’ comments (R1-R4) below. As requested by the editor, edits are highlighted in yellow in the updated LaTeX file.
R3: “L. 131 (P. 5) g = 1.89; The estimated g-values are rather small for related molecular conductors (the chalcogen donor salts, in particular). Although the present authors refer to the small effective mass of the compound, they should add some comments on the validity of thus estmated g-value.”
-An expanded comment has been written regarding the reasoning behind the validity of this claim (See: McKenzie). In addition, the new conclusion suggests future experiments to further verify our reported value and discusses limitations to our current accuracy.
R3: “L. 177 (P. 7) Orbital critical field calculations. It appeared ambiguous to this reviewer that which values in Table 1 should be compared with H_c2_vertical. H0_orb(T)? If so, please state explicitly in the main text.”
-We believe the order of the columns in Table 1 best matches the comparisons outlined in the text. H_c2_parallel (Col.#2) is compared with H_c2_perp (Col.#3), showcasing the remarkable anisotropy. H_c2_perp (Col.#3) is then compared with H_orb (Col.#4), Equation 3 in the text. The approximation in Equation 3 should be satisfied if H_orb (Col.#4) << H_P (Col.#5). Based on the reviewer’s suggestion, we have commented explicitly between Equations 4 and 5 that the calculated H_orb is close to our measured H_c2_perp value, due to the orbital effect being the dominant cause of the destruction of superconductivity.
R3: “L. 198 (P. 7) The conclusion may be unclear. It appears to this reviewer that the manuscript does not have conclusion in the present form.”
-An expanded conclusion has been added to the revised manuscript, in part following the request to extend the document over 4000 words. In this new conclusion is a summary of important findings and a suggestion for future experiments on this material, both to confirm the accuracy of the data presented here and to acquire more data that has not yet been collected.
R3: “There are some mistypo in letters representing physical parameters like B, g and T, which are not written in italic in some sentences and figures.”
-We have done our best to edit text to be of the accepted standard of Italicized vs. Roman upright.
Reviewer 4 Report
Some changes and additions should be made to improve the perception of the manuscript.
L3. I don't think the reference in the abstract is necessary.
L7. What is “FFLO state”
L15. For the first phrase, references are needed to support the statement about many crystalline organic substances.
L63. It is necessary to provide a description of the synthesis experiment in the article or in supplementary materials to the article. It is also necessary to supplement the materials with evidence of the desired substance obtaining.
L67. What is the orientation of a single crystal in a measuring cell? Along what crystallographic axis?
Author Response
We thank the reviewers for their careful review of our manuscript and the timely responses that were provided. We address the referees’ comments (R1-R4) below. As requested by the editor, edits are highlighted in yellow in the updated LaTeX file.
R4: “L3. I don't think the reference in the abstract is necessary.”
-We agree, and the reference was removed from the abstract.
R4: “L7. What is “FFLO state?”
-We removed the abbreviation from the abstract, as “FFLO” was not previously defined, and inserted “inhomogeneous superconductivity” as replacement. “FFLO” is defined later in the main text.
R4: “L15. For the first phrase, references are needed to support the statement about many crystalline organic substances.”
-We agree. References were added to support this claim.
R4: “What is the orientation of a single crystal in a measuring cell? Along what crystallographic axis?”
-Despite being diligent about labeling if the magnetic field orientation is parallel or perpendicular to the layers during the experiment, we agree that it is unclear which crystal axes these orientations correspond to. We added clarification to the end of the introduction that “parallel” means “in the a/b plane” and “perpendicular” means “in the c axis”.
R4: “It is necessary to provide a description of the synthesis experiment in the article or in supplementary materials to the article. It is also necessary to supplement the materials with evidence of the desired substance obtaining.”
-We added that the crystal was synthesized by electrocrystallization method, but the synthesis is otherwise well described in the reference provided (Martin, et al.).
Round 2
Reviewer 4 Report
Accept in present form
