Band Structure and Physical Properties of α-STF₂I₃: Dirac Electrons in Disordered Conduction Sheets

Round 1

Reviewer 1 Report

 

 

 

 

The manuscript by Naito and Doi makes the interesting proposition that $\alpha$-STF$_2$I_3$ is a zero bandgap semiconductor with Dirac electrons.
To a large extent, this conclusion is based on the band calculation of reference 22. The present experimental manuscript
is an attempt to confirm the conclusion of reference 22, but actually ends up raising several questions that are not clearly answered in the
present version of the manuscript. The last sentence of the manuscript is a good example of the latter: ``This means that ... between relativistic
and non-relativistic fermions ... between Fermi liquids and correlated electron systems.'' Do the authors believe or not believe that
the behavior of $\alpha$-STF$_2$I_3$ reflect Dirac cones? Clarification is needed. The resistivity behavior below 100 K clearly argues against
Dirac electron description; the T-dependence of the optical conductivity on the other hand is very suggestive. At the same time, the authors
state that the magnetic susceptibility reflects strong correlation behavior. Do the correlations give an energy gap that obliterates the
Dirac cone? More explicit discussions by the authors of these issues are warranted.

This referee was also unable to understand the discussion of the UV absorption spectrum. The authors state that the absorption band
with maximum at 326 nm corresponds to the HOMO-LUMO transition in the STF monomer but also state that excitations at wavelength 375 nm
``corresponds to direct perturbation to the carriers around the Fermi level''. How do we see this? Is the latter conclusion based on experiment
or on the calculation of ref 22?

In section 3.1.1 first paragraph the word ``monotonous'' should be replaced with ``monotonic''
[``monotonous'' means dull, tedious, and repetitious; lacking in variety and interest]. Two lines below ``magnetic'' is misspelt [``magentic''].

 

Author Response

Please see the attachment, "Reply to the Reviewer 1.pdf"

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript of Naito and Doi contains a large amount of interesting data on alpha-(BEDT-STF)2I3 that is a sister compound of alpha-(BEDT-TTF)2I3 which is well known for its charge order transition at 135 K and linear band dispersion under pressure and at low temperatures similar to Dirac electrons.
The authors cover structural and magnetic properties with transport and optical measurements. Some of the data are new, some are taken from literature for comparison, others are recorded a quarter of a century ago, but remain unpublished since.
This allows the authors to have a new look on the conductivity spectra and
analyse them in the view of lindear band dispersion.
Independent whether these data provide are similar or different to previous
reports, they would provide an interesting basis for a nice and useful paper/review.

Unfortunately the authors completely fail of doing so.
The dc resistivity of new crystals is similar to previous reports, which is good.
The sample-to-sample difference shown in Fig. A2 is quite common, as well
as the heating/cooling hysteresis and jump in the dc-curve, which can be explained by cracks.
The magnetic properties of rather standard, too, and discussed in a reasonable way.
This is not the case of the absorption spectra shown in Fig. A4.
The photo-conductivity data basically show a heating effect upon UV irradiation, which is not surprising, but only mentioned in the last section.
The ancient optical spectra are plotted in a very unfortunate way that does not allow to see any of the important features. If the data are available in a digital form, they should be plotted differently.

At the end, there is no real conclusion drawn, but some interesting outlook given.

The worst part of the manuscript is the abstract, which does not give a concise description of what to expect and what the results are, but list details of experimental findings and open questions: this is not an abstract.

In addition:
the authors should try to put more reference where necessary (e.g. the x-ray data discussed at the bottom of page 2). Most important, the references do not always cover the broad effort and achievements of the international community but is very much confined to the Japanese colleagues next to the authors.
We call for well balanced citations.

To summarize:
we encourage the authors to spend considerably more efforts to present their new and old results in comparison with literature data in a more concise way. We suggest to restart completely from scratch.
The separation of data in the main part and in the appendix is not always solved the best possible way. Most important the presentation, analysis and discussion of the results has to be improved substantially. If done so, they authors should be able to provide more physical insight and depth, despite the shortfalls of the available material and lack of theory, they are aware of.

Author Response

Please see the attachment, "Reply to Reviewer 2.pdf"

Author Response File: Author Response.pdf

Reviewer 3 Report

Recent calculations have shown that the salt α-ST₂I₃ has a band structure with Dirac cones at ambient pressure and temperature. Similar band structure was observed in the salt α-ET₂I₃ under high pressure, where existence of the Dirac electrons was established. Because of the possibility of finding the Dirac electrons at ambient pressure and temperature the salt α-ST₂I₃ is a very interesting molecular conductor. The band structure calculation of α-ST₂I₃ suggest that the electrical conductivity should exhibit a large anisotropy. For this reason measurements of the electrical conductivity anisotropy seem to be very important investigations which could provide arguments for the presence of Dirac electrons. I think that both the specific salt α-ST₂I₃ and the problem of Dirac electrons are very interesting and hot topic, nevertheless I am not convinced that the reviewed paper really reports new and valuable  experimental data.

In my opinion in comparison with previous studies the electrical conductivity data under dark do not give anything new and interesting. Moreover, there is no information about the anisotropy which – as results from the band structure calculations – should be considerable. As a matter of fact, in the experimental part is mentioned that “… alignment of the four gold wires on the crystal surface was different between measurements…” but is not sure that the electrical conductivity was measured along important crystallographic axes. It seems to me that he electrical conductivity data under UV-radiation are useless. All the observed modifications of the resistivity are a consequence of thermal effects caused by irradiation. The temperature T_x has no physical meaning. If the thermal contacts between the sample and the cryostat cold finger were different or the thickness of electrodes was different, i.e. heat transport efficiency was different, it can be expected that T_x would be different.  The only conclusion resulting from these data is as follows: no photoconductivity effect is seen. I think that showing the reflectance spectra of α-ST₂I₃ from Ref. [20] without any more detailed discussion has no sense. These spectra give nearly nothing related to the Dirac electrons. In the region 6000-1500 cm^-1 the spectra are more or less similar to the spectra of many other two-dimensional conducting ET salts, in which the Dirac electrons do not exist. In the paper [33] about a Dirac semimetal by D. Neubauer et al. the optical conductivity in the region 2000-8000 cm^-1 was fitted be a power low. For α-ST₂I₃ no fit was done. By the way I would like to mention that it is rather strange to see that the reflectance is larger than 100% in Fig. 5 – information about an offset in the figure caption does not change this impression. I wonder why references [28-32] are given. The abstract is not well-written. The paper sense is lost between too many detailed information. In many places, especially in 1. Introduction and Materials and Methods, the text should be divided into sections – reading would be easier. The English should be also improved.

In conclusion, because of the above reasons I do not recommend to accept the paper for publication in Crystals.

Author Response

Please see the attachment, "Reply to Reviewer 3.pdf"

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors did improve the manuscript, although I feel there are still enormous possibilities of analyzing and discussing the results.

The English language has to be improved! At present, the level is just too low.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The paper has been considerably modified. Practically, now it is a new paper. Thank you very much for taking into account all my critical remarks. Nevertheless, I still have some remarks and comments which I think should be considered before the final acceptance for publication in Crystals.

1. Page 2, line 46. Is it necessary to give so many references (citation [6-46]) assigned to the salt α-ET₂I₃? This is not a review paper. In my opinion only the most important and necessary references should be given. The same remark applies to the citation [47-75] in the line 49. Generally, the paper is not improved by so many additional references – most of them are not necessary.
2. Page 4, line 109. I wonder what does mean the following sentence: “…latter direction made an angle of ~0 to ~90 degree with the a-axis, depending on the crystal.” How it is possible that the b-axis (i.e. latter direction) forms so various angles with the a-axis?
3. Page 5, line 161. I think that “nor” should be replaced by “and”.
4. Page 5, line 175. Now, in the corrected paper information that the electrical resistivity was measured “under dark” is not necessary.
5. Page 8, Fig.7. The optical conductivity at the top of band at about 1000 cm^-1 for T = 25 K (Fig. 7c) is about 350 S/cm and in the Fig. 7d (upper part) it is about 550 S/cm (these are the same data). Moreover, I wonder why Fig. 7d is presented? This figure is prepared from copies of Fig. 7c and Fig. 7b but displayed in a different conductivity scale. I think that Fig. 7d is shown to convince readers that the optical conductivity is nearly constant. However, this is an approximation only if we compare with Fig. 7b and 7c (straight lines fit the data so well in Fig. 7d because of the conductivity scale). I suggest to plot these straight lines in Fig. 7b and 7c and additionally discuss the data. The Fig. 7d is not necessary.
6. Page 9, Discussion. I am not convinced that Dirac electrons really exist in the salt α-STF₂I₃. The possibility of existence of these electrons is strongly suggested by the calculated band structure (Fig. 2), nevertheless the paper does not provide strong arguments that it is really true. On the other hand, the chapter Discussion is written is such way that a reader can think that it is proved that that the Dirac electrons exist. May be it would be better to write about a "possibility of existence of the Dirac electrons"?

Author Response

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