Peer review of: Turbulent hydrodynamics in strongly correlated Kagome metals (2020)

Editorial Note: This manuscript has been previously reviewed at another journal that is not operating a 
transparent peer review scheme. This document only contains reviewer comments and rebuttal letters 
for versions considered at Nature Communications. 

REVIEWERS' COMMENTS: 

Reviewer #1 (Remarks to the Author): 

I have read the revised version of the manuscript as well as the authors' responses to all three referee 
reports. These responses are detailed and I believe that the authors put a large amount of effort in 
addressing most of the issues raised to the best of their ability and to a very high standard. Naturally, 
limitations on what could be done (and be reasonably expected) exit because this paper crosses a 
number of subfields of physics (condensed matter physics, AdS/CFT, materials, ...). But this also 
makes the paper interesting and thought provoking. As a result of these detailed revisions, it is my 
opinion that the paper deserves to be published by Nature Communications. 



Reviewer #2 (Remarks to the Author): 

Although I feel the authors have done a competent job addressing most of the remarks 
of the previous referees, the central objection expressed in my previous report still 
remains. So, all I can do here is to expand on this point in greater detail. 

Hydrodynamic behavior in the condensed matter physics context is unusual because of 
the special conditions needed for it to be realized. To date, evidence for this has 
only been found in materials exhibiting nearly ideal conduction electron behavior, 
such as graphene and PdCoO2. In these materials, highly dispersive s-p electrons 
are involved in the conduction process. There are potentially other materials out 
there as well worthy of investigation. For instance, there are copper organics 
where dispersive conduction electrons are on a kagome lattice, and these materials 
even exhibit superconductivity (Huang et al, Angew. Chem Int. Ed. 57, 146 (2018)). 
So, had the authors picked one of these materials to study, I would have regarded 
this as a solid piece of work worthy of being considered for one of the Nature 
journals. 

Instead, the authors chose to study the hypothetical material Sc-hebertsmithite. 
Certainly, at the time Mazin et al published their paper (Ref. 6), there was some 
optimism that perhaps conducting versions of herbertsmithite might exist. But 
recent work has dashed this hope. In the work of Kelly et al (Phys. Rev X 6, 
041007 (2016)), almost two electrons per formula unit were introduced into 
herbertsmithite, yet the material remained insulating. In subsequent theory work by 
Liu et al (Phys. Rev. Lett. 121, 186402), this was understood to be due to polaron 
formation. And the one attempt to do as the authors suggested, which was to 
substitute the 2+ interlayer ions by 3+ ions by Puphal et al (Phys. Status 
Solidi B 256, 1800663 (2019)), also did not lead to conducting behavior. Although 
it is true that substitution by Ga was not complete (only 80%), the fact is, the best 
single crystals of herbertsmithite itself also have incomplete substitution (85%). 
Despite the authors' assertion about ionic sizes, the situation in the magnesium 
version of herbertsmithite is not that much different, despite the ionic size 
difference between Mg and Zn. 
In retrospect, this should not have been a surprise. Unlike cuprates that exhibit 
mobile carriers (though it would be extremely doubtful that these would ever be 
in the hydrodynamic regime as well), most other 3d transition metal oxides do not. 
Cuprates are special in that they are strongly in the charge transfer regime, 
thus promoting mobile carrier formation. But this is not the case for the 
herbertsmithite class of materials. Because of the replacement of 2- oxygen 
by 1- ions (OH, Cl), the energetics are completely different. In particular, one 
finds a large band gap of order 4 eV in herbertsmithite, more comparable to NiO, say, 
than undoped cuprates. I would bet the authors an expensive bottle of wine 
that even if Sc-herbertsmithite could be synthesized, it will not be conducting. 
Because of this, I just don't see the rationale behind the present paper, and 
unfortunately cannot recommend its publication in Nature Communications. I say 
"unfortunately" because I personally found the paper to be very thought provoking.