Peer review of: In situ nanoscale imaging of moiré superlattices in twisted van der Waals heterostructures (2020)


RR2
 REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author):
The authors successfully addressed my concerns. I would like to suggest to publish the manuscript.
Reviewer #2 (Remarks to the Author):
The authors have successfully addressed all my concerns and comments. I can now recommend it
 for publication.
Reviewer #3 (Remarks to the Author):   
 The authors responses are not completely satisfactory. I do not see any issues with most of their
 findings, specifically the very nice observation of hBN polaritons reflecting off of the domain-walls
 in TBG and the assignment that the latter reflection is caused by the spatially inhomogeneous
 optical conductivity at the domain wall. I emphasize, again, that I believe the latter observation is
 both interesting and important for the community. I, however, have remaining concerns with the
 presentation of the key scientific messages and feel that significant revisions are still needed. The
 two main issues I have are detailed below:
 1. The authors have stated that the morie pattern has been imaged in a fully encapsulated sample
 with minimal doping without the aid of polaritons in the superstrate within their own work, see Fig
 1d. So, the statement that <4 nm hBN superstrates and highly doped samples are needed to
 visualize the morie in encapsulated devices without the hBN polaritions appears to be disproved in
 the same report where it is claimed. If I have accurately assessed the situation, I believe these
 claims should be modified.
 2. My major technical concern is: it is still not clear if the specific form of the frequency dependent
 optical conductivity reported in Fig. 2d is fully supported by the data in this report. In the rebuttal
 the authors have stated: “The resonance at 0.2 eV alone is not essential to explain our findings”.
 Yet, the main text gives the impression that the frequency dependent optical conductivity reported
 in Fig. 2d is fully supported by the analysis and data presented in this work. Given the authors
 quoted response, it appears that even the gross features of the optical conductivity, such as the
 0.2 eV peak, are not, yet, an experimental fact.
 If the authors do not wish to claim to have experimentally determined the frequency dependent
 optical conductivity at the domain wall, I would feel much more comfortable if that were crystal
 clear from reading the text. The figure caption should clearly state that the conductivity is coming
 from a theoretical calculation. Further, adding brief discussion of the specific attributes of the
 conductivity which will require further study and experimental support would add tremendous
 clarity for the reader.
 If the authors do wish to claim to have experimentally determined specific aspects of the
 frequency dependent optical conductivity at the domain wall, then more support is needed. From
 the experimental side if the authors cannot provide hyperspectral linescans with FTIR then
 frequency dependent nano-imaging data, presented already in Fig. 3 and Fig. 4, can be analyzed
 where drift and signal to noise issues mentioned by the authors are readily overcome. The
 available data points of the near field amplitude from imaging data, normalized as
 S_{DW/BN}/S_{AB/BN}, can be overlaid on the FTIR data, normalized in the same manner, to
 fully support proper normalization and null influence of the propagating polariton. The result can
 be compared to nano-optical spectra calculated with the optical conductivity from the tight binding
 calculation. Finally, the authors should make crystal clear why gross features that they are

 claiming to determine, like the 0.2 eV peak, are absolutely necessary to explain their data. A few less severe issues are also present in the current version:
  I understand that poles of Im{Rp} can be compared to the wavevector of propagating modes and
 that the high-q frequency dependence of Im{Rp} can bear qualitative resemblance to
 experimental near-field spectra in certain cases. As is, I am still not sure what specific attributes of
 the frequency dependent optical conductivity at the domain wall are supported by the comparison
 of Rp and the data. As the authors claim, “We get a more complete picture by performing
 simulations that include the tip-sample interactions.” So, why does Rp need to be discussed at all?
 The colorbars for Fig 1c, d are missing; labels for the signals shown with false color maps are
 missing throughout the text.
 As the near-field amplitude signals in Fig. 2b were normalized to SiO2, the units are not arbitrary
 and the normalization procedure, while mentioned in the rebuttal, is still not mentioned in the text
 or SOM.
 The twist angle of the device in Fig. 3 is missing from the figure caption. The excitation frequency
 used in Fig 3. d-f is missing.