Peer review of: Gapless spin liquid in a square-kagome lattice antiferromagnet (2017)





RR3
 REVIEWERS' COMMENTS:
Reviewer #4 (Remarks to the Author):
  I went through the revised manuscript, the comments from the reviewers and the answers to the
 comments.
 There were several points raised by the referees which have been answered satisfactorily to by the
 authors, according to the referees but there seemed to be some remaining controversy about the
 muSR experiment which I address below. At the end of this report, please find a few additional
 remarks after I read the manuscript.
 Indeed, after a first really misleading response by the authors to explain the difference between
 asymmetries in different cryogenic set ups, the authors have referred to a technical report dated 2013
 showing that the measured asymmetry in the dilution refrigerator (DR) is consistent with their data.
 Yet, this does not explain why the asymmetry is larger in the He4 cryostat, for a given set of detectors
 suc has that from the D1 beamline, usually it goes in the opposite way as the multiplicity of walls in
 the DR usually leads to a selection of higher energy for the emitted positrons then an increased
 asymmetry. There are two possibilities for explaining this:
 An upgrade occurred in the set-up of D1 instrument, 0.18 is the asymmetry before the upgrade
 [Journal of Physics: Conference Series551(2014) 012063] and it was about 0.205 [JPS Conf. Proc. ,
 011062 (2018)for the S1 spectrometer], anyway < 0.24 after the upgrade of D1 identical to that of
 the S1 spectrometer. The authors should check with the beamline scientist whether this qualifies for
 explaining partly the Asymmetry difference, if the experiments in the two setups were performed
 before and after the upgrade.
 An alternative and very likely additional possibility is that the sample mounted in the He4 cryostat was
 much more massive leading to an increased asymmetry for a given set-up.
 Anyhow, I believe that such details are not commonly mentioned neither in the main body of
 manuscripts nor in Supplementary Materials, even in full muSR papers. I agree that the third
 reviewer’s criticisms should have been handled in a proper manner once being raised, his question
 was clear and definitely calling for a much more expert answer, the least I can say, than the first
 given one. Overall, the DR experiment per se clearly shows that all the asymmetry is detected from 58
 mK up to 5K, hence there is no hidden frozen component and the fully detected signal is purely
 dynamical at base-T, the major conclusion from the muSR study – unless one parasitic phase would
 freeze at very high temperatures but it would have been detected through x-rays.
 In passing, I have two comments:
 (i) references to gapped vs ungapped discussion in herbertsmithite look completely out of date. Better
 use the more appropriate NMR references: M. Fu, Science, P. Khuntia, Nature Physics
 (ii) disqualifying the quantum spin liquid physics in herbertsmithite by advocating the scenario
 disorder from K. Kawamura Sensei, is not correct, as the amount of requested disorder Is certainly not
 present in herbertsmithite.
 (iii) the Curie tail in the susceptibility has not received any explanation other than free spins. Yet
 attributing to grain surface effect does not make sense in a few % ratio for standard powers, hence
 this interesting square-kagome lattice antiferromagnet suffers from some defects, a common feature
 to all these kagome-based materials up to now. This does not obscure at all the fact that the discovery
 of this material is a nice achievement.

 (iv) the fluctuating field seems quite small from the decoupling experiment. This could have been
 discussed in the paper.