> Re: BN13654 > Elastic properties of hidden order in URu 2Si 2 are reproduced by a > staggered nematic > by Jaron Kent-Dobias, Michael Matty, and B. J. Ramshaw > > Dear Jaron Kent-Dobias, > > The above manuscript has been reviewed by two of our referees. > Comments from the reports appear below. > > These comments suggest that the present manuscript is not suitable for > publication in the Physical Review. > > Yours sincerely, > > Sarma Kancharla > Associate Editor > Physical Review B > Email: prb@aps.org > https://journals.aps.org/prb/ > > NEWS FROM THE PHYSICAL REVIEW JOURNALS > > Announcing a new Physical Review journal: PRX Quantum > https://go.aps.org/2Q0uVwP > > Celebrating 50 years of Physical Review A, B, C, and D > https://go.aps.org/2IDjVBM > > APS is offering promotional open access fee discounts on most > journals in 2020 > https://go.aps.org/3388lYG > > ---------------------------------------------------------------------- > Report of the First Referee -- BN13654/Kent-Dobias > ---------------------------------------------------------------------- > > The work deals with a purely phenomenological model for the “hidden” > order parameter if URu2Si2, with particular emphasis on the expected > elastic properties. The work might eventually be suitable for Phys. > Rev. B, but some aspects are not clear to me. > > The main result is Fig. 2, where the behavior around TN is difficult > to see. I suggest to add zooms on that crucial T-range, where it seems > to me that there is a qualitative difference between model and > experiments. The justification given by the Authors (“mean field > theory—which is based on a small-eta expansion—will not work > quantitatively far below the transition where eta has a large nonzero > value and higher powers in the free energy become important”) does not > look plausible as the disagreement does not appear to develop slowly > as T decreases, but appears immediately below TN, where eta is small. The disagreement between the theory at low temperature is resolved by the addition of an additional interaction in the mean-field free energy of the form ε²η², now shown in Fig. 2 as a thin black line. > Is it not clear how discriminatory is the agreement above TN in 2a, 2b > and 2c. Are calculation results robust over a wide range of fitting > parameters, or does the agreement result from a fine-tuning? (e.g., > the presence of a maximum at 120 K in 2b). [Not exactly sure what this means.] > Is it possible to say something about the c/a ratio, which displays a > non-trivial T-dependence? [Not sure what this means either. Is this asking about the ratio of lattice constants?] > At last, I understand that the model is meant to be purely > phenomenological, but given the plethora of publications on URu2Si2 > over 30 years, where any conceivable order parameter has been proposed > as candidate, the Authors should make a connection between their > abstract OP and possible physical realizations. For instance, in the > simplest framework of localized f-electrons, what ionic moments would > fit the present proposal? [Mike, can you help with this?] > ---------------------------------------------------------------------- > Report of the Second Referee -- BN13654/Kent-Dobias > ---------------------------------------------------------------------- > > In this paper, possible elastic properties of URu2Si2 are studied with > focusing on the long-standing hidden order (HO) problem. The authors > introduce a generic form of the free energy density for the elastic > energy, a modulated order parameter, and their mutual coupling, and > analyze the temperature dependences of the elastic constants by > minimizing the free energy. It is shown that the B1g component > exhibits a remarkable softening with decreasing temperature and a cusp > singularity at the HO transition point, and these results are compared > with recent ultrasound experiments. From the comparison, the authors > conclude that the HO phase of URu2Si2 originates from the modulated > B1g order parameter. > > In the course of evaluation, the referee does not recommend the paper > to be published in PRB, mainly based on the following reason. > > 1) The scheme for the coupled strains in this paper is quite standard > within the mean-field treatment and does not provide a novel > theoretical advance. > > 2) One can generically expect several sources for softening elastic > constants. For example, the authors in ref.25 also succeeded in the > quantitative fits in the framework of a 4f crystal field model for T > > T_HO. Thus, the fitting is not regarded as the decisive evidence on > the validity of the model. [Not sure how to respond to this; Brad?] > 3) The agreement of C[B1g] in the region T the cusp at T_HO seems qualitatively consistent with the experiment. > Moreover, the referee expects that even a cusp structure in the > elastic constants is not unique to this model; it can be obtained from > more general models beyond the linear coupling (4), within the > mean-field level. Therefore, the referee thinks that this analysis > does not lead uniquely to the authors' arguments on the realization of > the B1g order parameter. The disagreement between the theory at low temperature is resolved by the addition of an additional interaction in the mean-field free energy of the form ε²η², now shown in Fig. 2 as a thin black line. While terms like this provide cusp-like features in the modulus for each strain symmetry, they cannot explain the 1/ΔT softening seen in the high-temperature phase, since their contribution to the response function is zero above T_c. > 4) The most important point in the HO problem is the microscopic > identification of symmetry breaking and the order parameter. In spite > of the long history in research over almost 40 years, there is no > experimental evidence of the formation of any superlattice structure > at least at ambient pressure. So, the proposed modulated order is not > consistent with the absence or identification of symmetry breaking. > The authors do not provide any resolution on that point which is the > most relevant in this problem. The articles below provide experimental evidence for the formation of superlattice structure along the c-axis at ambient pressure. [I pulled these from our citation on the estimate for q_*. Do they actually provide the evidence we need?] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.127002 https://journals.aps.org/prb/abstract/10.1103/PhysRevB.43.12809 https://journals.jps.jp/doi/10.1143/JPSJ.79.064719 https://www.nature.com/articles/nphys522