Condensed Matter

In their Letter, Borda, Cai, and de Koning report the results of ab initio simulations of dislocations responsible for the giant plasticity. The authors claim key insights into the recent interpretations of (i) the giant plasticity and (ii) the mass flow junction experiments. The purpose of this Comment is clarifying the role of dislocations in the mass flow in conjunction with explaining that the part (ii) of the claim is misleading.

The comment explains that the preprint arXiv:1503.0042 has not presented persuasive theoretical or experimental arguments of existence of spin supercurrents in a magnon condensate prepared in a room temperature yttrium-iron-garnet magnetic film because the authors did not check known criteria for existence of spin supercurrents in magnetically ordered materials. Also they did not compare their supercurrent interpretation with a competing and more realistic scenario of transport by spin diffusion.

Recently the spectacular result was derived quantum mechanically that the total angular momentum of photons in light beams with finite lateral extensions can have half-integer quantum numbers. In a circularly polarized Gauss light beam it is half of the spin angular momentum which it would have in a respective infinitely extended wave. In another paper it was shown by a classical calculation that the magnetic moment induced by such a beam in a metal is a factor of two smaller than the one induced by a respective infinitely extended wave. Since the system's angular momentum is proportional to its magnetic moment it could be assumed that the classical result for the magnetic moment reflects the transfer of the total angular momenta of the beam photons to the metal. Here we show that there is no hint that this is indeed the case.

This comment regards a central aspect of the referred-to paper, the issue of convergence of the large coordination-number expansion. Perturbation expansions of expressions containing a large number of parameters are generally invalid due to the non-analyticity of the expanded expressions. I refer to recent work where these issues are analyzed and discussed in detail in relation to a benchmark example of a cluster model. As discussed therein, methods which are uncontrollable and for which their convergence is not foreseeable are not only useless but can mislead, particularly if models derived from them are used to interpret experiments.

Loubeyre, Occelli, and Dumas (LOD) [1] claim to have produced metallic hydrogen (MH) at a pressure of 425 GPa, without the necessary supporting evidence of an insulator to metal transition. The paper is much ado about nothing. Most of the results have been reported earlier. Zha, Liu, and Hemley [2] studied hydrogen at low temperature up to 360 GPa in 2012; they reported absorption studies up to 0.1eV. Eremets et al [3] studied dense hydrogen up to 480 GPa using standard bevel diamonds. They reported darkening of the sample and electrical conductivity in which they reported semi-metallic behavior around 440 GPa. In 2016 Dias, Noked, and Silvera [4] reported hydrogen was opaque at 420 GPa. In 2017 Dias and Silvera observed atomic metallic hydrogen at 495 GPa in the temperature range 5.5-83 K [5].

In a recently published article [1], Ranga P. Dias & Isaac F. Silvera have reported the visual evidence of metallic hydrogen concomitantly with its characterization at a pressure of 495 GPa and low temperatures. We have expressed serious doubts of such a conclusion when interviewed to comment on this publication [2,3]. In the following comment, we would like to detail the reasons, based on experimental evidences obtained by us and by other groups worldwide that sustain our skepticism. We have identified two main flaws in this paper, as discussed in details below: the pressure is largely overestimated; the origin of the sample reflectivity and the analysis of the reflectance can be seriously questioned.

The paper shows that the kinetic equations considered in [1], equilibrium distribution obtained in [1], and results and conclusions obtained on the basis of the kinetic equation derived in [1] do not correspond to the mixed Bose-Fermi statistics. Moreover, it is shown that the kinetic equation corresponding to the case when the copies of the system are characterized by different values of the fraction of the Fermi-like moves is incorrect. We present a correct kinetic equation for the mixture of the Bose and Fermi moves and obtained the equilibrium distribution for the case when the probability of the Fermi moves is higher or equal to that of the Bose moves.

It is shown that the potential of Keesom which depends on temperature cannot be used for calculate the second virial coefficient of polar molecules and the formulae for second virial coefficient of [2] have no molecular statistical mechanical base for water and carbon dioxide. The contradictions and errors of [2] are discussed.

We showed that the equations (3), (4), (5) and (6), used in the paper Total and fractional densities of states from caloric relations by S. F. Chekmarev and S. V. Krivov, Phys. Rev. E 57 2445 (1998), are incorrect, the data, presented in the paper by lines on Figs. 1, (3a) and (3b), are not correct, the data presented by the symbols on Figs. 3(a) and 3(b) in the paper are made manually (false), all conclusions made in the paper have no sense, the assertion in the paper that the molecular dynamics simulations sample the potential energy surface not uniformly, but according to the fractional densities of state for the isomers is incorrect. We showed also that the total and fractional densities of states obtained in the paper from caloric relations are not equal to that of microcanonical ensemble of clusters, the ensemble of clusters used in the paper does not represent the microcanonical ensemble of clusters.

It is shown that the isobaric heat capacity of chalcogenides , , , and can be described by the Debye and Einstein models for the phonon frequency spectrum within their uncertainties; the models give the results for the isochoric heat capacity which are close to each other; the models give the close results for the difference between the isobaric and isochoric heat capacities; the isobaric heat capacities of the isostructural , , and as the functions of the temperature reduced to the Debye (Einstein) temperature are described by single Debay (Einstein) equation for the isobaric heat capacity; the isochoric heat capacities of , , , and (which has another structure than , , and [1]) as the functions of the temperature reduced to the Debye (Einstein) temperature are described by the Debye (Einstein) equation for the isochoric heat capacity. It is shown also that the Debye and Einstein equations for the isochoric heat capacity of , , , and give the same results if the means of the squares of the frequencies of the Debye and Einstein spectra are equal to each other, and the Debye and Einstein equations for the isobaric heat capacity of , , and as the functions of the temperature reduced to the Debye or Einstein temperature give the same results.