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Register a new userComment on Collective modes and gapped momentum states in liquid Ga: Experiment, theory, and simulation
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We show that the presented in Phys.Rev.B, v.101, 214312 (2020) theoretical expressions for longitudinal current spectral function $C^L(k,omega)$ and dispersion of collective excitations are not correct. Indeed, they are not compatible with the continuum limit and $C^L(k,omegato 0)$ contradicts the continuity equation.
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We show, that the theoretical expression for the dispersion of collective excitations reported in [Phys. Rev. B {bf 103}, 099901 (2021)], at variance with what was claimed in the paper, does not account for the energy fluctuations and does not tend in the long-wavelegth limit to the correct hydrodynamic dispersion law.
In a recent paper, S. Singh and K. Tankeshwar (ST), [Phys. Rev. E textbf{67}, 012201 (2003)], proposed a new interpretation of the collective dynamics in liquid metals, and, in particular, of the relaxation mechanisms ruling the density fluctuations propagation. At variance with both the predictions of the current literature and the results of recent Inelastic X-ray Scattering (IXS) experiments, ST associate the quasielastic component of the $S(Q,omega)$ to the thermal relaxation, as it holds in an ordinary adiabatic hydrodynamics valid for non-conductive liquids and in the $Q to 0$ limit. We show here that this interpretation leads to a non-physical behaviour of different thermodynamic and transport parameters.
We discuss the validity of recent results in [Phys. Rev. Lett. 125, 125501 (2020)] on an universal relation between the heat capacity and dispersions of collective excitations in liquids.
We comment on three incorrect claims in the paper by Fomin et al (arXiv:1507.06094) concerning the generalized hydrodynamic methodology and positive sound dispersion in fluids.
Inelastic x-ray scattering (IXS) measurements of the dynamic structure factor in liquid Na57K43, sensitive to the atomic-scale coarse graining, reveal a sound velocity value exceeding the long wavelength, continuum value and indicate the coexistence of two phonon-like modes. Applying Generalized Collective Mode (GCM) analysis scheme, we show that the positive dispersion of the sound velocity occurs in a wavelength region below the crossover from hydrodynamic to atom-type excitations and, therefore, it can not be explained as sound propagation over the light specie (Na) network. The present result experimentally proves the existence of positive dispersion in a binary mixture due to a relaxation process, as opposed to fast sound phenomena.
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