Spectra of collective excitations of particles in normal and superfluid liquids

V.V. Brazhkin^†
Vereshchagin Institute for High Pressure Physics, Russian Academy of Sciences, Kaluzhskoe shosse 14, Troitsk, Moscow, 108840, Russian Federation

We consider the specific features of collective excitations in disordered media, mainly in liquids. We discuss the difference between proper excitations in disordered media and plane harmonic waves, and the related problem of effective excitation damping. A detailed comparative analysis of the spectra of collective excitations in normal liquids and in superfluid helium is carried out. The aspects discussed include the phonon and roton parts of the spectrum, the phenomenon of 'fast sound,' shear waves, and the 'mixing in' of longitudinal and transverse excitations in liquids. Excitations in liquids are shown to determine the thermodynamic properties in both normal and superfluid states. Despite the qualitative similarity of the spectra of superfluid helium and ordinary liquids, they show significant differences, which have not yet received a proper theoretical description in our opinion. We emphasize the major differences between superfluid helium and superfluid Bose condensates of ensembles of ultracold atoms. A hypothesis is put forward regarding the possible major role that the high zero-point energy of helium atoms at ultralow temperatures plays both in understanding the features of the excitation spectra and in the superfluidity phenomenon itself.

Keywords: collective excitations, liquids, superfluid helium, heat capacity
PACS: 47.37.+q, 62.60.+v, 63.50.−x, 65.40.Ba, 67.25.−k (all)
DOI: 10.3367/UFNe.2025.01.039852
URL: https://ufn.ru/en/articles/2025/5/c/
Web of Science

001524725300004
Scopus

2-s2.0-105008972525
ADS NASA

2025PhyU...68..490B
Citation: Brazhkin V V "Spectra of collective excitations of particles in normal and superfluid liquids" Phys. Usp. 68 490–511 (2025)

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Received: 25th, November 2024, revised: 10th, January 2025, accepted: 28th, January 2025

Оригинал: Бражкин В В «Спектры коллективных возбуждений частиц в нормальной и сверхтекучей жидкости» УФН 195 519–542 (2025); DOI: 10.3367/UFNr.2025.01.039852

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