Universality of phonon-roton spectrum in liquids and superfluidity of He II

TL;DR

This paper argues that the phonon-roton spectrum is a universal feature of liquids, caused by short-range order, and predicts an additional collective excitation branch in superfluid helium based on superfluidity criteria.

Contribution

It demonstrates the universality of the phonon-roton spectrum in liquids and predicts a new excitation branch in superfluid helium.

Findings

Phonon-roton spectrum is universal in liquids.

Roton minimum arises from short-range order.

A new collective excitation branch is predicted in He II.

Abstract

Based on numerous experimental data on inelastic neutron and X-ray scattering in liquids, we assert that the phonon-roton spectrum of elementary excitations, predicted by Landau for superfluid helium, is a universal property of the liquid state. We show that the existence of the roton minimum in the spectrum of collective excitations is caused by the short-range order in liquids. Using the superfluidity criterion, we assume that one more branch of collective excitations should exist in He II, whose energy spectrum differs from the phonon-roton spectrum.