Universality of rotons in liquids, generalized superfluidity criterion and helons in helium II

TL;DR

This paper proposes that rotons are universal excitations in liquids and introduces a generalized superfluidity criterion, emphasizing the role of temperature-dependent excitation spectra and a new gapped spectrum associated with superfluid transition.

Contribution

It introduces a new perspective that rotons are universal in liquids and generalizes the superfluidity criterion to include multiple excitation branches, supported by experimental and numerical evidence.

Findings

Rotons are universal excitations in liquids, not specific to superfluid helium.

A new gapped excitation spectrum appears only in the superfluid state.

The generalized criterion accounts for multiple excitation branches affecting superfluidity.

Abstract

An analysis of experimental data shows that, in addition to phonon--roton excitations in superfluid helium, there necessarily exist at least one branch of elementary excitations whose energy spectrum strongly depends on temperature. On this basis, the Landau superfluidity criterion is generalized for several branches of elementary excitations, taking into account that the critical velocity should vanish during the phase transition of liquid helium from the superfluid state to the normal state. We suppose that a new spectrum of excitations with a gap, depending on interparticle interaction, corresponds to the transition of helium to superfluid state. This gap exists only in superfluid state and disappears at the transition temperature to normal state. The phonon-roton branch of excitations has no crucial influence on superfluidity. Rotons, as well as phonons, are not the specific excitations for the superfluid helium, but exist as universal excitations in liquid state, and form the continuous branch of excitations in different liquids. This point of view is confirmed experimentally and numerically.