an elementary excitation (quasiparticle) in superfluid ⁴He with comparatively high energy & and momentum p. The dependence of the energy of the quasiparticles on the momentum ρ (dispersion relation) in superfluid ⁴He is shown in Figure 1.

Figure 1. Dependence on momentum of the energy of quasj-particles in superfluid helium

I. E. Tamm suggested the name “roton” for quasiparticles with momentum close to p₀ In the linear region of (p) the quasiparticles are called phonons; quasiparticles in ⁴He with any value of ρ are sometimes called phonons. Roton spectra have been determined with high accuracy by means of inelastic neutron scattering in superfluid ⁴He.

Rotons play an important role in the properties of superfluid helium at T ≳ 0.6°K. They are responsible for the existence of components of heat capacity, entropy, normal density, and other properties that vary exponentially with temperature. The kinetic properties of superfluid ⁴He—for example, viscosity and sound absorption—have been explained on the basis of collisions and interconversion of rotons and phonons.