Roton

In theoretical physics, a roton is an elementary excitation, or quasiparticle, in superfluid helium-4. The dispersion relation of elementary excitations in this superfluid shows a linear increase from the origin, but exhibits first a maximum and then a minimum in energy as the momentum increases. Excitations with momenta in the linear region are called phonons; those with momenta close to the minimum are called rotons. Excitations with momenta near the maximum are sometimes called maxons.

The term "roton" is also used for the quantised eigenmode of a freely rotating molecule.

Originally, the roton spectrum was phenomenologically introduced by Lev Landau. Currently there exist different models which try to explain the roton spectrum, with a different degree of success and fundamentality. The requirement for any model of such kind is that it must explain not only the shape of the spectrum itself but also other related observables, such as the speed of sound and structure factor of superfluid helium-4. Microwave and Bragg spectroscopy has been conducted on helium to study roton spectrum.

Bose–Einstein condensation of rotons has been proposed and studied, but not yet detected.

...
Wikipedia