![]() In a conventional fluid, sound waves are strongly damped, and this linear regime therefore represents the complete dispersion spectrum: at higher perturbation energies the phonons cannot be kept coherent and the excitation dies out. For small momenta this relationship is linear, and the slope of the line gives the speed of sound in the material. For example, when a sound wave (or phonon) travels in a material, the dispersion relation defines the relationship between the energy and the momentum of the phonon. The result is an experimental tour de force, establishing the superfluid dispersion relation to an unprecedented precision.Ī material’s dispersion relation describes how the material responds to a perturbation. Now, Henri Godfin at Grenoble Alpes University, France, and colleagues have carried out a new study of superfluid helium’s excitations that provides important constraints that could support or rule out certain theoretical models of superfluidity. Understanding and predicting such behaviors has been an active line of research since they were first observed in the 1930s. These fluctuations give rise to a menagerie of effects that can be both entertaining (a cup of liquid helium can empty itself) and potentially problematic (“superleaks” can compromise low-temperature experiments). ×Īt low temperatures, helium is the ultimate quantum fluid, or “superfluid,” exhibiting behaviors that are dominated by quantum-mechanical fluctuations. The minimum of the superfluid spectrum (yellow) corresponds to roton excitations. ![]() ![]() In the linear part of the spectrum, excitations take the form of phonons in both cases. APS/ Alan Stonebraker Figure 1: Schematic energy-momentum dispersion relations for (left) a normal fluid and (right) superfluid helium.
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