Mar 11, 2013
Phonons-Rotons interactions in superfluid Helium
Contact : Bjorn Fåk

dispersion relation of superfluid helium excitations.

Measurements of elementary excitations of superfluid helium performed at INAC show the importance of process in three excitations processes in Bose condensates.


Liquid helium study is an extremely rich subject that can address many issues of quantum and statistical physics. 4He shows below 2.2 K a phase that is characterized by a very high thermal conduction and a superfluid state, that is to say without measurable viscosity. In this state a macroscopic fraction of 4He atoms stands in a fundamental quantum state called Bose condensate. Elementary excitations of this phase are characterized by the dispersion relation (Fig.) between the energy E and wave vector Q. A small Q, the classical mode of vibration called phonon is revealed by a linear relationship between energy and wave vector. At Q = 2 Å-1, another mode appears. Predicted in the early 1940s by Landau who called this mode roton, it has been experimentally observed nearly 20 years later by inelastic neutron scattering.


INAC researchers have studied the interactions between rotons as well as between rotons and phonons by accurate measurements of the temperature dependence of the excitation energy and its lifetime using the technique of resonant spin echo neutron. The measurements show that the energy and lifetime of the roton vary differently depending on the temperature, in contrast to the early theoretical prediction that considered only four particles interaction processes. New theoretical work realized at INAC show that it is necessary to take into account three excitations processes like the decay of a roton in another roton and phonon. Recognition of the importance of these three excitations interactions should have implications for the understanding of other systems involving condensation of bosons, as in semiconductors (excitons), ultra-cold gases or spintronics (magnons).


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