A new regime of sound
attenuation in a superfluid
Sound waves are ubiquitous in physics, in particular in the
macroscopic quantum world where all superfluids of neutral
particles have an acoustic excitation branch. Sound waves are in
this case composed of particles named phonons. At low
temperature phonons are the only microscopic degrees of freedom
of the superfluid, and the understanding of the phonon dynamics
and their interactions is a crucial step in the understanding of
the system properties, including transport phenomena,
temperature dependent viscosity, attenuation of sound and
macroscopic quantum coherence.
We can distinguish two types of superfluids depending on whether
the phonon excitation branch bends upwards or downwards for
increasing momentum. For upward bending, dominant interactions
among phonons at low temperature are 3-phonon processes. This is
the case for atomic Bose-Einstein condensates and superfluid
liquid helium at standard pressure. The attenuation of sound in
this kind of superfluid was calculated by Beliaev and Landau and
it was already measured in experiments. For downward bending,
phonon interactions occur mainly via 4-phonon processes as
understood by Landau and Khalatnikov in 1949. However the
attenuation of sound has not been calculated for almost 70 years
as this case was seen as exotic.
A collaboration between two groups of LKB has led to the first
quantitative prediction of the attenuation of sound in the
downward bending case. The effect, that has not yet been
investigated experimentally, may be disclosed for the first time
in two systems available at Laboratoire Kastler Brossel: liquid
helium under pressure and strongly interacting pair-condensed
Fermi gases. This would open a new era in the exploration of
low-temperature dynamics of macroscopically coherent quantum
many-body systems.
Publication
"Landau-Khalatnikov
phonon damping in strongly interacting Fermi gases", H.
Kurkjian, Y.
Castin, A.
Sinatra, EPL 116, 40002 (28 December 2016)
Contact
Alice Sinatra,
alice.sinatra@lkb.ens.fr