Excitations of superfluid helium in porous media

The elementary excitations of superfluid He, characterised by the phonon-roton dispersion curve, have been studied in great detail over the last forty years since the first observation of the roton in a pioneering neutron-scattering experiment in Stockholm in 1957 [1]. The interest today is turning towards the effect of disorder and confinement on these excitations, when e.g. helium is immersed in porous media. The key word is here “dirty bosons”. We have studied the microscopic dynamics of superfluid He in porous media using the IN12 and IN6 spectrometers at the ILL. These measurements suggest that the modifications of the macroscopic properties (specific heat, superfluid fraction) are related to the existence of two-dimensional (2D) excitations rather than to modifications of the three-dimensional (3D) bulk excitations, such as the roton.