Microscopic origins of superfluidity in confined geometries

LIQUID helium provides a convenient model system in which to study the effects of disorder on strongly interacting media such as superfluids(1,2) and superconductors(3). Confinement of liquid helium in porous glasses profoundly affects its behaviour, even to the extent of changing the universality class of the normal-to-superfluid phase transition(1,4,5). Although the effects of disorder on macroscopic fluid properties (such as the superfluid fraction) have been studied extensively(6-11), the microscopic processes underlying these effects have received much less attention. For example, little is known theoretically(12) or experimentally(13,14) about the effects of disorder on the spectrum of elementary superfluid excitations, which reflects the dynamics of the system on a microscopic scale, Here we report inelastic neutron scattering measurements of the collective excitation spectrum for He-4 confined in porous aerogel glass. Near the superfluid transition temperature, the behaviour of the superfluid phase is governed by rotons (elementary excitations that are often compared to microscopic vortex rings), which we find to exhibit an increased effective mass and a decreased lifetime in the disordered system, relative to the unconfined superfluid. No theoretical predictions for these changes exist, and their origin is unclear; nevertheless, the disorder-induced changes in the microscopic collective excitation spectrum account fully for the observed changes in macroscopic fluid behaviour.