The many facets of small liquid helium-4 clusters

Abstract:  Helium is the only element that remains liquid under normal pressure down to zero temperature. Below 2.17K, the bosonic isotope helium-4 undergoes a phase transition to a superfluid. Motivated by this intriguing bulk behavior, the properties of finite-sized helium droplets have been studied extensively over the past 25 years or so. A number of properties of liquid helium-4 droplets are, just as those of nuclei, well described by the liquid drop model. The existence of the extremely fragile helium dimer was proven experimentally in 1994 in diffraction grating experiments. Since then, appreciable effort has gone into creating and characterizing trimers, tetramers and larger clusters. The ground state and excited state of the helium trimer are particularly interesting since these systems are candidates for Efimov states. The existence of Efimov states, which are unique due to scale invariance and an associated limit cycle, was predicted in 1971. However, till recently, Efimov states had -- although their existence had been confirmed experimentally -- not been imaged directly. Recently, ingenious experimental advances made it possible to directly image the quantum mechanical density distribution of helium dimers and trimers. I will review some of these experiments and related theoretical calculations that led to the experimental detection of the excited helium trimer Efimov state.