Evidence for superfluidity in para-hydrogen clusters inside helium-4 droplets at 0.15 kelvin

A linear carbonyl sulfide (OCS) molecule surrounded by 14 to 16 para-hydrog en (pH(2)) molecules, or similar numbers of ortha-deuterium (oD(2)) molecul es, within large helium-4 (He-4) droplets and inside mixed He-4/He-3 drople ts was investigated by infrared spectroscopy. In the pure He-4 droplets (0. 38 kelvin), both systems exhibited spectral features that indicate the exci tation of angular momentum around the OCS axis. In the colder He-4/He-3 dro plets (0.15 kelvin), these features remained in the oD(2) cluster spectra b ut disappeared in the pH(2) spectra, indicating that the angular momentum i s no Longer excited. These results are consistent with the onset of superfl uidity, thereby providing the first evidence for superfluidity in a Liquid other then helium.