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-hydrogen (pH(2)) molecules, or similar numbers of ortha-deuterium (oD(2)) molecules, within large helium-4 (He-4) droplets and inside mixed He-4/He-3 droplets was investigated by infrared spectroscopy. In the pure He-4 droplets (0.38 kelvin), both systems exhibited spectral features that indicate the excitation of angular momentum around the OCS axis. In the colder He-4/He-3 droplets (0.15 kelvin), these features remained in the oD(2) cluster spectra but disappeared in the pH(2) spectra, indicating that the angular momentum is no Longer excited. These results are consistent with the onset of superfluidity, thereby providing the first evidence for superfluidity in a Liquid other then helium.