Evidence for Superfluidity in Para-Hydrogen Clusters Inside Helium-4 Droplets at 0.15 Kelvin

Abstract

A linear carbonyl sulfide (OCS) molecule surrounded by 14 to 16 para-hydrogen (pH 2 ) molecules, or similar numbers of ortho-deuterium (oD 2 ) molecules, within large helium-4 ( 4 He) droplets and inside mixed 4 He/ 3 He droplets was investigated by infrared spectroscopy. In the pure 4 He droplets (0.38 kelvin), both systems exhibited spectral features that indicate the excitation of angular momentum around the OCS axis. In the colder 4 He/ 3 He 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 than helium.