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

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Science  01 Sep 2000:
Vol. 289, Issue 5484, pp. 1532-1535
DOI: 10.1126/science.289.5484.1532

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A linear carbonyl sulfide (OCS) molecule surrounded by 14 to 16 para-hydrogen (pH2) molecules, or similar numbers of ortho-deuterium (oD2) molecules, within large helium-4 (4He) droplets and inside mixed4He/3He droplets was investigated by infrared spectroscopy. In the pure 4He droplets (0.38 kelvin), both systems exhibited spectral features that indicate the excitation of angular momentum around the OCS axis. In the colder4He/3He droplets (0.15 kelvin), these features remained in the oD2 cluster spectra but disappeared in the pH2 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.

  • * Present address: Ruhr-University Bochum, Physical Chemistry II, D-44780 Bochum, Germany.

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