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

摘要

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 (~4He) droplets and inside mixed ~4He/~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 colder ~4He/~3He 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.