4He superfluidity on hydrogen and C60
The authors report measurements of the real 4He superfluid density, sigma s, as a function of temperature and coverage on hydrogen and C60 substrates using a high-Q Al oscillator. Sub-kelvin measurements of the helium mass loading of a 780 nm thick C60 film deposited on the active surface of the oscillator show that helium does not significantly permeate the interstitial sites of the C60 matrix, in either its superfluid or normal phase. In fact, the Kosterlitz-Thouless superfluid transition is observed on the C60 at very modest mass loadings. The superfluid behaviour on C60 is found to be very similar to that observed for helium on solid hydrogen, namely a suppression of sigma s(Tc) in transitions near 1 K and excess vapour dissipation in the normal phase.