Elastic Anomaly of Helium, Hydrogen and Neon Films on Disordered Substrate

We report systematic elasticity measurements of helium (⁴He and ³He), hydrogen (H₂, HD, D₂) and neon films physisorbed on a porous glass substrate. The elastic constant and dissipation as a function of temperature were measured by means of torsional oscillator technique with changing the film coverage. We have found that films of all species (⁴He, ³He, H₂, HD, D₂, Ne) showed an increase of the elastic constant accompanied by a dissipation peak at low temperatures, which we call the elastic anomaly. This suggests that any atomic and molecular films adsorbed on disordered substrates exhibit a crossover from a soft state at high temperature to a stiff state at low temperature. Only ⁴He and ³He films exhibit insulator–(super)fluid quantum phase transitions at critical coverages corresponding to about 2 atomic layers. Helium films below those critical coverages are in a gapped and compressible ground state. Hydrogen films show multiple dissipation peaks depending on the film coverage. The elastic anomaly of neon has single dissipation peak, and the peak temperature did not decrease below 5 K which signifies quantum phase transition does not occur in neon films.