Helium films show various quantum phases that undergo quantum phase transitions by changing coverage n. We found anomalous elastic phenomena in bosonic He4 and fermionic He3 films adsorbed on a glass substrate. The films stiffen under ac strain at low temperature with an excess dissipation. The onset temperature of the stiffening decreases to 0 K as n approaches a critical coverage nc. The elastic anomaly is explained by thermal activation of helium atoms from the localized to extended states with a distributed energy gap. We determine the energy band structure of helium films from elasticity. The ground states of He4 and He3 at n<nc, which are possibly a sort of Mott insulator or Mott glass, are identically gapped and compressible.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics