No abstract

Helium films show various quantum phases that undergo quantum phase transitions by changing coverage n. We found anomalous elastic phenomena in bosonic 4 He and fermionic 3 He 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 for the first time the energy band structure of helium films from elasticity. The ground states of 4 He and 3 He at n < nc are identically gapped and compressible, which are possibly a sort of Mott insulator or Mott glass.

Molecular hydrogen is a fascinating candidate for quantum fluid showing bosonic and fermionic superfluidity. We have studied diffusion dynamics of thin films of H2, HD and D2 adsorbed on a glass substrate by measurements of elasticity. The elasticity shows multiple anomalies well below bulk triple point. They are attributed to three different diffusion mechanisms of admolecules and their "freezing" into localized state: classical thermal diffusion of vacancies, quantum tunneling of vacancies, and diffusion of molecules in the uppermost surface. The surface diffusion is active down to 1 K, below which the molecules become localized. This suggests that the surface layer of hydrogen films is on the verge of quantum phase transition to superfluid state.

We report the critical behavior of steady quantum turbulence in superfluid 4 He. By using a thin vibrating wire with no bridge vortices in the superfluid, we find that lifetime of turbulent state for a given driving force reveals an exponential distribution. The mean lifetime estimated from the distribution decreases exponentially but greatly below a critical injection power. We estimate the vortex line density in the turbulent state and find that this critical behavior arises when the interdistance between vortex lines becomes as large as the turbulent region.Superfluid turbulence has attracted renewed experimental and theoretical interest as a result of recent investigations. 1 Turbulence in superfluid 4 He at very low temperatures consists only of a tangle of quantized vortex lines that defines superfluid flow. Hence, the energy flux in superfluid turbulence is described by the motions of the vortex lines. The energy of flows circulating vortex cores cascades from large vortex loops to smaller loops by reconnection ͑Richardson cascade͒; Kelvin waves with scales smaller than the vortex line spacing form along vortex lines and transfer energy from large wavelengths to smaller wavelengths ͑Kelvin wave cascade͒. 2-4 Consequently, the turbulence energy cascades from large scales to small scales, eventually dissipating at high wave numbers. This process can be observed in the decay of the density of vortex lines in superfluid helium 5,6 even at very low temperatures where the normal-fluid component is almost absent.Motions of vortex lines are also manifested in the continuous generation of turbulence by oscillating structures in superfluid 4 He. Experimental studies using oscillating spheres, 7 wires, 8,9 grids, 10 and tuning forks 11,12 indicate that superfluid turbulence can be generated at oscillating velocities above a critical velocity of about 50 mm/s. Vortex lines form bridges between a structure and its surrounding boundaries and they are shaken by the oscillation, developing into turbulence at velocities exceeding the critical velocity. 13,14 In a previous study, we found that a vibrating wire in the absence of bridge vortices also generates turbulence after vortex rings are applied from a vortex ring generator to the wire. 15 In a turbulent state, the vibrating wire continues to generate vortex lines even when further vortex rings are not applied. At sufficiently high driving forces, the generation seems to continue indefinitely. The wire velocity remains constant during the generation of vortex lines. However, the generation will stop suddenly when the driving force is reduced. After that, the vibrating wire cannot generate turbulence even at high velocities. This behavior is in marked contrast to the responses of other oscillating structures with bridge vortices, with respect to intermittent switching between turbulent flow and laminar flow or, more precisely, potential flow. 16,17 A vibrating wire with no bridge vortices determines the lifetime of the turbulent state in the turbulentto-laminar transitio...

No abstract