3) Macroscopic Quantum Tunneling

The phenomenon of macroscopic quantum tunneling (MQT) has attracted attention of experimentalists and theorists for many years. The most extraordinary feature of it is that a macroscopic object with many degrees of freedom manifests nonclassical behavior calculable only using quantum mechanics. MQT has been observed to play a role in a number of physical processes, e.g., in superfluids and in superconductors, as well as in magnetic systems.

There are very few undisputed examples of macroscopic quantum tunneling up to date. This is one of the main reasons for a large on-going experimental effort in this field. We have investigated MQT in relation with nucleation problems in solid and superfluid helium. Contrary to the common problems caused by dissipation in MQT, the helium experiments suffer easily from heterogeneity due to confining walls. Hence, extreme care has to be exercized to eliminate spurious effects and to reveal features generic to homogeneous nucleation.

- Nucleation of 3He-B from the A-phase: A Cosmic-Ray Effect?  [PRL 54, 245 (1985)]

- 4He-crystallization via quantum tunneling  [PRL 77, 2514 (1996)].

- Vortex unpinning by quantum tunneling  [PRL 81, 3451 (1998)].