Nucleation of the $\mathrm{AB}$ transition in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$: Surface effects and baked Alaska

The first-order phase transition between the $A$ and $B$ phases of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has remained an outstanding mystery in helium physics for nearly 20 years. The small difference in bulk free energies between the two phases, combined with the relatively large surface energy associated with the $\mathrm{AB}$ interface, leads to an anomalously large critical radius for nucleation, of order 1 μm, suggesting a lifetime for the super-cooled $A$ phase against homogeneous nucleation far beyond the age of the universe. Yet anisotropy of the high-temperature phase minimizes the depairing effects of surfaces, thus making conventional heterogeneous nucleation unlikely. Recent experiments have been reported that lend support to one of the more exotic nucleation mechanisms ever proposed: Leggett's "baked Alaska" model, in which the $B$ phase is nucleated by cosmic rays penetrating the supercooled $A$ phase. The results of these experiments are discussed, along with the prospects for future work.