Nucleation and Interfacial Coupling between Pure and Dirty Superfluid Phases of [Formula presented]

The nucleation of the first-order phase transition of superfluid [Formula presented] from superfluid [Formula presented] is quite remarkable since it requires a seed of the order of a micron. We have studied this nucleation for [Formula presented] confined to a very dilute silica aerogel. This dirty superfluid behaves in a manner similar to previous reports for the pure superfluid. But we have discovered a novel magnetically driven nucleation switch acting on the pure superfluid- [Formula presented] phase. Last, we find the surprising result that the proximity effect between the pure and dirty superfluids at their interface is insufficient to nucleate the [Formula presented] phase in either superfluid.