Because helium-3 is so light, and because, as a fermion, it can’t crowd into low energy levels, ensembles of 3He atoms have unusually large kinetic energy in the ground state. When that energy is partitioned among two dimensions instead of three, it’s thought to be sufficient to overcome van der Waals attractions and prevent the system from condensing into a 2D liquid—even at absolute zero.
It came as a surprise, then, when in a 1985 experiment1 by Bidyut Bhattacharyya and Francis Gasparini (State University of New York at Buffalo) 3He appeared to form a quasi-2D liquid. The researchers had added 3He to a thin film of 4He and chilled the mixture to millikelvin temperatures. In such a system, the 3He atoms strongly prefer to sit at the film’s surface, and because 4He becomes a superfluid, they can move about that surface almost as...
