When a three-dimensional (3D) system is flattened into a 2D “pancake,” interactions between its constituents play an enhanced role. To study these effects, atomic physicists trap atoms so that their motion is restricted to a 2D plane. The rule of thumb is that both the chemical potential and the temperature of the system must be well below the strong confinement in the transverse direction (perpendicular to the plane). Now, Dyke et al. show that the rule is a bit more subtle. As they increased the number of atoms in the trap at a fixed interaction strength, the size of the cloud in the transverse direction increased suddenly. This suggests that the system left the strictly 2D regime, even when the conditions above were met.
Phys. Rev. A 93, 011603(R) (2016).