Quantum degeneracy in mesoscopic matter: Casimir effect and Bose-Einstein condensation

The pressure of ground-state phonons is an acoustic analog to the famous Casimir pressure of zero-point photons. In the experiments here, the authors demonstrate acoustic Casimir pressure in solid monolayer ${}³$He adsorbed on a nanotube. They also demonstrate pushing out thermal phonons with decreasing temperature or length. Another manifestation of quantum degeneracy is Bose-Einstein condensation of vacancies in dimer solid ${}³$He and supersolidity, i.e., nondissipative mass current. The condensation is detected by an increase of dissipation and decoupling of mass.