Evidence for a Common Physical Origin of the Landau and BEC Theories of Superfluidity
There are two renowned theories of superfluidity in liquid 4 He, quite different and each with specific domains of application. In the first, the Landau theory, superflow follows from the existence of a well-defined collective mode supported by dense liquid 4 He, the phonon-roton mode. In the second, superflow is a manifestation of Bose-Einstein condensation (BEC) and phase coherence in the liquid. We present combined measurements of superfluidity, BEC and phonon-roton (P-R) modes in liquid 4 He confined in the porous medium MCM-41. The results integrate the two theories by showing that well-defined P-R modes exist where there is BEC. The two are common properties of a Bose condensed liquid and either can be used as a basis of a theory of superfluidity. In addition, the confinement and disorder suppresses the critical temperature for superfluidity, T c , below that for BEC creating a localized BEC phase consisting of islands of BEC and P-R modes. This phase is much like the pseudogap phase in the cuprate superconductors.