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Phonon-roton modes in liquid 4 He coincide with Bose-Einstein condensation

“…The first picture is that they are collective density modes of a strongly interacting cold Bose liquid. This picture was initiated by Landau [1,2] and developed by Feynman [3] and others [4][5][6][7]. It forms the basis used today to calculate P-R mode energies with high accuracy [8][9][10].…”